1 '\" t 2 .\" Copyright (c) 1994, 2015, Oracle and/or its affiliates. All rights reserved. 3 .\" 4 .\" DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 5 .\" 6 .\" This code is free software; you can redistribute it and/or modify it 7 .\" under the terms of the GNU General Public License version 2 only, as 8 .\" published by the Free Software Foundation. 9 .\" 10 .\" This code is distributed in the hope that it will be useful, but WITHOUT 11 .\" ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 .\" FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 .\" version 2 for more details (a copy is included in the LICENSE file that 14 .\" accompanied this code). 15 .\" 16 .\" You should have received a copy of the GNU General Public License version 17 .\" 2 along with this work; if not, write to the Free Software Foundation, 18 .\" Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 .\" 20 .\" Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 .\" or visit www.oracle.com if you need additional information or have any 22 .\" questions. 23 .\" 24 .\" Arch: generic 25 .\" Software: JDK 8 26 .\" Date: 03 March 2015 27 .\" SectDesc: Basic Tools 28 .\" Title: java.1 29 .\" 30 .if n .pl 99999 31 .TH java 1 "03 March 2015" "JDK 8" "Basic Tools" 32 .\" ----------------------------------------------------------------- 33 .\" * Define some portability stuff 34 .\" ----------------------------------------------------------------- 35 .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 36 .\" http://bugs.debian.org/507673 37 .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html 38 .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 39 .ie \n(.g .ds Aq \(aq 40 .el .ds Aq ' 41 .\" ----------------------------------------------------------------- 42 .\" * set default formatting 43 .\" ----------------------------------------------------------------- 44 .\" disable hyphenation 45 .nh 46 .\" disable justification (adjust text to left margin only) 47 .ad l 48 .\" ----------------------------------------------------------------- 49 .\" * MAIN CONTENT STARTS HERE * 50 .\" ----------------------------------------------------------------- 51 52 .SH NAME 53 java \- Launches a Java application\&. 54 .SH SYNOPSIS 55 .sp 56 .nf 57 58 \fBjava\fR [\fIoptions\fR] \fIclassname\fR [\fIargs\fR] 59 .fi 60 .nf 61 62 \fBjava\fR [\fIoptions\fR] \fB\-jar\fR \fIfilename\fR [\fIargs\fR] 63 .fi 64 .sp 65 .TP 66 \fIoptions\fR 67 Command-line options separated by spaces\&. See Options\&. 68 .TP 69 \fIclassname\fR 70 The name of the class to be launched\&. 71 .TP 72 \fIfilename\fR 73 The name of the Java Archive (JAR) file to be called\&. Used only with the \f3-jar\fR option\&. 74 .TP 75 \fIargs\fR 76 The arguments passed to the \f3main()\fR method separated by spaces\&. 77 .SH DESCRIPTION 78 The \f3java\fR command starts a Java application\&. It does this by starting the Java Runtime Environment (JRE), loading the specified class, and calling that class\&'s \f3main()\fR method\&. The method must be declared \fIpublic\fR and \fIstatic\fR, it must not return any value, and it must accept a \f3String\fR array as a parameter\&. The method declaration has the following form: 79 .sp 80 .nf 81 \f3public static void main(String[] args)\fP 82 .fi 83 .nf 84 \f3\fP 85 .fi 86 .sp 87 The \f3java\fR command can be used to launch a JavaFX application by loading a class that either has a \f3main()\fR method or that extends \f3javafx\&.application\&.Application\fR\&. In the latter case, the launcher constructs an instance of the \f3Application\fR class, calls its \f3init()\fR method, and then calls the \f3start(javafx\&.stage\&.Stage)\fR method\&. 88 .PP 89 By default, the first argument that is not an option of the \f3java\fR command is the fully qualified name of the class to be called\&. If the \f3-jar\fR option is specified, its argument is the name of the JAR file containing class and resource files for the application\&. The startup class must be indicated by the \f3Main-Class\fR manifest header in its source code\&. 90 .PP 91 The JRE searches for the startup class (and other classes used by the application) in three sets of locations: the bootstrap class path, the installed extensions, and the user\(cqs class path\&. 92 .PP 93 Arguments after the class file name or the JAR file name are passed to the \f3main()\fR method\&. 94 .SH OPTIONS 95 The \f3java\fR command supports a wide range of options that can be divided into the following categories: 96 .TP 0.2i 97 \(bu 98 Standard Options 99 .TP 0.2i 100 \(bu 101 Non-Standard Options 102 .TP 0.2i 103 \(bu 104 Advanced Runtime Options 105 .TP 0.2i 106 \(bu 107 Advanced JIT Compiler Options 108 .TP 0.2i 109 \(bu 110 Advanced Serviceability Options 111 .TP 0.2i 112 \(bu 113 Advanced Garbage Collection Options 114 .PP 115 Standard options are guaranteed to be supported by all implementations of the Java Virtual Machine (JVM)\&. They are used for common actions, such as checking the version of the JRE, setting the class path, enabling verbose output, and so on\&. 116 .PP 117 Non-standard options are general purpose options that are specific to the Java HotSpot Virtual Machine, so they are not guaranteed to be supported by all JVM implementations, and are subject to change\&. These options start with \f3-X\fR\&. 118 .PP 119 Advanced options are not recommended for casual use\&. These are developer options used for tuning specific areas of the Java HotSpot Virtual Machine operation that often have specific system requirements and may require privileged access to system configuration parameters\&. They are also not guaranteed to be supported by all JVM implementations, and are subject to change\&. Advanced options start with \f3-XX\fR\&. 120 .PP 121 To keep track of the options that were deprecated or removed in the latest release, there is a section named Deprecated and Removed Options at the end of the document\&. 122 .PP 123 Boolean options are used to either enable a feature that is disabled by default or disable a feature that is enabled by default\&. Such options do not require a parameter\&. Boolean \f3-XX\fR options are enabled using the plus sign (\f3-XX:+\fR\fIOptionName\fR) and disabled using the minus sign (\f3-XX:-\fR\fIOptionName\fR)\&. 124 .PP 125 For options that require an argument, the argument may be separated from the option name by a space, a colon (:), or an equal sign (=), or the argument may directly follow the option (the exact syntax differs for each option)\&. If you are expected to specify the size in bytes, you can use no suffix, or use the suffix \f3k\fR or \f3K\fR for kilobytes (KB), \f3m\fR or \f3M\fR for megabytes (MB), \f3g\fR or \f3G\fR for gigabytes (GB)\&. For example, to set the size to 8 GB, you can specify either \f38g\fR, \f38192m\fR, \f38388608k\fR, or \f38589934592\fR as the argument\&. If you are expected to specify the percentage, use a number from 0 to 1 (for example, specify \f30\&.25\fR for 25%)\&. 126 .SS STANDARD\ OPTIONS 127 These are the most commonly used options that are supported by all implementations of the JVM\&. 128 .TP 129 -agentlib:\fIlibname\fR[=\fIoptions\fR] 130 .br 131 Loads the specified native agent library\&. After the library name, a comma-separated list of options specific to the library can be used\&. 132 133 If the option \f3-agentlib:foo\fR is specified, then the JVM attempts to load the library named \f3libfoo\&.so\fR in the location specified by the \f3LD_LIBRARY_PATH\fR system variable (on OS X this variable is \f3DYLD_LIBRARY_PATH\fR)\&. 134 135 The following example shows how to load the heap profiling tool (HPROF) library and get sample CPU information every 20 ms, with a stack depth of 3: 136 .sp 137 .nf 138 \f3\-agentlib:hprof=cpu=samples,interval=20,depth=3\fP 139 .fi 140 .nf 141 \f3\fP 142 .fi 143 .sp 144 145 146 The following example shows how to load the Java Debug Wire Protocol (JDWP) library and listen for the socket connection on port 8000, suspending the JVM before the main class loads: 147 .sp 148 .nf 149 \f3\-agentlib:jdwp=transport=dt_socket,server=y,address=8000\fP 150 .fi 151 .nf 152 \f3\fP 153 .fi 154 .sp 155 156 157 For more information about the native agent libraries, refer to the following: 158 .RS 159 .TP 0.2i 160 \(bu 161 The \f3java\&.lang\&.instrument\fR package description at http://docs\&.oracle\&.com/javase/8/docs/api/java/lang/instrument/package-summary\&.html 162 .TP 0.2i 163 \(bu 164 Agent Command Line Options in the JVM Tools Interface guide at http://docs\&.oracle\&.com/javase/8/docs/platform/jvmti/jvmti\&.html#starting 165 .RE 166 167 .TP 168 -agentpath:\fIpathname\fR[=\fIoptions\fR] 169 .br 170 Loads the native agent library specified by the absolute path name\&. This option is equivalent to \f3-agentlib\fR but uses the full path and file name of the library\&. 171 .TP 172 -client 173 .br 174 Selects the Java HotSpot Client VM\&. The 64-bit version of the Java SE Development Kit (JDK) currently ignores this option and instead uses the Server JVM\&. 175 176 For default JVM selection, see Server-Class Machine Detection at http://docs\&.oracle\&.com/javase/8/docs/technotes/guides/vm/server-class\&.html 177 .TP 178 -D\fIproperty\fR=\fIvalue\fR 179 .br 180 Sets a system property value\&. The \fIproperty\fR variable is a string with no spaces that represents the name of the property\&. The \fIvalue\fR variable is a string that represents the value of the property\&. If \fIvalue\fR is a string with spaces, then enclose it in quotation marks (for example \f3-Dfoo="foo bar"\fR)\&. 181 .TP 182 -d32 183 .br 184 Runs the application in a 32-bit environment\&. If a 32-bit environment is not installed or is not supported, then an error will be reported\&. By default, the application is run in a 32-bit environment unless a 64-bit system is used\&. 185 .TP 186 -d64 187 .br 188 Runs the application in a 64-bit environment\&. If a 64-bit environment is not installed or is not supported, then an error will be reported\&. By default, the application is run in a 32-bit environment unless a 64-bit system is used\&. 189 190 Currently only the Java HotSpot Server VM supports 64-bit operation, and the \f3-server\fR option is implicit with the use of \f3-d64\fR\&. The \f3-client\fR option is ignored with the use of \f3-d64\fR\&. This is subject to change in a future release\&. 191 .TP 192 .nf 193 -disableassertions[:[\fIpackagename\fR]\&.\&.\&.|:\fIclassname\fR], -da[:[\fIpackagename\fR]\&.\&.\&.|:\fIclassname\fR] 194 .br 195 .fi 196 Disables assertions\&. By default, assertions are disabled in all packages and classes\&. 197 198 With no arguments, \f3-disableassertions\fR (\f3-da\fR) disables assertions in all packages and classes\&. With the \fIpackagename\fR argument ending in \f3\&.\&.\&.\fR, the switch disables assertions in the specified package and any subpackages\&. If the argument is simply \f3\&.\&.\&.\fR, then the switch disables assertions in the unnamed package in the current working directory\&. With the \fIclassname\fR argument\f3\fR, the switch disables assertions in the specified class\&. 199 200 The \f3-disableassertions\fR (\f3-da\fR) option applies to all class loaders and to system classes (which do not have a class loader)\&. There is one exception to this rule: if the option is provided with no arguments, then it does not apply to system classes\&. This makes it easy to disable assertions in all classes except for system classes\&. The \f3-disablesystemassertions\fR option enables you to disable assertions in all system classes\&. 201 202 To explicitly enable assertions in specific packages or classes, use the \f3-enableassertions\fR (\f3-ea\fR) option\&. Both options can be used at the same time\&. For example, to run the \f3MyClass\fR application with assertions enabled in package \f3com\&.wombat\&.fruitbat\fR (and any subpackages) but disabled in class \f3com\&.wombat\&.fruitbat\&.Brickbat\fR, use the following command: 203 .sp 204 .nf 205 \f3java \-ea:com\&.wombat\&.fruitbat\&.\&.\&. \-da:com\&.wombat\&.fruitbat\&.Brickbat MyClass\fP 206 .fi 207 .nf 208 \f3\fP 209 .fi 210 .sp 211 212 .TP 213 -disablesystemassertions, -dsa 214 .br 215 Disables assertions in all system classes\&. 216 .TP 217 .nf 218 -enableassertions[:[\fIpackagename\fR]\&.\&.\&.|:\fIclassname\fR], -ea[:[\fIpackagename\fR]\&.\&.\&.|:\fIclassname\fR] 219 .br 220 .fi 221 Enables assertions\&. By default, assertions are disabled in all packages and classes\&. 222 223 With no arguments, \f3-enableassertions\fR (\f3-ea\fR) enables assertions in all packages and classes\&. With the \fIpackagename\fR argument ending in \f3\&.\&.\&.\fR, the switch enables assertions in the specified package and any subpackages\&. If the argument is simply \f3\&.\&.\&.\fR, then the switch enables assertions in the unnamed package in the current working directory\&. With the \fIclassname\fR argument\f3\fR, the switch enables assertions in the specified class\&. 224 225 The \f3-enableassertions\fR (\f3-ea\fR) option applies to all class loaders and to system classes (which do not have a class loader)\&. There is one exception to this rule: if the option is provided with no arguments, then it does not apply to system classes\&. This makes it easy to enable assertions in all classes except for system classes\&. The \f3-enablesystemassertions\fR option provides a separate switch to enable assertions in all system classes\&. 226 227 To explicitly disable assertions in specific packages or classes, use the \f3-disableassertions\fR (\f3-da\fR) option\&. If a single command contains multiple instances of these switches, then they are processed in order before loading any classes\&. For example, to run the \f3MyClass\fR application with assertions enabled only in package \f3com\&.wombat\&.fruitbat\fR (and any subpackages) but disabled in class \f3com\&.wombat\&.fruitbat\&.Brickbat\fR, use the following command: 228 .sp 229 .nf 230 \f3java \-ea:com\&.wombat\&.fruitbat\&.\&.\&. \-da:com\&.wombat\&.fruitbat\&.Brickbat MyClass\fP 231 .fi 232 .nf 233 \f3\fP 234 .fi 235 .sp 236 237 .TP 238 -enablesystemassertions, -esa 239 .br 240 Enables assertions in all system classes\&. 241 .TP 242 -help, -? 243 .br 244 Displays usage information for the \f3java\fR command without actually running the JVM\&. 245 .TP 246 -jar \fIfilename\fR 247 .br 248 Executes a program encapsulated in a JAR file\&. The \fIfilename\fR argument is the name of a JAR file with a manifest that contains a line in the form \f3Main-Class:\fR\fIclassname\fR that defines the class with the \f3public static void main(String[] args)\fR method that serves as your application\&'s starting point\&. 249 250 When you use the \f3-jar\fR option, the specified JAR file is the source of all user classes, and other class path settings are ignored\&. 251 252 For more information about JAR files, see the following resources: 253 .RS 254 .TP 0.2i 255 \(bu 256 jar(1) 257 .TP 0.2i 258 \(bu 259 The Java Archive (JAR) Files guide at http://docs\&.oracle\&.com/javase/8/docs/technotes/guides/jar/index\&.html 260 .TP 0.2i 261 \(bu 262 Lesson: Packaging Programs in JAR Files at http://docs\&.oracle\&.com/javase/tutorial/deployment/jar/index\&.html 263 .RE 264 265 .TP 266 -javaagent:\fIjarpath\fR[=\fIoptions\fR] 267 .br 268 Loads the specified Java programming language agent\&. For more information about instrumenting Java applications, see the \f3java\&.lang\&.instrument\fR package description in the Java API documentation at http://docs\&.oracle\&.com/javase/8/docs/api/java/lang/instrument/package-summary\&.html 269 .TP 270 -jre-restrict-search 271 .br 272 Includes user-private JREs in the version search\&. 273 .TP 274 -no-jre-restrict-search 275 .br 276 Excludes user-private JREs from the version search\&. 277 .TP 278 -server 279 .br 280 Selects the Java HotSpot Server VM\&. The 64-bit version of the JDK supports only the Server VM, so in that case the option is implicit\&. 281 282 For default JVM selection, see Server-Class Machine Detection at http://docs\&.oracle\&.com/javase/8/docs/technotes/guides/vm/server-class\&.html 283 .TP 284 -showversion 285 .br 286 Displays version information and continues execution of the application\&. This option is equivalent to the \f3-version\fR option except that the latter instructs the JVM to exit after displaying version information\&. 287 .TP 288 -splash:\fIimgname\fR 289 .br 290 Shows the splash screen with the image specified by \fIimgname\fR\&. For example, to show the \f3splash\&.gif\fR file from the \f3images\fR directory when starting your application, use the following option: 291 .sp 292 .nf 293 \f3\-splash:images/splash\&.gif\fP 294 .fi 295 .nf 296 \f3\fP 297 .fi 298 .sp 299 300 .TP 301 -verbose:class 302 .br 303 Displays information about each loaded class\&. 304 .TP 305 -verbose:gc 306 .br 307 Displays information about each garbage collection (GC) event\&. 308 .TP 309 -verbose:jni 310 .br 311 Displays information about the use of native methods and other Java Native Interface (JNI) activity\&. 312 .TP 313 -version 314 .br 315 Displays version information and then exits\&. This option is equivalent to the \f3-showversion\fR option except that the latter does not instruct the JVM to exit after displaying version information\&. 316 .TP 317 -version:\fIrelease\fR 318 .br 319 Specifies the release version to be used for running the application\&. If the version of the \f3java\fR command called does not meet this specification and an appropriate implementation is found on the system, then the appropriate implementation will be used\&. 320 321 The \fIrelease\fR argument specifies either the exact version string, or a list of version strings and ranges separated by spaces\&. A \fIversion string\fR is the developer designation of the version number in the following form: \f31\&.\fR\fIx\fR\f3\&.0_\fR\fIu\fR (where \fIx\fR is the major version number, and \fIu\fR is the update version number)\&. A \fIversion range\fR is made up of a version string followed by a plus sign (\f3+\fR) to designate this version or later, or a part of a version string followed by an asterisk (\f3*\fR) to designate any version string with a matching prefix\&. Version strings and ranges can be combined using a space for a logical \fIOR\fR combination, or an ampersand (\f3&\fR) for a logical \fIAND\fR combination of two version strings/ranges\&. For example, if running the class or JAR file requires either JRE 6u13 (1\&.6\&.0_13), or any JRE 6 starting from 6u10 (1\&.6\&.0_10), specify the following: 322 .sp 323 .nf 324 \f3\-version:"1\&.6\&.0_13 1\&.6* & 1\&.6\&.0_10+"\fP 325 .fi 326 .nf 327 \f3\fP 328 .fi 329 .sp 330 331 332 Quotation marks are necessary only if there are spaces in the \fIrelease\fR parameter\&. 333 334 For JAR files, the preference is to specify version requirements in the JAR file manifest rather than on the command line\&. 335 .SS NON-STANDARD\ OPTIONS 336 These options are general purpose options that are specific to the Java HotSpot Virtual Machine\&. 337 .TP 338 -X 339 .br 340 Displays help for all available \f3-X\fR options\&. 341 .TP 342 -Xbatch 343 .br 344 Disables background compilation\&. By default, the JVM compiles the method as a background task, running the method in interpreter mode until the background compilation is finished\&. The \f3-Xbatch\fR flag disables background compilation so that compilation of all methods proceeds as a foreground task until completed\&. 345 346 This option is equivalent to \f3-XX:-BackgroundCompilation\fR\&. 347 .TP 348 -Xbootclasspath:\fIpath\fR 349 .br 350 Specifies a list of directories, JAR files, and ZIP archives separated by colons (:) to search for boot class files\&. These are used in place of the boot class files included in the JDK\&. 351 352 \fI\fRDo not deploy applications that use this option to override a class in \f3rt\&.jar\fR, because this violates the JRE binary code license\&. 353 .TP 354 -Xbootclasspath/a:\fIpath\fR 355 .br 356 Specifies a list of directories, JAR files, and ZIP archives separated by colons (:) to append to the end of the default bootstrap class path\&. 357 358 Do not deploy applications that use this option to override a class in \f3rt\&.jar\fR, because this violates the JRE binary code license\&. 359 .TP 360 -Xbootclasspath/p:\fIpath\fR 361 .br 362 Specifies a list of directories, JAR files, and ZIP archives separated by colons (:) to prepend to the front of the default bootstrap class path\&. 363 364 Do not deploy applications that use this option to override a class in \f3rt\&.jar\fR, because this violates the JRE binary code license\&. 365 .TP 366 -Xcheck:jni 367 .br 368 Performs additional checks for Java Native Interface (JNI) functions\&. Specifically, it validates the parameters passed to the JNI function and the runtime environment data before processing the JNI request\&. Any invalid data encountered indicates a problem in the native code, and the JVM will terminate with an irrecoverable error in such cases\&. Expect a performance degradation when this option is used\&. 369 .TP 370 -Xcomp 371 .br 372 Forces compilation of methods on first invocation\&. By default, the Client VM (\f3-client\fR) performs 1,000 interpreted method invocations and the Server VM (\f3-server\fR) performs 10,000 interpreted method invocations to gather information for efficient compilation\&. Specifying the \f3-Xcomp\fR option disables interpreted method invocations to increase compilation performance at the expense of efficiency\&. 373 374 You can also change the number of interpreted method invocations before compilation using the \f3-XX:CompileThreshold\fR option\&. 375 .TP 376 -Xdebug 377 .br 378 Does nothing\&. Provided for backward compatibility\&. 379 .TP 380 -Xdiag 381 .br 382 Shows additional diagnostic messages\&. 383 .TP 384 -Xfuture 385 .br 386 Enables strict class-file format checks that enforce close conformance to the class-file format specification\&. Developers are encouraged to use this flag when developing new code because the stricter checks will become the default in future releases\&. 387 .TP 388 -Xint 389 .br 390 Runs the application in interpreted-only mode\&. Compilation to native code is disabled, and all bytecode is executed by the interpreter\&. The performance benefits offered by the just in time (JIT) compiler are not present in this mode\&. 391 .TP 392 -Xinternalversion 393 .br 394 Displays more detailed JVM version information than the \f3-version\fR option, and then exits\&. 395 .TP 396 -Xloggc:\fIfilename\fR 397 .br 398 Sets the file to which verbose GC events information should be redirected for logging\&. The information written to this file is similar to the output of \f3-verbose:gc\fR with the time elapsed since the first GC event preceding each logged event\&. The \f3-Xloggc\fR option overrides \f3-verbose:gc\fR if both are given with the same \f3java\fR command\&. 399 400 Example: 401 .sp 402 .nf 403 \f3\-Xloggc:garbage\-collection\&.log\fP 404 .fi 405 .nf 406 \f3\fP 407 .fi 408 .sp 409 410 .TP 411 -Xmaxjitcodesize=\fIsize\fR 412 .br 413 Specifies the maximum code cache size (in bytes) for JIT-compiled code\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. By default, the value is set to 48 MB: 414 .sp 415 .nf 416 \f3\-Xmaxjitcodesize=48m\fP 417 .fi 418 .nf 419 \f3\fP 420 .fi 421 .sp 422 423 424 This option is equivalent to \f3-XX:ReservedCodeCacheSize\fR\&. 425 .TP 426 -Xmixed 427 .br 428 Executes all bytecode by the interpreter except for hot methods, which are compiled to native code\&. 429 .TP 430 -Xmn\fIsize\fR 431 .br 432 Sets the initial and maximum size (in bytes) of the heap for the young generation (nursery)\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. 433 434 The young generation region of the heap is used for new objects\&. GC is performed in this region more often than in other regions\&. If the size for the young generation is too small, then a lot of minor garbage collections will be performed\&. If the size is too large, then only full garbage collections will be performed, which can take a long time to complete\&. Oracle recommends that you keep the size for the young generation between a half and a quarter of the overall heap size\&. 435 436 The following examples show how to set the initial and maximum size of young generation to 256 MB using various units: 437 .sp 438 .nf 439 \f3\-Xmn256m\fP 440 .fi 441 .nf 442 \f3\-Xmn262144k\fP 443 .fi 444 .nf 445 \f3\-Xmn268435456\fP 446 .fi 447 .nf 448 \f3\fP 449 .fi 450 .sp 451 452 453 Instead of the \f3-Xmn\fR option to set both the initial and maximum size of the heap for the young generation, you can use \f3-XX:NewSize\fR to set the initial size and \f3-XX:MaxNewSize\fR to set the maximum size\&. 454 .TP 455 -Xms\fIsize\fR 456 .br 457 Sets the initial size (in bytes) of the heap\&. This value must be a multiple of 1024 and greater than 1 MB\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. 458 459 The following examples show how to set the size of allocated memory to 6 MB using various units: 460 .sp 461 .nf 462 \f3\-Xms6291456\fP 463 .fi 464 .nf 465 \f3\-Xms6144k\fP 466 .fi 467 .nf 468 \f3\-Xms6m\fP 469 .fi 470 .nf 471 \f3\fP 472 .fi 473 .sp 474 475 476 If you do not set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation\&. The initial size of the heap for the young generation can be set using the \f3-Xmn\fR option or the \f3-XX:NewSize\fR option\&. 477 .TP 478 -Xmx\fIsize\fR 479 .br 480 Specifies the maximum size (in bytes) of the memory allocation pool in bytes\&. This value must be a multiple of 1024 and greater than 2 MB\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. The default value is chosen at runtime based on system configuration\&. For server deployments, \f3-Xms\fR and \f3-Xmx\fR are often set to the same value\&. See the section "Ergonomics" in \fIJava SE HotSpot Virtual Machine Garbage Collection Tuning Guide\fR at http://docs\&.oracle\&.com/javase/8/docs/technotes/guides/vm/gctuning/index\&.html\&. 481 482 The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units: 483 .sp 484 .nf 485 \f3\-Xmx83886080\fP 486 .fi 487 .nf 488 \f3\-Xmx81920k\fP 489 .fi 490 .nf 491 \f3\-Xmx80m\fP 492 .fi 493 .nf 494 \f3\fP 495 .fi 496 .sp 497 498 499 The \f3-Xmx\fR option is equivalent to \f3-XX:MaxHeapSize\fR\&. 500 .TP 501 -Xnoclassgc 502 .br 503 Disables garbage collection (GC) of classes\&. This can save some GC time, which shortens interruptions during the application run\&. 504 505 When you specify \f3-Xnoclassgc\fR at startup, the class objects in the application will be left untouched during GC and will always be considered live\&. This can result in more memory being permanently occupied which, if not used carefully, will throw an out of memory exception\&. 506 .TP 507 -Xprof 508 .br 509 Profiles the running program and sends profiling data to standard output\&. This option is provided as a utility that is useful in program development and is not intended to be used in production systems\&. 510 .TP 511 -Xrs 512 .br 513 Reduces the use of operating system signals by the JVM\&. 514 515 Shutdown hooks enable orderly shutdown of a Java application by running user cleanup code (such as closing database connections) at shutdown, even if the JVM terminates abruptly\&. 516 517 The JVM catches signals to implement shutdown hooks for unexpected termination\&. The JVM uses \f3SIGHUP\fR, \f3SIGINT\fR, and \f3SIGTERM\fR to initiate the running of shutdown hooks\&. 518 519 The JVM uses a similar mechanism to implement the feature of dumping thread stacks for debugging purposes\&. The JVM uses \f3SIGQUIT\fR to perform thread dumps\&. 520 521 Applications embedding the JVM frequently need to trap signals such as \f3SIGINT\fR or \f3SIGTERM\fR, which can lead to interference with the JVM signal handlers\&. The \f3-Xrs\fR option is available to address this issue\&. When \f3-Xrs\fR is used, the signal masks for \f3SIGINT\fR, \f3SIGTERM\fR, \f3SIGHUP\fR, and \f3SIGQUIT\fR are not changed by the JVM, and signal handlers for these signals are not installed\&. 522 523 There are two consequences of specifying \f3-Xrs\fR: 524 .RS 525 .TP 0.2i 526 \(bu 527 \f3SIGQUIT\fR thread dumps are not available\&. 528 .TP 0.2i 529 \(bu 530 User code is responsible for causing shutdown hooks to run, for example, by calling \f3System\&.exit()\fR when the JVM is to be terminated\&. 531 .RE 532 533 .TP 534 -Xshare:\fImode\fR 535 .br 536 Sets the class data sharing mode\&. Possible \fImode\fR arguments for this option include the following: 537 .RS 538 .TP 539 auto 540 Use shared class data if possible\&. This is the default value for Java HotSpot 32-Bit Client VM\&. 541 .TP 542 on 543 Require the use of class data sharing\&. Print an error message and exit if class data sharing cannot be used\&. 544 .TP 545 off 546 Do not use shared class data\&. This is the default value for Java HotSpot 32-Bit Server VM, Java HotSpot 64-Bit Client VM, and Java HotSpot 64-Bit Server VM\&. 547 .TP 548 dump 549 Manually generate the class data sharing archive\&. 550 .RE 551 552 .TP 553 -XshowSettings:\fIcategory\fR 554 .br 555 Shows settings and continues\&. Possible \fIcategory\fR arguments for this option include the following: 556 .RS 557 .TP 558 all 559 Shows all categories of settings\&. This is the default value\&. 560 .TP 561 locale 562 Shows settings related to locale\&. 563 .TP 564 properties 565 Shows settings related to system properties\&. 566 .TP 567 vm 568 Shows the settings of the JVM\&. 569 .RE 570 571 .TP 572 -Xss\fIsize\fR 573 .br 574 Sets the thread stack size (in bytes)\&. Append the letter \f3k\fR or \f3K\fR to indicate KB, \f3m\fR or \f3M\fR to indicate MB, \f3g\fR or \f3G\fR to indicate GB\&. The default value depends on the platform: 575 .RS 576 .TP 0.2i 577 \(bu 578 Linux/ARM (32-bit): 320 KB 579 .TP 0.2i 580 \(bu 581 Linux/i386 (32-bit): 320 KB 582 .TP 0.2i 583 \(bu 584 Linux/x64 (64-bit): 1024 KB 585 .TP 0.2i 586 \(bu 587 OS X (64-bit): 1024 KB 588 .TP 0.2i 589 \(bu 590 Oracle Solaris/i386 (32-bit): 320 KB 591 .TP 0.2i 592 \(bu 593 Oracle Solaris/x64 (64-bit): 1024 KB 594 .TP 0.2i 595 \(bu 596 Windows: depends on virtual memory 597 .RE 598 599 600 The following examples set the thread stack size to 1024 KB in different units: 601 .sp 602 .nf 603 \f3\-Xss1m\fP 604 .fi 605 .nf 606 \f3\-Xss1024k\fP 607 .fi 608 .nf 609 \f3\-Xss1048576\fP 610 .fi 611 .nf 612 \f3\fP 613 .fi 614 .sp 615 616 617 This option is equivalent to \f3-XX:ThreadStackSize\fR\&. 618 .TP 619 -Xusealtsigs 620 .br 621 Use alternative signals instead of \f3SIGUSR1\fR and \f3SIGUSR2\fR for JVM internal signals\&. This option is equivalent to \f3-XX:+UseAltSigs\fR\&. 622 .TP 623 -Xverify:\fImode\fR 624 .br 625 Sets the mode of the bytecode verifier\&. Bytecode verification helps to troubleshoot some problems, but it also adds overhead to the running application\&. Possible \fImode\fR arguments for this option include the following: 626 .RS 627 .TP 628 none 629 Do not verify the bytecode\&. This reduces startup time and also reduces the protection provided by Java\&. 630 .TP 631 remote 632 Verify those classes that are not loaded by the bootstrap class loader\&. This is the default behavior if you do not specify the \f3-Xverify\fR option\&. 633 .TP 634 all 635 Verify all classes\&. 636 .RE 637 638 .SS ADVANCED\ RUNTIME\ OPTIONS 639 These options control the runtime behavior of the Java HotSpot VM\&. 640 .TP 641 -XX:+DisableAttachMechanism 642 .br 643 Enables the option that disables the mechanism that lets tools attach to the JVM\&. By default, this option is disabled, meaning that the attach mechanism is enabled and you can use tools such as \f3jcmd\fR, \f3jstack\fR, \f3jmap\fR, and \f3jinfo\fR\&. 644 .TP 645 -XX:ErrorFile=\fIfilename\fR 646 .br 647 Specifies the path and file name to which error data is written when an irrecoverable error occurs\&. By default, this file is created in the current working directory and named \f3hs_err_pid\fR\fIpid\fR\f3\&.log\fR where \fIpid\fR is the identifier of the process that caused the error\&. The following example shows how to set the default log file (note that the identifier of the process is specified as \f3%p\fR): 648 .sp 649 .nf 650 \f3\-XX:ErrorFile=\&./hs_err_pid%p\&.log\fP 651 .fi 652 .nf 653 \f3\fP 654 .fi 655 .sp 656 657 658 The following example shows how to set the error log to \f3/var/log/java/java_error\&.log\fR: 659 .sp 660 .nf 661 \f3\-XX:ErrorFile=/var/log/java/java_error\&.log\fP 662 .fi 663 .nf 664 \f3\fP 665 .fi 666 .sp 667 668 669 If the file cannot be created in the specified directory (due to insufficient space, permission problem, or another issue), then the file is created in the temporary directory for the operating system\&. The temporary directory is \f3/tmp\fR\&. 670 .TP 671 -XX:+FailOverToOldVerifier 672 .br 673 Enables automatic failover to the old verifier when the new type checker fails\&. By default, this option is disabled and it is ignored (that is, treated as disabled) for classes with a recent bytecode version\&. You can enable it for classes with older versions of the bytecode\&. 674 .TP 675 -XX:LargePageSizeInBytes=\fIsize\fR 676 .br 677 On Solaris, sets the maximum size (in bytes) for large pages used for Java heap\&. The \fIsize\fR argument must be a power of 2 (2, 4, 8, 16, \&.\&.\&.)\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. By default, the size is set to 0, meaning that the JVM chooses the size for large pages automatically\&. 678 679 The following example illustrates how to set the large page size to 4 megabytes (MB): 680 .sp 681 .nf 682 \f3\-XX:LargePageSizeInBytes=4m\fP 683 .fi 684 .nf 685 \f3\fP 686 .fi 687 .sp 688 689 .TP 690 -XX:MaxDirectMemorySize=\fIsize\fR 691 .br 692 Sets the maximum total size (in bytes) of the New I/O (the \f3java\&.nio\fR package) direct-buffer allocations\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. By default, the size is set to 0, meaning that the JVM chooses the size for NIO direct-buffer allocations automatically\&. 693 694 The following examples illustrate how to set the NIO size to 1024 KB in different units: 695 .sp 696 .nf 697 \f3\-XX:MaxDirectMemorySize=1m\fP 698 .fi 699 .nf 700 \f3\-XX:MaxDirectMemorySize=1024k\fP 701 .fi 702 .nf 703 \f3\-XX:MaxDirectMemorySize=1048576\fP 704 .fi 705 .nf 706 \f3\fP 707 .fi 708 .sp 709 710 .TP 711 -XX:NativeMemoryTracking=\fImode\fR 712 .br 713 Specifies the mode for tracking JVM native memory usage\&. Possible \fImode\fR arguments for this option include the following: 714 .RS 715 .TP 716 off 717 Do not track JVM native memory usage\&. This is the default behavior if you do not specify the \f3-XX:NativeMemoryTracking\fR option\&. 718 .TP 719 summary 720 Only track memory usage by JVM subsystems, such as Java heap, class, code, and thread\&. 721 .TP 722 detail 723 In addition to tracking memory usage by JVM subsystems, track memory usage by individual \f3CallSite\fR, individual virtual memory region and its committed regions\&. 724 .RE 725 726 .TP 727 -XX:ObjectAlignmentInBytes=\fIalignment\fR 728 .br 729 Sets the memory alignment of Java objects (in bytes)\&. By default, the value is set to 8 bytes\&. The specified value should be a power of two, and must be within the range of 8 and 256 (inclusive)\&. This option makes it possible to use compressed pointers with large Java heap sizes\&. 730 731 The heap size limit in bytes is calculated as: 732 733 \f34GB * ObjectAlignmentInBytes\fR 734 735 Note: As the alignment value increases, the unused space between objects will also increase\&. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes\&. 736 .TP 737 -XX:OnError=\fIstring\fR 738 .br 739 Sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs\&. If the string contains spaces, then it must be enclosed in quotation marks\&. 740 741 \fI\fRThe following example shows how the \f3-XX:OnError\fR option can be used to run the \f3gcore\fR command to create the core image, and the debugger is started to attach to the process in case of an irrecoverable error (the \f3%p\fR designates the current process): 742 .sp 743 .nf 744 \f3\-XX:OnError="gcore %p;dbx \- %p"\fP 745 .fi 746 .nf 747 \f3\fP 748 .fi 749 .sp 750 751 .TP 752 -XX:OnOutOfMemoryError=\fIstring\fR 753 .br 754 Sets a custom command or a series of semicolon-separated commands to run when an \f3OutOfMemoryError\fR exception is first thrown\&. If the string contains spaces, then it must be enclosed in quotation marks\&. For an example of a command string, see the description of the \f3-XX:OnError\fR option\&. 755 .TP 756 -XX:+PerfDataSaveToFile 757 .br 758 If enabled, saves jstat(1) binary data when the Java application exits\&. This binary data is saved in a file named \f3hsperfdata_\fR\fI<pid>\fR, where \fI<pid>\fR is the process identifier of the Java application you ran\&. Use \f3jstat\fR to display the performance data contained in this file as follows: 759 .sp 760 .nf 761 \f3jstat \-class file:///\fI<path>\fR/hsperfdata_\fI<pid>\fR\fP 762 .fi 763 .nf 764 \f3jstat \-gc file:///\fI<path>\fR/hsperfdata_\fI<pid>\fR\fP 765 .fi 766 .sp 767 768 .TP 769 -XX:+PrintCommandLineFlags 770 .br 771 Enables printing of ergonomically selected JVM flags that appeared on the command line\&. It can be useful to know the ergonomic values set by the JVM, such as the heap space size and the selected garbage collector\&. By default, this option is disabled and flags are not printed\&. 772 .TP 773 -XX:+PrintNMTStatistics 774 .br 775 Enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see \f3-XX:NativeMemoryTracking\fR)\&. By default, this option is disabled and native memory tracking data is not printed\&. 776 .TP 777 -XX:+RelaxAccessControlCheck 778 .br 779 Decreases the amount of access control checks in the verifier\&. By default, this option is disabled, and it is ignored (that is, treated as disabled) for classes with a recent bytecode version\&. You can enable it for classes with older versions of the bytecode\&. 780 .TP 781 -XX:+ShowMessageBoxOnError 782 .br 783 Enables displaying of a dialog box when the JVM experiences an irrecoverable error\&. This prevents the JVM from exiting and keeps the process active so that you can attach a debugger to it to investigate the cause of the error\&. By default, this option is disabled\&. 784 .TP 785 -XX:ThreadStackSize=\fIsize\fR 786 .br 787 Sets the thread stack size (in bytes)\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. The default value depends on the platform: 788 .RS 789 .TP 0.2i 790 \(bu 791 Linux/ARM (32-bit): 320 KB 792 .TP 0.2i 793 \(bu 794 Linux/i386 (32-bit): 320 KB 795 .TP 0.2i 796 \(bu 797 Linux/x64 (64-bit): 1024 KB 798 .TP 0.2i 799 \(bu 800 OS X (64-bit): 1024 KB 801 .TP 0.2i 802 \(bu 803 Oracle Solaris/i386 (32-bit): 320 KB 804 .TP 0.2i 805 \(bu 806 Oracle Solaris/x64 (64-bit): 1024 KB 807 .TP 0.2i 808 \(bu 809 Windows: depends on virtual memory 810 .RE 811 812 813 The following examples show how to set the thread stack size to 1024 KB in different units: 814 .sp 815 .nf 816 \f3\-XX:ThreadStackSize=1m\fP 817 .fi 818 .nf 819 \f3\-XX:ThreadStackSize=1024k\fP 820 .fi 821 .nf 822 \f3\-XX:ThreadStackSize=1048576\fP 823 .fi 824 .nf 825 \f3\fP 826 .fi 827 .sp 828 829 830 This option is equivalent to \f3-Xss\fR\&. 831 .TP 832 -XX:+TraceClassLoading 833 .br 834 Enables tracing of classes as they are loaded\&. By default, this option is disabled and classes are not traced\&. 835 .TP 836 -XX:+TraceClassLoadingPreorder 837 .br 838 Enables tracing of all loaded classes in the order in which they are referenced\&. By default, this option is disabled and classes are not traced\&. 839 .TP 840 -XX:+TraceClassResolution 841 .br 842 Enables tracing of constant pool resolutions\&. By default, this option is disabled and constant pool resolutions are not traced\&. 843 .TP 844 -XX:+TraceClassUnloading 845 .br 846 Enables tracing of classes as they are unloaded\&. By default, this option is disabled and classes are not traced\&. 847 .TP 848 -XX:+TraceLoaderConstraints 849 .br 850 Enables tracing of the loader constraints recording\&. By default, this option is disabled and loader constraints recording is not traced\&. 851 .TP 852 -XX:+UseAltSigs 853 .br 854 Enables the use of alternative signals instead of \f3SIGUSR1\fR and \f3SIGUSR2\fR for JVM internal signals\&. By default, this option is disabled and alternative signals are not used\&. This option is equivalent to \f3-Xusealtsigs\fR\&. 855 .TP 856 -XX:-UseBiasedLocking 857 .br 858 Disables the use of biased locking\&. Some applications with significant amounts of uncontended synchronization may attain significant speedups with this flag enabled, whereas applications with certain patterns of locking may see slowdowns\&. For more information about the biased locking technique, see the example in Java Tuning White Paper at http://www\&.oracle\&.com/technetwork/java/tuning-139912\&.html#section4\&.2\&.5 859 860 By default, this option is enabled\&. 861 .TP 862 -XX:-UseCompressedOops 863 .br 864 Disables the use of compressed pointers\&. By default, this option is enabled, and compressed pointers are used when Java heap sizes are less than 32 GB\&. When this option is enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes less than 32 GB\&. This option works only for 64-bit JVMs\&. 865 866 It is also possible to use compressed pointers when Java heap sizes are greater than 32GB\&. See the \f3-XX:ObjectAlignmentInBytes\fR option\&. 867 .TP 868 -XX:+UseHugeTLBFS 869 .br 870 This option for Linux is the equivalent of specifying \f3-XX:+UseLargePages\fR\&. This option is disabled by default\&. This option pre-allocates all large pages up-front, when memory is reserved; consequently the JVM cannot dynamically grow or shrink large pages memory areas; see \f3-XX:UseTransparentHugePages\fR if you want this behavior\&. 871 872 For more information, see Large Pages\&. 873 .TP 874 -XX:+UseLargePages 875 .br 876 Enables the use of large page memory\&. By default, this option is disabled and large page memory is not used\&. 877 878 For more information, see Large Pages\&. 879 .TP 880 -XX:+UseMembar 881 .br 882 Enables issuing of membars on thread state transitions\&. This option is disabled by default on all platforms except ARM servers, where it is enabled\&. (It is recommended that you do not disable this option on ARM servers\&.) 883 .TP 884 -XX:+UsePerfData 885 .br 886 Enables the \f3perfdata\fR feature\&. This option is enabled by default to allow JVM monitoring and performance testing\&. Disabling it suppresses the creation of the \f3hsperfdata_userid\fR directories\&. To disable the \f3perfdata\fR feature, specify \f3-XX:-UsePerfData\fR\&. 887 .TP 888 -XX:+UseTransparentHugePages 889 .br 890 On Linux, enables the use of large pages that can dynamically grow or shrink\&. This option is disabled by default\&. You may encounter performance problems with transparent huge pages as the OS moves other pages around to create huge pages; this option is made available for experimentation\&. 891 892 For more information, see Large Pages\&. 893 .TP 894 -XX:+AllowUserSignalHandlers 895 .br 896 Enables installation of signal handlers by the application\&. By default, this option is disabled and the application is not allowed to install signal handlers\&. 897 .SS ADVANCED\ JIT\ COMPILER\ OPTIONS 898 These options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM\&. 899 .TP 900 -XX:+AggressiveOpts 901 .br 902 Enables the use of aggressive performance optimization features, which are expected to become default in upcoming releases\&. By default, this option is disabled and experimental performance features are not used\&. 903 .TP 904 -XX:AllocateInstancePrefetchLines=\fIlines\fR 905 .br 906 Sets the number of lines to prefetch ahead of the instance allocation pointer\&. By default, the number of lines to prefetch is set to 1: 907 .sp 908 .nf 909 \f3\-XX:AllocateInstancePrefetchLines=1\fP 910 .fi 911 .nf 912 \f3\fP 913 .fi 914 .sp 915 916 917 Only the Java HotSpot Server VM supports this option\&. 918 .TP 919 -XX:AllocatePrefetchDistance=\fIsize\fR 920 .br 921 Sets the size (in bytes) of the prefetch distance for object allocation\&. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object\&. Each Java thread has its own allocation point\&. 922 923 Negative values denote that prefetch distance is chosen based on the platform\&. Positive values are bytes to prefetch\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. The default value is set to -1\&. 924 925 The following example shows how to set the prefetch distance to 1024 bytes: 926 .sp 927 .nf 928 \f3\-XX:AllocatePrefetchDistance=1024\fP 929 .fi 930 .nf 931 \f3\fP 932 .fi 933 .sp 934 935 936 Only the Java HotSpot Server VM supports this option\&. 937 .TP 938 -XX:AllocatePrefetchInstr=\fIinstruction\fR 939 .br 940 Sets the prefetch instruction to prefetch ahead of the allocation pointer\&. Only the Java HotSpot Server VM supports this option\&. Possible values are from 0 to 3\&. The actual instructions behind the values depend on the platform\&. By default, the prefetch instruction is set to 0: 941 .sp 942 .nf 943 \f3\-XX:AllocatePrefetchInstr=0\fP 944 .fi 945 .nf 946 \f3\fP 947 .fi 948 .sp 949 950 951 Only the Java HotSpot Server VM supports this option\&. 952 .TP 953 -XX:AllocatePrefetchLines=\fIlines\fR 954 .br 955 Sets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code\&. The default value is 1 if the last allocated object was an instance, and 3 if it was an array\&. 956 957 The following example shows how to set the number of loaded cache lines to 5: 958 .sp 959 .nf 960 \f3\-XX:AllocatePrefetchLines=5\fP 961 .fi 962 .nf 963 \f3\fP 964 .fi 965 .sp 966 967 968 Only the Java HotSpot Server VM supports this option\&. 969 .TP 970 -XX:AllocatePrefetchStepSize=\fIsize\fR 971 .br 972 Sets the step size (in bytes) for sequential prefetch instructions\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. By default, the step size is set to 16 bytes: 973 .sp 974 .nf 975 \f3\-XX:AllocatePrefetchStepSize=16\fP 976 .fi 977 .nf 978 \f3\fP 979 .fi 980 .sp 981 982 983 Only the Java HotSpot Server VM supports this option\&. 984 .TP 985 -XX:AllocatePrefetchStyle=\fIstyle\fR 986 .br 987 Sets the generated code style for prefetch instructions\&. The \fIstyle\fR argument is an integer from 0 to 3: 988 .RS 989 .TP 990 0 991 Do not generate prefetch instructions\&. 992 .TP 993 1 994 Execute prefetch instructions after each allocation\&. This is the default parameter\&. 995 .TP 996 2 997 Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed\&. 998 .TP 999 3 1000 Use BIS instruction on SPARC for allocation prefetch\&. 1001 .RE 1002 1003 1004 Only the Java HotSpot Server VM supports this option\&. 1005 .TP 1006 -XX:+BackgroundCompilation 1007 .br 1008 Enables background compilation\&. This option is enabled by default\&. To disable background compilation, specify \f3-XX:-BackgroundCompilation\fR (this is equivalent to specifying \f3-Xbatch\fR)\&. 1009 .TP 1010 -XX:CICompilerCount=\fIthreads\fR 1011 .br 1012 Sets the number of compiler threads to use for compilation\&. By default, the number of threads is set to 2 for the server JVM, to 1 for the client JVM, and it scales to the number of cores if tiered compilation is used\&. The following example shows how to set the number of threads to 2: 1013 .sp 1014 .nf 1015 \f3\-XX:CICompilerCount=2\fP 1016 .fi 1017 .nf 1018 \f3\fP 1019 .fi 1020 .sp 1021 1022 .TP 1023 -XX:CodeCacheMinimumFreeSpace=\fIsize\fR 1024 .br 1025 Sets the minimum free space (in bytes) required for compilation\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. When less than the minimum free space remains, compiling stops\&. By default, this option is set to 500 KB\&. The following example shows how to set the minimum free space to 1024 MB: 1026 .sp 1027 .nf 1028 \f3\-XX:CodeCacheMinimumFreeSpace=1024m\fP 1029 .fi 1030 .nf 1031 \f3\fP 1032 .fi 1033 .sp 1034 1035 .TP 1036 -XX:CompileCommand=\fIcommand\fR,\fImethod\fR[,\fIoption\fR] 1037 .br 1038 Specifies a command to perform on a method\&. For example, to exclude the \f3indexOf()\fR method of the \f3String\fR class from being compiled, use the following: 1039 .sp 1040 .nf 1041 \f3\-XX:CompileCommand=exclude,java/lang/String\&.indexOf\fP 1042 .fi 1043 .nf 1044 \f3\fP 1045 .fi 1046 .sp 1047 1048 1049 Note that the full class name is specified, including all packages and subpackages separated by a slash (\f3/\fR)\&. For easier cut and paste operations, it is also possible to use the method name format produced by the \f3-XX:+PrintCompilation\fR and \f3-XX:+LogCompilation\fR options: 1050 .sp 1051 .nf 1052 \f3\-XX:CompileCommand=exclude,java\&.lang\&.String::indexOf\fP 1053 .fi 1054 .nf 1055 \f3\fP 1056 .fi 1057 .sp 1058 1059 1060 If the method is specified without the signature, the command will be applied to all methods with the specified name\&. However, you can also specify the signature of the method in the class file format\&. In this case, you should enclose the arguments in quotation marks, because otherwise the shell treats the semicolon as command end\&. For example, if you want to exclude only the \f3indexOf(String)\fR method of the \f3String\fR class from being compiled, use the following: 1061 .sp 1062 .nf 1063 \f3\-XX:CompileCommand="exclude,java/lang/String\&.indexOf,(Ljava/lang/String;)I"\fP 1064 .fi 1065 .nf 1066 \f3\fP 1067 .fi 1068 .sp 1069 1070 1071 You can also use the asterisk (*) as a wildcard for class and method names\&. For example, to exclude all \f3indexOf()\fR methods in all classes from being compiled, use the following: 1072 .sp 1073 .nf 1074 \f3\-XX:CompileCommand=exclude,*\&.indexOf\fP 1075 .fi 1076 .nf 1077 \f3\fP 1078 .fi 1079 .sp 1080 1081 1082 The commas and periods are aliases for spaces, making it easier to pass compiler commands through a shell\&. You can pass arguments to \f3-XX:CompileCommand\fR using spaces as separators by enclosing the argument in quotation marks: 1083 .sp 1084 .nf 1085 \f3\-XX:CompileCommand="exclude java/lang/String indexOf"\fP 1086 .fi 1087 .nf 1088 \f3\fP 1089 .fi 1090 .sp 1091 1092 1093 Note that after parsing the commands passed on the command line using the \f3-XX:CompileCommand\fR options, the JIT compiler then reads commands from the \f3\&.hotspot_compiler\fR file\&. You can add commands to this file or specify a different file using the \f3-XX:CompileCommandFile\fR option\&. 1094 1095 To add several commands, either specify the \f3-XX:CompileCommand\fR option multiple times, or separate each argument with the newline separator (\f3\en\fR)\&. The following commands are available: 1096 .RS 1097 .TP 1098 break 1099 Set a breakpoint when debugging the JVM to stop at the beginning of compilation of the specified method\&. 1100 .TP 1101 compileonly 1102 Exclude all methods from compilation except for the specified method\&. As an alternative, you can use the \f3-XX:CompileOnly\fR option, which allows to specify several methods\&. 1103 .TP 1104 dontinline 1105 Prevent inlining of the specified method\&. 1106 .TP 1107 exclude 1108 Exclude the specified method from compilation\&. 1109 .TP 1110 help 1111 Print a help message for the \f3-XX:CompileCommand\fR option\&. 1112 .TP 1113 inline 1114 Attempt to inline the specified method\&. 1115 .TP 1116 log 1117 Exclude compilation logging (with the \f3-XX:+LogCompilation\fR option) for all methods except for the specified method\&. By default, logging is performed for all compiled methods\&. 1118 .TP 1119 option 1120 This command can be used to pass a JIT compilation option to the specified method in place of the last argument (\fIoption\fR)\&. The compilation option is set at the end, after the method name\&. For example, to enable the \f3BlockLayoutByFrequency\fR option for the \f3append()\fR method of the \f3StringBuffer\fR class, use the following: 1121 .sp 1122 .nf 1123 \f3\-XX:CompileCommand=option,java/lang/StringBuffer\&.append,BlockLayoutByFrequency\fP 1124 .fi 1125 .nf 1126 \f3\fP 1127 .fi 1128 .sp 1129 1130 1131 You can specify multiple compilation options, separated by commas or spaces\&. 1132 .TP 1133 print 1134 Print generated assembler code after compilation of the specified method\&. 1135 .TP 1136 quiet 1137 Do not print the compile commands\&. By default, the commands that you specify with the -\f3XX:CompileCommand\fR option are printed; for example, if you exclude from compilation the \f3indexOf()\fR method of the \f3String\fR class, then the following will be printed to standard output: 1138 .sp 1139 .nf 1140 \f3CompilerOracle: exclude java/lang/String\&.indexOf\fP 1141 .fi 1142 .nf 1143 \f3\fP 1144 .fi 1145 .sp 1146 1147 1148 You can suppress this by specifying the \f3-XX:CompileCommand=quiet\fR option before other \f3-XX:CompileCommand\fR options\&. 1149 .RE 1150 1151 .TP 1152 -XX:CompileCommandFile=\fIfilename\fR 1153 .br 1154 Sets the file from which JIT compiler commands are read\&. By default, the \f3\&.hotspot_compiler\fR file is used to store commands performed by the JIT compiler\&. 1155 1156 Each line in the command file represents a command, a class name, and a method name for which the command is used\&. For example, this line prints assembly code for the \f3toString()\fR method of the \f3String\fR class: 1157 .sp 1158 .nf 1159 \f3print java/lang/String toString\fP 1160 .fi 1161 .nf 1162 \f3\fP 1163 .fi 1164 .sp 1165 1166 1167 For more information about specifying the commands for the JIT compiler to perform on methods, see the \f3-XX:CompileCommand\fR option\&. 1168 .TP 1169 -XX:CompileOnly=\fImethods\fR 1170 .br 1171 Sets the list of methods (separated by commas) to which compilation should be restricted\&. Only the specified methods will be compiled\&. Specify each method with the full class name (including the packages and subpackages)\&. For example, to compile only the \f3length()\fR method of the \f3String\fR class and the \f3size()\fR method of the \f3List\fR class, use the following: 1172 .sp 1173 .nf 1174 \f3\-XX:CompileOnly=java/lang/String\&.length,java/util/List\&.size\fP 1175 .fi 1176 .nf 1177 \f3\fP 1178 .fi 1179 .sp 1180 1181 1182 Note that the full class name is specified, including all packages and subpackages separated by a slash (\f3/\fR)\&. For easier cut and paste operations, it is also possible to use the method name format produced by the \f3-XX:+PrintCompilation\fR and \f3-XX:+LogCompilation\fR options: 1183 .sp 1184 .nf 1185 \f3\-XX:CompileOnly=java\&.lang\&.String::length,java\&.util\&.List::size\fP 1186 .fi 1187 .nf 1188 \f3\fP 1189 .fi 1190 .sp 1191 1192 1193 Although wildcards are not supported, you can specify only the class or package name to compile all methods in that class or package, as well as specify just the method to compile methods with this name in any class: 1194 .sp 1195 .nf 1196 \f3\-XX:CompileOnly=java/lang/String\fP 1197 .fi 1198 .nf 1199 \f3\-XX:CompileOnly=java/lang\fP 1200 .fi 1201 .nf 1202 \f3\-XX:CompileOnly=\&.length\fP 1203 .fi 1204 .nf 1205 \f3\fP 1206 .fi 1207 .sp 1208 1209 .TP 1210 -XX:CompileThreshold=\fIinvocations\fR 1211 .br 1212 Sets the number of interpreted method invocations before compilation\&. By default, in the server JVM, the JIT compiler performs 10,000 interpreted method invocations to gather information for efficient compilation\&. For the client JVM, the default setting is 1,500 invocations\&. This option is ignored when tiered compilation is enabled; see the option \f3-XX:+TieredCompilation\fR\&. The following example shows how to set the number of interpreted method invocations to 5,000: 1213 .sp 1214 .nf 1215 \f3\-XX:CompileThreshold=5000\fP 1216 .fi 1217 .nf 1218 \f3\fP 1219 .fi 1220 .sp 1221 1222 1223 You can completely disable interpretation of Java methods before compilation by specifying the \f3-Xcomp\fR option\&. 1224 .TP 1225 -XX:+DoEscapeAnalysis 1226 .br 1227 Enables the use of escape analysis\&. This option is enabled by default\&. To disable the use of escape analysis, specify \f3-XX:-DoEscapeAnalysis\fR\&. Only the Java HotSpot Server VM supports this option\&. 1228 .TP 1229 -XX:InitialCodeCacheSize=\fIsize\fR 1230 .br 1231 Sets the initial code cache size (in bytes)\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. The default value is set to 500 KB\&. The initial code cache size should be not less than the system\&'s minimal memory page size\&. The following example shows how to set the initial code cache size to 32 KB: 1232 .sp 1233 .nf 1234 \f3\-XX:InitialCodeCacheSize=32k\fP 1235 .fi 1236 .nf 1237 \f3\fP 1238 .fi 1239 .sp 1240 1241 .TP 1242 -XX:+Inline 1243 .br 1244 Enables method inlining\&. This option is enabled by default to increase performance\&. To disable method inlining, specify \f3-XX:-Inline\fR\&. 1245 .TP 1246 -XX:InlineSmallCode=\fIsize\fR 1247 .br 1248 Sets the maximum code size (in bytes) for compiled methods that should be inlined\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. Only compiled methods with the size smaller than the specified size will be inlined\&. By default, the maximum code size is set to 1000 bytes: 1249 .sp 1250 .nf 1251 \f3\-XX:InlineSmallCode=1000\fP 1252 .fi 1253 .nf 1254 \f3\fP 1255 .fi 1256 .sp 1257 1258 .TP 1259 -XX:+LogCompilation 1260 .br 1261 Enables logging of compilation activity to a file named \f3hotspot\&.log\fR in the current working directory\&. You can specify a different log file path and name using the \f3-XX:LogFile\fR option\&. 1262 1263 By default, this option is disabled and compilation activity is not logged\&. The \f3-XX:+LogCompilation\fR option has to be used together with the \f3-XX:UnlockDiagnosticVMOptions\fR option that unlocks diagnostic JVM options\&. 1264 1265 You can enable verbose diagnostic output with a message printed to the console every time a method is compiled by using the \f3-XX:+PrintCompilation\fR option\&. 1266 .TP 1267 -XX:MaxInlineSize=\fIsize\fR 1268 .br 1269 Sets the maximum bytecode size (in bytes) of a method to be inlined\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. By default, the maximum bytecode size is set to 35 bytes: 1270 .sp 1271 .nf 1272 \f3\-XX:MaxInlineSize=35\fP 1273 .fi 1274 .nf 1275 \f3\fP 1276 .fi 1277 .sp 1278 1279 .TP 1280 -XX:MaxNodeLimit=\fInodes\fR 1281 .br 1282 Sets the maximum number of nodes to be used during single method compilation\&. By default, the maximum number of nodes is set to 65,000: 1283 .sp 1284 .nf 1285 \f3\-XX:MaxNodeLimit=65000\fP 1286 .fi 1287 .nf 1288 \f3\fP 1289 .fi 1290 .sp 1291 1292 .TP 1293 -XX:MaxTrivialSize=\fIsize\fR 1294 .br 1295 Sets the maximum bytecode size (in bytes) of a trivial method to be inlined\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. By default, the maximum bytecode size of a trivial method is set to 6 bytes: 1296 .sp 1297 .nf 1298 \f3\-XX:MaxTrivialSize=6\fP 1299 .fi 1300 .nf 1301 \f3\fP 1302 .fi 1303 .sp 1304 1305 .TP 1306 -XX:+OptimizeStringConcat 1307 .br 1308 Enables the optimization of \f3String\fR concatenation operations\&. This option is enabled by default\&. To disable the optimization of \f3String\fR concatenation operations, specify \f3-XX:-OptimizeStringConcat\fR\&. Only the Java HotSpot Server VM supports this option\&. 1309 .TP 1310 -XX:+PrintAssembly 1311 .br 1312 Enables printing of assembly code for bytecoded and native methods by using the external \f3disassembler\&.so\fR library\&. This enables you to see the generated code, which may help you to diagnose performance issues\&. 1313 1314 By default, this option is disabled and assembly code is not printed\&. The \f3-XX:+PrintAssembly\fR option has to be used together with the \f3-XX:UnlockDiagnosticVMOptions\fR option that unlocks diagnostic JVM options\&. 1315 .TP 1316 -XX:+PrintCompilation 1317 .br 1318 Enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled\&. This enables you to see which methods actually get compiled\&. By default, this option is disabled and diagnostic output is not printed\&. 1319 1320 You can also log compilation activity to a file by using the \f3-XX:+LogCompilation\fR option\&. 1321 .TP 1322 -XX:+PrintInlining 1323 .br 1324 Enables printing of inlining decisions\&. This enables you to see which methods are getting inlined\&. 1325 1326 By default, this option is disabled and inlining information is not printed\&. The \f3-XX:+PrintInlining\fR option has to be used together with the \f3-XX:+UnlockDiagnosticVMOptions\fR option that unlocks diagnostic JVM options\&. 1327 .TP 1328 -XX:ReservedCodeCacheSize=\fIsize\fR 1329 .br 1330 Sets the maximum code cache size (in bytes) for JIT-compiled code\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. This option has a limit of 2 GB; otherwise, an error is generated\&. The maximum code cache size should not be less than the initial code cache size; see the option \f3-XX:InitialCodeCacheSize\fR\&. This option is equivalent to \f3-Xmaxjitcodesize\fR\&. 1331 .TP 1332 -XX:RTMAbortRatio=\fIabort_ratio\fR 1333 .br 1334 The RTM abort ratio is specified as a percentage (%) of all executed RTM transactions\&. If a number of aborted transactions becomes greater than this ratio, then the compiled code will be deoptimized\&. This ratio is used when the \f3-XX:+UseRTMDeopt\fR option is enabled\&. The default value of this option is 50\&. This means that the compiled code will be deoptimized if 50% of all transactions are aborted\&. 1335 .TP 1336 -XX:RTMRetryCount=\fInumber_of_retries\fR 1337 .br 1338 RTM locking code will be retried, when it is aborted or busy, the number of times specified by this option before falling back to the normal locking mechanism\&. The default value for this option is 5\&. The \f3-XX:UseRTMLocking\fR option must be enabled\&. 1339 .TP 1340 -XX:+TieredCompilation 1341 .br 1342 Enables the use of tiered compilation\&. By default, this option is enabled\&. Only the Java HotSpot Server VM supports this option\&. 1343 .TP 1344 -XX:+UseAES 1345 .br 1346 Enables hardware-based AES intrinsics for Intel, AMD, and SPARC hardware\&. Intel Westmere (2010 and newer), AMD Bulldozer (2011 and newer), and SPARC (T4 and newer) are the supported hardware\&. UseAES is used in conjunction with UseAESIntrinsics\&. 1347 .TP 1348 -XX:+UseAESIntrinsics 1349 .br 1350 UseAES and UseAESIntrinsics flags are enabled by default and are supported only for Java HotSpot Server VM 32-bit and 64-bit\&. To disable hardware-based AES intrinsics, specify \f3-XX:-UseAES -XX:-UseAESIntrinsics\fR\&. For example, to enable hardware AES, use the following flags: 1351 .sp 1352 .nf 1353 \f3\-XX:+UseAES \-XX:+UseAESIntrinsics\fP 1354 .fi 1355 .nf 1356 \f3\fP 1357 .fi 1358 .sp 1359 1360 1361 To support UseAES and UseAESIntrinsics flags for 32-bit and 64-bit use \f3-server\fR option to choose Java HotSpot Server VM\&. These flags are not supported on Client VM\&. 1362 .TP 1363 -XX:+UseCodeCacheFlushing 1364 .br 1365 Enables flushing of the code cache before shutting down the compiler\&. This option is enabled by default\&. To disable flushing of the code cache before shutting down the compiler, specify \f3-XX:-UseCodeCacheFlushing\fR\&. 1366 .TP 1367 -XX:+UseCondCardMark 1368 .br 1369 Enables checking of whether the card is already marked before updating the card table\&. This option is disabled by default and should only be used on machines with multiple sockets, where it will increase performance of Java applications that rely heavily on concurrent operations\&. Only the Java HotSpot Server VM supports this option\&. 1370 .TP 1371 -XX:+UseRTMDeopt 1372 .br 1373 Auto-tunes RTM locking depending on the abort ratio\&. This ratio is specified by \f3-XX:RTMAbortRatio\fR option\&. If the number of aborted transactions exceeds the abort ratio, then the method containing the lock will be deoptimized and recompiled with all locks as normal locks\&. This option is disabled by default\&. The \f3-XX:+UseRTMLocking\fR option must be enabled\&. 1374 .TP 1375 -XX:+UseRTMLocking 1376 .br 1377 Generate Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler\&. This option is disabled by default\&. Options related to RTM are only available for the Java HotSpot Server VM on x86 CPUs that support Transactional Synchronization Extensions (TSX)\&. 1378 1379 RTM is part of Intel\&'s TSX, which is an x86 instruction set extension and facilitates the creation of multithreaded applications\&. RTM introduces the new instructions \f3XBEGIN\fR, \f3XABORT\fR, \f3XEND\fR, and \f3XTEST\fR\&. The \f3XBEGIN\fR and \f3XEND\fR instructions enclose a set of instructions to run as a transaction\&. If no conflict is found when running the transaction, the memory and register modifications are committed together at the \f3XEND\fR instruction\&. The \f3XABORT\fR instruction can be used to explicitly abort a transaction and the \f3XEND\fR instruction to check if a set of instructions are being run in a transaction\&. 1380 1381 A lock on a transaction is inflated when another thread tries to access the same transaction, thereby blocking the thread that did not originally request access to the transaction\&. RTM requires that a fallback set of operations be specified in case a transaction aborts or fails\&. An RTM lock is a lock that has been delegated to the TSX\&'s system\&. 1382 1383 RTM improves performance for highly contended locks with low conflict in a critical region (which is code that must not be accessed by more than one thread concurrently)\&. RTM also improves the performance of coarse-grain locking, which typically does not perform well in multithreaded applications\&. (Coarse-grain locking is the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, while fine-grained locking is the strategy of trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible\&.) Also, for lightly contended locks that are used by different threads, RTM can reduce false cache line sharing, also known as cache line ping-pong\&. This occurs when multiple threads from different processors are accessing different resources, but the resources share the same cache line\&. As a result, the processors repeatedly invalidate the cache lines of other processors, which forces them to read from main memory instead of their cache\&. 1384 .TP 1385 -XX:+UseSHA 1386 .br 1387 Enables hardware-based intrinsics for SHA crypto hash functions for SPARC hardware\&. \f3UseSHA\fR is used in conjunction with the \f3UseSHA1Intrinsics\fR, \f3UseSHA256Intrinsics\fR, and \f3UseSHA512Intrinsics\fR options\&. 1388 1389 The \f3UseSHA\fR and \f3UseSHA*Intrinsics\fR flags are enabled by default, and are supported only for Java HotSpot Server VM 64-bit on SPARC T4 and newer\&. 1390 1391 This feature is only applicable when using the \f3sun\&.security\&.provider\&.Sun\fR provider for SHA operations\&. 1392 1393 To disable all hardware-based SHA intrinsics, specify \f3-XX:-UseSHA\fR\&. To disable only a particular SHA intrinsic, use the appropriate corresponding option\&. For example: \f3-XX:-UseSHA256Intrinsics\fR\&. 1394 .TP 1395 -XX:+UseSHA1Intrinsics 1396 .br 1397 Enables intrinsics for SHA-1 crypto hash function\&. 1398 .TP 1399 -XX:+UseSHA256Intrinsics 1400 .br 1401 Enables intrinsics for SHA-224 and SHA-256 crypto hash functions\&. 1402 .TP 1403 -XX:+UseSHA512Intrinsics 1404 .br 1405 Enables intrinsics for SHA-384 and SHA-512 crypto hash functions\&. 1406 .TP 1407 -XX:+UseSuperWord 1408 .br 1409 Enables the transformation of scalar operations into superword operations\&. This option is enabled by default\&. To disable the transformation of scalar operations into superword operations, specify \f3-XX:-UseSuperWord\fR\&. Only the Java HotSpot Server VM supports this option\&. 1410 .SS ADVANCED\ SERVICEABILITY\ OPTIONS 1411 These options provide the ability to gather system information and perform extensive debugging\&. 1412 .TP 1413 -XX:+ExtendedDTraceProbes 1414 .br 1415 Enables additional \f3dtrace\fR tool probes that impact the performance\&. By default, this option is disabled and \f3dtrace\fR performs only standard probes\&. 1416 .TP 1417 -XX:+HeapDumpOnOutOfMemory 1418 .br 1419 Enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a \f3java\&.lang\&.OutOfMemoryError\fR exception is thrown\&. You can explicitly set the heap dump file path and name using the \f3-XX:HeapDumpPath\fR option\&. By default, this option is disabled and the heap is not dumped when an \f3OutOfMemoryError\fR exception is thrown\&. 1420 .TP 1421 -XX:HeapDumpPath=\fIpath\fR 1422 .br 1423 Sets the path and file name for writing the heap dump provided by the heap profiler (HPROF) when the \f3-XX:+HeapDumpOnOutOfMemoryError\fR option is set\&. By default, the file is created in the current working directory, and it is named \f3java_pid\fR\fIpid\fR\f3\&.hprof\fR where \fIpid\fR is the identifier of the process that caused the error\&. The following example shows how to set the default file explicitly (\f3%p\fR represents the current process identificator): 1424 .sp 1425 .nf 1426 \f3\-XX:HeapDumpPath=\&./java_pid%p\&.hprof\fP 1427 .fi 1428 .nf 1429 \f3\fP 1430 .fi 1431 .sp 1432 1433 1434 \fI\fRThe following example shows how to set the heap dump file to \f3/var/log/java/java_heapdump\&.hprof\fR: 1435 .sp 1436 .nf 1437 \f3\-XX:HeapDumpPath=/var/log/java/java_heapdump\&.hprof\fP 1438 .fi 1439 .nf 1440 \f3\fP 1441 .fi 1442 .sp 1443 1444 .TP 1445 -XX:LogFile=\fIpath\fR 1446 .br 1447 Sets the path and file name where log data is written\&. By default, the file is created in the current working directory, and it is named \f3hotspot\&.log\fR\&. 1448 1449 \fI\fRThe following example shows how to set the log file to \f3/var/log/java/hotspot\&.log\fR: 1450 .sp 1451 .nf 1452 \f3\-XX:LogFile=/var/log/java/hotspot\&.log\fP 1453 .fi 1454 .nf 1455 \f3\fP 1456 .fi 1457 .sp 1458 1459 .TP 1460 -XX:+PrintClassHistogram 1461 .br 1462 \fI\fREnables printing of a class instance histogram after a \f3Control+C\fR event (\f3SIGTERM\fR)\&. By default, this option is disabled\&. 1463 1464 Setting this option is equivalent to running the \f3jmap -histo\fR command, or the \f3jcmd\fR\fIpid\fR\f3GC\&.class_histogram\fR command, where \fIpid\fR is the current Java process identifier\&. 1465 .TP 1466 -XX:+PrintConcurrentLocks 1467 1468 1469 Enables printing of \f3java\&.util\&.concurrent\fR locks after a \f3Control+C\fR event (\f3SIGTERM\fR)\&. By default, this option is disabled\&. 1470 1471 Setting this option is equivalent to running the \f3jstack -l\fR command or the \f3jcmd\fR\fIpid\fR\f3Thread\&.print -l\fR command, where \fIpid\fR is the current Java process identifier\&. 1472 .TP 1473 -XX:+UnlockDiagnosticVMOptions 1474 .br 1475 Unlocks the options intended for diagnosing the JVM\&. By default, this option is disabled and diagnostic options are not available\&. 1476 .SS ADVANCED\ GARBAGE\ COLLECTION\ OPTIONS 1477 These options control how garbage collection (GC) is performed by the Java HotSpot VM\&. 1478 .TP 1479 -XX:+AggressiveHeap 1480 .br 1481 Enables Java heap optimization\&. This sets various parameters to be optimal for long-running jobs with intensive memory allocation, based on the configuration of the computer (RAM and CPU)\&. By default, the option is disabled and the heap is not optimized\&. 1482 .TP 1483 -XX:+AlwaysPreTouch 1484 .br 1485 Enables touching of every page on the Java heap during JVM initialization\&. This gets all pages into the memory before entering the \f3main()\fR method\&. The option can be used in testing to simulate a long-running system with all virtual memory mapped to physical memory\&. By default, this option is disabled and all pages are committed as JVM heap space fills\&. 1486 .TP 1487 -XX:+CMSClassUnloadingEnabled 1488 .br 1489 Enables class unloading when using the concurrent mark-sweep (CMS) garbage collector\&. This option is enabled by default\&. To disable class unloading for the CMS garbage collector, specify \f3-XX:-CMSClassUnloadingEnabled\fR\&. 1490 .TP 1491 -XX:CMSExpAvgFactor=\fIpercent\fR 1492 .br 1493 Sets the percentage of time (0 to 100) used to weight the current sample when computing exponential averages for the concurrent collection statistics\&. By default, the exponential averages factor is set to 25%\&. The following example shows how to set the factor to 15%: 1494 .sp 1495 .nf 1496 \f3\-XX:CMSExpAvgFactor=15\fP 1497 .fi 1498 .nf 1499 \f3\fP 1500 .fi 1501 .sp 1502 1503 .TP 1504 -XX:CMSInitiatingOccupancyFraction=\fIpercent\fR 1505 .br 1506 Sets the percentage of the old generation occupancy (0 to 100) at which to start a CMS collection cycle\&. The default value is set to -1\&. Any negative value (including the default) implies that \f3-XX:CMSTriggerRatio\fR is used to define the value of the initiating occupancy fraction\&. 1507 1508 The following example shows how to set the occupancy fraction to 20%: 1509 .sp 1510 .nf 1511 \f3\-XX:CMSInitiatingOccupancyFraction=20\fP 1512 .fi 1513 .nf 1514 \f3\fP 1515 .fi 1516 .sp 1517 1518 .TP 1519 -XX:+CMSScavengeBeforeRemark 1520 .br 1521 Enables scavenging attempts before the CMS remark step\&. By default, this option is disabled\&. 1522 .TP 1523 -XX:CMSTriggerRatio=\fIpercent\fR 1524 .br 1525 Sets the percentage (0 to 100) of the value specified by \f3-XX:MinHeapFreeRatio\fR that is allocated before a CMS collection cycle commences\&. The default value is set to 80%\&. 1526 1527 The following example shows how to set the occupancy fraction to 75%: 1528 .sp 1529 .nf 1530 \f3\-XX:CMSTriggerRatio=75\fP 1531 .fi 1532 .nf 1533 \f3\fP 1534 .fi 1535 .sp 1536 1537 .TP 1538 -XX:ConcGCThreads=\fIthreads\fR 1539 .br 1540 Sets the number of threads used for concurrent GC\&. The default value depends on the number of CPUs available to the JVM\&. 1541 1542 For example, to set the number of threads for concurrent GC to 2, specify the following option: 1543 .sp 1544 .nf 1545 \f3\-XX:ConcGCThreads=2\fP 1546 .fi 1547 .nf 1548 \f3\fP 1549 .fi 1550 .sp 1551 1552 .TP 1553 -XX:+DisableExplicitGC 1554 .br 1555 Enables the option that disables processing of calls to \f3System\&.gc()\fR\&. This option is disabled by default, meaning that calls to \f3System\&.gc()\fR are processed\&. If processing of calls to \f3System\&.gc()\fR is disabled, the JVM still performs GC when necessary\&. 1556 .TP 1557 -XX:+ExplicitGCInvokesConcurrent 1558 .br 1559 Enables invoking of concurrent GC by using the \f3System\&.gc()\fR request\&. This option is disabled by default and can be enabled only together with the \f3-XX:+UseConcMarkSweepGC\fR option\&. 1560 .TP 1561 -XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses 1562 .br 1563 Enables invoking of concurrent GC by using the \f3System\&.gc()\fR request and unloading of classes during the concurrent GC cycle\&. This option is disabled by default and can be enabled only together with the \f3-XX:+UseConcMarkSweepGC\fR option\&. 1564 .TP 1565 -XX:G1HeapRegionSize=\fIsize\fR 1566 .br 1567 Sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector\&. The value can be between 1 MB and 32 MB\&. The default region size is determined ergonomically based on the heap size\&. 1568 1569 The following example shows how to set the size of the subdivisions to 16 MB: 1570 .sp 1571 .nf 1572 \f3\-XX:G1HeapRegionSize=16m\fP 1573 .fi 1574 .nf 1575 \f3\fP 1576 .fi 1577 .sp 1578 1579 .TP 1580 -XX:+G1PrintHeapRegions 1581 .br 1582 Enables the printing of information about which regions are allocated and which are reclaimed by the G1 collector\&. By default, this option is disabled\&. 1583 .TP 1584 -XX:G1ReservePercent=\fIpercent\fR 1585 .br 1586 Sets the percentage of the heap (0 to 50) that is reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector\&. By default, this option is set to 10%\&. 1587 1588 The following example shows how to set the reserved heap to 20%: 1589 .sp 1590 .nf 1591 \f3\-XX:G1ReservePercent=20\fP 1592 .fi 1593 .nf 1594 \f3\fP 1595 .fi 1596 .sp 1597 1598 .TP 1599 -XX:InitialHeapSize=\fIsize\fR 1600 .br 1601 Sets the initial size (in bytes) of the memory allocation pool\&. This value must be either 0, or a multiple of 1024 and greater than 1 MB\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. The default value is chosen at runtime based on system configuration\&. See the section "Ergonomics" in \fIJava SE HotSpot Virtual Machine Garbage Collection Tuning Guide\fR at http://docs\&.oracle\&.com/javase/8/docs/technotes/guides/vm/gctuning/index\&.html\&. 1602 1603 The following examples show how to set the size of allocated memory to 6 MB using various units: 1604 .sp 1605 .nf 1606 \f3\-XX:InitialHeapSize=6291456\fP 1607 .fi 1608 .nf 1609 \f3\-XX:InitialHeapSize=6144k\fP 1610 .fi 1611 .nf 1612 \f3\-XX:InitialHeapSize=6m\fP 1613 .fi 1614 .nf 1615 \f3\fP 1616 .fi 1617 .sp 1618 1619 1620 If you set this option to 0, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation\&. The size of the heap for the young generation can be set using the \f3-XX:NewSize\fR option\&. 1621 .TP 1622 -XX:InitialSurvivorRatio=\fIratio\fR 1623 .br 1624 Sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the \f3-XX:+UseParallelGC\fR and/or -\f3XX:+UseParallelOldGC\fR options)\&. Adaptive sizing is enabled by default with the throughput garbage collector by using the \f3-XX:+UseParallelGC\fR and \f3-XX:+UseParallelOldGC\fR options, and survivor space is resized according to the application behavior, starting with the initial value\&. If adaptive sizing is disabled (using the \f3-XX:-UseAdaptiveSizePolicy\fR option), then the \f3-XX:SurvivorRatio\fR option should be used to set the size of the survivor space for the entire execution of the application\&. 1625 1626 The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R): 1627 .sp 1628 .nf 1629 \f3S=Y/(R+2)\fP 1630 .fi 1631 .nf 1632 \f3\fP 1633 .fi 1634 .sp 1635 1636 1637 The 2 in the equation denotes two survivor spaces\&. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size\&. 1638 1639 By default, the initial survivor space ratio is set to 8\&. If the default value for the young generation space size is used (2 MB), the initial size of the survivor space will be 0\&.2 MB\&. 1640 1641 The following example shows how to set the initial survivor space ratio to 4: 1642 .sp 1643 .nf 1644 \f3\-XX:InitialSurvivorRatio=4\fP 1645 .fi 1646 .nf 1647 \f3\fP 1648 .fi 1649 .sp 1650 1651 .TP 1652 -XX:InitiatingHeapOccupancyPercent=\fIpercent\fR 1653 .br 1654 Sets the percentage of the heap occupancy (0 to 100) at which to start a concurrent GC cycle\&. It is used by garbage collectors that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (for example, the G1 garbage collector)\&. 1655 1656 By default, the initiating value is set to 45%\&. A value of 0 implies nonstop GC cycles\&. The following example shows how to set the initiating heap occupancy to 75%: 1657 .sp 1658 .nf 1659 \f3\-XX:InitiatingHeapOccupancyPercent=75\fP 1660 .fi 1661 .nf 1662 \f3\fP 1663 .fi 1664 .sp 1665 1666 .TP 1667 -XX:MaxGCPauseMillis=\fItime\fR 1668 .br 1669 Sets a target for the maximum GC pause time (in milliseconds)\&. This is a soft goal, and the JVM will make its best effort to achieve it\&. By default, there is no maximum pause time value\&. 1670 1671 The following example shows how to set the maximum target pause time to 500 ms: 1672 .sp 1673 .nf 1674 \f3\-XX:MaxGCPauseMillis=500\fP 1675 .fi 1676 .nf 1677 \f3\fP 1678 .fi 1679 .sp 1680 1681 .TP 1682 -XX:MaxHeapSize=\fIsize\fR 1683 .br 1684 Sets the maximum size (in byes) of the memory allocation pool\&. This value must be a multiple of 1024 and greater than 2 MB\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. The default value is chosen at runtime based on system configuration\&. For server deployments, \f3-XX:InitialHeapSize\fR and \f3-XX:MaxHeapSize\fR are often set to the same value\&. See the section "Ergonomics" in \fIJava SE HotSpot Virtual Machine Garbage Collection Tuning Guide\fR at http://docs\&.oracle\&.com/javase/8/docs/technotes/guides/vm/gctuning/index\&.html\&. 1685 1686 The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units: 1687 .sp 1688 .nf 1689 \f3\-XX:MaxHeapSize=83886080\fP 1690 .fi 1691 .nf 1692 \f3\-XX:MaxHeapSize=81920k\fP 1693 .fi 1694 .nf 1695 \f3\-XX:MaxHeapSize=80m\fP 1696 .fi 1697 .nf 1698 \f3\fP 1699 .fi 1700 .sp 1701 1702 1703 On Oracle Solaris 7 and Oracle Solaris 8 SPARC platforms, the upper limit for this value is approximately 4,000 MB minus overhead amounts\&. On Oracle Solaris 2\&.6 and x86 platforms, the upper limit is approximately 2,000 MB minus overhead amounts\&. On Linux platforms, the upper limit is approximately 2,000 MB minus overhead amounts\&. 1704 1705 The \f3-XX:MaxHeapSize\fR option is equivalent to \f3-Xmx\fR\&. 1706 .TP 1707 -XX:MaxHeapFreeRatio=\fIpercent\fR 1708 .br 1709 Sets the maximum allowed percentage of free heap space (0 to 100) after a GC event\&. If free heap space expands above this value, then the heap will be shrunk\&. By default, this value is set to 70%\&. 1710 1711 The following example shows how to set the maximum free heap ratio to 75%: 1712 .sp 1713 .nf 1714 \f3\-XX:MaxHeapFreeRatio=75\fP 1715 .fi 1716 .nf 1717 \f3\fP 1718 .fi 1719 .sp 1720 1721 .TP 1722 -XX:MaxMetaspaceSize=\fIsize\fR 1723 .br 1724 Sets the maximum amount of native memory that can be allocated for class metadata\&. By default, the size is not limited\&. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system\&. 1725 1726 The following example shows how to set the maximum class metadata size to 256 MB: 1727 .sp 1728 .nf 1729 \f3\-XX:MaxMetaspaceSize=256m\fP 1730 .fi 1731 .nf 1732 \f3\fP 1733 .fi 1734 .sp 1735 1736 .TP 1737 -XX:MaxNewSize=\fIsize\fR 1738 .br 1739 Sets the maximum size (in bytes) of the heap for the young generation (nursery)\&. The default value is set ergonomically\&. 1740 .TP 1741 -XX:MaxTenuringThreshold=\fIthreshold\fR 1742 .br 1743 Sets the maximum tenuring threshold for use in adaptive GC sizing\&. The largest value is 15\&. The default value is 15 for the parallel (throughput) collector, and 6 for the CMS collector\&. 1744 1745 The following example shows how to set the maximum tenuring threshold to 10: 1746 .sp 1747 .nf 1748 \f3\-XX:MaxTenuringThreshold=10\fP 1749 .fi 1750 .nf 1751 \f3\fP 1752 .fi 1753 .sp 1754 1755 .TP 1756 -XX:MetaspaceSize=\fIsize\fR 1757 .br 1758 Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded\&. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used\&. The default size depends on the platform\&. 1759 .TP 1760 -XX:MinHeapFreeRatio=\fIpercent\fR 1761 .br 1762 Sets the minimum allowed percentage of free heap space (0 to 100) after a GC event\&. If free heap space falls below this value, then the heap will be expanded\&. By default, this value is set to 40%\&. 1763 1764 The following example shows how to set the minimum free heap ratio to 25%: 1765 .sp 1766 .nf 1767 \f3\-XX:MinHeapFreeRatio=25\fP 1768 .fi 1769 .nf 1770 \f3\fP 1771 .fi 1772 .sp 1773 1774 .TP 1775 -XX:NewRatio=\fIratio\fR 1776 .br 1777 Sets the ratio between young and old generation sizes\&. By default, this option is set to 2\&. The following example shows how to set the young/old ratio to 1: 1778 .sp 1779 .nf 1780 \f3\-XX:NewRatio=1\fP 1781 .fi 1782 .nf 1783 \f3\fP 1784 .fi 1785 .sp 1786 1787 .TP 1788 -XX:NewSize=\fIsize\fR 1789 .br 1790 Sets the initial size (in bytes) of the heap for the young generation (nursery)\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. 1791 1792 The young generation region of the heap is used for new objects\&. GC is performed in this region more often than in other regions\&. If the size for the young generation is too low, then a large number of minor GCs will be performed\&. If the size is too high, then only full GCs will be performed, which can take a long time to complete\&. Oracle recommends that you keep the size for the young generation between a half and a quarter of the overall heap size\&. 1793 1794 The following examples show how to set the initial size of young generation to 256 MB using various units: 1795 .sp 1796 .nf 1797 \f3\-XX:NewSize=256m\fP 1798 .fi 1799 .nf 1800 \f3\-XX:NewSize=262144k\fP 1801 .fi 1802 .nf 1803 \f3\-XX:NewSize=268435456\fP 1804 .fi 1805 .nf 1806 \f3\fP 1807 .fi 1808 .sp 1809 1810 1811 The \f3-XX:NewSize\fR option is equivalent to \f3-Xmn\fR\&. 1812 .TP 1813 -XX:ParallelGCThreads=\fIthreads\fR 1814 .br 1815 Sets the number of threads used for parallel garbage collection in the young and old generations\&. The default value depends on the number of CPUs available to the JVM\&. 1816 1817 For example, to set the number of threads for parallel GC to 2, specify the following option: 1818 .sp 1819 .nf 1820 \f3\-XX:ParallelGCThreads=2\fP 1821 .fi 1822 .nf 1823 \f3\fP 1824 .fi 1825 .sp 1826 1827 .TP 1828 -XX:+ParallelRefProcEnabled 1829 .br 1830 Enables parallel reference processing\&. By default, this option is disabled\&. 1831 .TP 1832 -XX:+PrintAdaptiveSizePolicy 1833 .br 1834 Enables printing of information about adaptive generation sizing\&. By default, this option is disabled\&. 1835 .TP 1836 -XX:+PrintGC 1837 .br 1838 Enables printing of messages at every GC\&. By default, this option is disabled\&. 1839 .TP 1840 -XX:+PrintGCApplicationConcurrentTime 1841 .br 1842 Enables printing of how much time elapsed since the last pause (for example, a GC pause)\&. By default, this option is disabled\&. 1843 .TP 1844 -XX:+PrintGCApplicationStoppedTime 1845 .br 1846 Enables printing of how much time the pause (for example, a GC pause) lasted\&. By default, this option is disabled\&. 1847 .TP 1848 -XX:+PrintGCDateStamps 1849 .br 1850 Enables printing of a date stamp at every GC\&. By default, this option is disabled\&. 1851 .TP 1852 -XX:+PrintGCDetails 1853 .br 1854 Enables printing of detailed messages at every GC\&. By default, this option is disabled\&. 1855 .TP 1856 -XX:+PrintGCTaskTimeStamps 1857 .br 1858 Enables printing of time stamps for every individual GC worker thread task\&. By default, this option is disabled\&. 1859 .TP 1860 -XX:+PrintGCTimeStamps 1861 .br 1862 Enables printing of time stamps at every GC\&. By default, this option is disabled\&. 1863 .TP 1864 -XX:+PrintStringDeduplicationStatistics 1865 .br 1866 Prints detailed deduplication statistics\&. By default, this option is disabled\&. See the \f3-XX:+UseStringDeduplication\fR option\&. 1867 .TP 1868 -XX:+PrintTenuringDistribution 1869 .br 1870 Enables printing of tenuring age information\&. The following is an example of the output: 1871 .sp 1872 .nf 1873 \f3Desired survivor size 48286924 bytes, new threshold 10 (max 10)\fP 1874 .fi 1875 .nf 1876 \f3\- age 1: 28992024 bytes, 28992024 total\fP 1877 .fi 1878 .nf 1879 \f3\- age 2: 1366864 bytes, 30358888 total\fP 1880 .fi 1881 .nf 1882 \f3\- age 3: 1425912 bytes, 31784800 total\fP 1883 .fi 1884 .nf 1885 \f3\&.\&.\&.\fP 1886 .fi 1887 .nf 1888 \f3\fP 1889 .fi 1890 .sp 1891 1892 1893 Age 1 objects are the youngest survivors (they were created after the previous scavenge, survived the latest scavenge, and moved from eden to survivor space)\&. Age 2 objects have survived two scavenges (during the second scavenge they were copied from one survivor space to the next)\&. And so on\&. 1894 1895 In the preceding example, 28 992 024 bytes survived one scavenge and were copied from eden to survivor space, 1 366 864 bytes are occupied by age 2 objects, etc\&. The third value in each row is the cumulative size of objects of age n or less\&. 1896 1897 By default, this option is disabled\&. 1898 .TP 1899 -XX:+ScavengeBeforeFullGC 1900 .br 1901 Enables GC of the young generation before each full GC\&. This option is enabled by default\&. Oracle recommends that you \fIdo not\fR disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space\&. To disable GC of the young generation before each full GC, specify \f3-XX:-ScavengeBeforeFullGC\fR\&. 1902 .TP 1903 -XX:SoftRefLRUPolicyMSPerMB=\fItime\fR 1904 .br 1905 Sets the amount of time (in milliseconds) a softly reachable object is kept active on the heap after the last time it was referenced\&. The default value is one second of lifetime per free megabyte in the heap\&. The \f3-XX:SoftRefLRUPolicyMSPerMB\fR option accepts integer values representing milliseconds per one megabyte of the current heap size (for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM)\&. This difference means that the Client VM tends to flush soft references rather than grow the heap, whereas the Server VM tends to grow the heap rather than flush soft references\&. In the latter case, the value of the \f3-Xmx\fR option has a significant effect on how quickly soft references are garbage collected\&. 1906 1907 The following example shows how to set the value to 2\&.5 seconds: 1908 .sp 1909 .nf 1910 \f3\-XX:SoftRefLRUPolicyMSPerMB=2500\fP 1911 .fi 1912 .nf 1913 \f3\fP 1914 .fi 1915 .sp 1916 1917 .TP 1918 -XX:StringDeduplicationAgeThreshold=\fIthreshold\fR 1919 .br 1920 \f3String\fR objects reaching the specified age are considered candidates for deduplication\&. An object\&'s age is a measure of how many times it has survived garbage collection\&. This is sometimes referred to as tenuring; see the \f3-XX:+PrintTenuringDistribution\fR option\&. Note that \f3String\fR objects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication\&. The default value for this option is \f33\fR\&. See the \f3-XX:+UseStringDeduplication\fR option\&. 1921 .TP 1922 -XX:SurvivorRatio=\fIratio\fR 1923 .br 1924 Sets the ratio between eden space size and survivor space size\&. By default, this option is set to 8\&. The following example shows how to set the eden/survivor space ratio to 4: 1925 .sp 1926 .nf 1927 \f3\-XX:SurvivorRatio=4\fP 1928 .fi 1929 .nf 1930 \f3\fP 1931 .fi 1932 .sp 1933 1934 .TP 1935 -XX:TargetSurvivorRatio=\fIpercent\fR 1936 .br 1937 Sets the desired percentage of survivor space (0 to 100) used after young garbage collection\&. By default, this option is set to 50%\&. 1938 1939 The following example shows how to set the target survivor space ratio to 30%: 1940 .sp 1941 .nf 1942 \f3\-XX:TargetSurvivorRatio=30\fP 1943 .fi 1944 .nf 1945 \f3\fP 1946 .fi 1947 .sp 1948 1949 .TP 1950 -XX:TLABSize=\fIsize\fR 1951 .br 1952 Sets the initial size (in bytes) of a thread-local allocation buffer (TLAB)\&. Append the letter \f3k\fR or \f3K\fR to indicate kilobytes, \f3m\fR or \f3M\fR to indicate megabytes, \f3g\fR or \f3G\fR to indicate gigabytes\&. If this option is set to 0, then the JVM chooses the initial size automatically\&. 1953 1954 The following example shows how to set the initial TLAB size to 512 KB: 1955 .sp 1956 .nf 1957 \f3\-XX:TLABSize=512k\fP 1958 .fi 1959 .nf 1960 \f3\fP 1961 .fi 1962 .sp 1963 1964 .TP 1965 -XX:+UseAdaptiveSizePolicy 1966 .br 1967 Enables the use of adaptive generation sizing\&. This option is enabled by default\&. To disable adaptive generation sizing, specify \f3-XX:-UseAdaptiveSizePolicy\fR and set the size of the memory allocation pool explicitly (see the \f3-XX:SurvivorRatio\fR option)\&. 1968 .TP 1969 -XX:+UseCMSInitiatingOccupancyOnly 1970 .br 1971 Enables the use of the occupancy value as the only criterion for initiating the CMS collector\&. By default, this option is disabled and other criteria may be used\&. 1972 .TP 1973 -XX:+UseConcMarkSweepGC 1974 .br 1975 Enables the use of the CMS garbage collector for the old generation\&. Oracle recommends that you use the CMS garbage collector when application latency requirements cannot be met by the throughput (\f3-XX:+UseParallelGC\fR) garbage collector\&. The G1 garbage collector (\f3-XX:+UseG1GC\fR) is another alternative\&. 1976 1977 By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM\&. When this option is enabled, the \f3-XX:+UseParNewGC\fR option is automatically set and you should not disable it, because the following combination of options has been deprecated in JDK 8: \f3-XX:+UseConcMarkSweepGC -XX:-UseParNewGC\fR\&. 1978 .TP 1979 -XX:+UseG1GC 1980 .br 1981 Enables the use of the garbage-first (G1) garbage collector\&. It is a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM\&. It meets GC pause time goals with high probability, while maintaining good throughput\&. The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (stable and predictable pause time below 0\&.5 seconds)\&. 1982 1983 By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM\&. 1984 .TP 1985 -XX:+UseGCOverheadLimit 1986 .br 1987 Enables the use of a policy that limits the proportion of time spent by the JVM on GC before an \f3OutOfMemoryError\fR exception is thrown\&. This option is enabled, by default and the parallel GC will throw an \f3OutOfMemoryError\fR if more than 98% of the total time is spent on garbage collection and less than 2% of the heap is recovered\&. When the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress\&. To disable this option, specify \f3-XX:-UseGCOverheadLimit\fR\&. 1988 .TP 1989 -XX:+UseNUMA 1990 .br 1991 Enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application\&'s use of lower latency memory\&. By default, this option is disabled and no optimization for NUMA is made\&. The option is only available when the parallel garbage collector is used (\f3-XX:+UseParallelGC\fR)\&. 1992 .TP 1993 -XX:+UseParallelGC 1994 .br 1995 Enables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors\&. 1996 1997 By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM\&. If it is enabled, then the \f3-XX:+UseParallelOldGC\fR option is automatically enabled, unless you explicitly disable it\&. 1998 .TP 1999 -XX:+UseParallelOldGC 2000 .br 2001 Enables the use of the parallel garbage collector for full GCs\&. By default, this option is disabled\&. Enabling it automatically enables the \f3-XX:+UseParallelGC\fR option\&. 2002 .TP 2003 -XX:+UseParNewGC 2004 .br 2005 Enables the use of parallel threads for collection in the young generation\&. By default, this option is disabled\&. It is automatically enabled when you set the \f3-XX:+UseConcMarkSweepGC\fR option\&. Using the \f3-XX:+UseParNewGC\fR option without the \f3-XX:+UseConcMarkSweepGC\fR option was deprecated in JDK 8\&. 2006 .TP 2007 -XX:+UseSerialGC 2008 .br 2009 Enables the use of the serial garbage collector\&. This is generally the best choice for small and simple applications that do not require any special functionality from garbage collection\&. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM\&. 2010 .TP 2011 -XX:+UseSHM 2012 .br 2013 On Linux, enables the JVM to use shared memory to setup large pages\&. 2014 2015 For more information, see Large Pages\&. 2016 .TP 2017 -XX:+UseStringDeduplication 2018 .br 2019 Enables string deduplication\&. By default, this option is disabled\&. To use this option, you must enable the garbage-first (G1) garbage collector\&. See the \f3-XX:+UseG1GC\fR option\&. 2020 2021 \fIString deduplication\fR reduces the memory footprint of \f3String\fR objects on the Java heap by taking advantage of the fact that many \f3String\fR objects are identical\&. Instead of each \f3String\fR object pointing to its own character array, identical \f3String\fR objects can point to and share the same character array\&. 2022 .TP 2023 -XX:+UseTLAB 2024 .br 2025 Enables the use of thread-local allocation blocks (TLABs) in the young generation space\&. This option is enabled by default\&. To disable the use of TLABs, specify \f3-XX:-UseTLAB\fR\&. 2026 .SS DEPRECATED\ AND\ REMOVED\ OPTIONS 2027 These options were included in the previous release, but have since been considered unnecessary\&. 2028 .TP 2029 -Xincgc 2030 .br 2031 Enables incremental garbage collection\&. This option was deprecated in JDK 8 with no replacement\&. 2032 .TP 2033 -Xrun\fIlibname\fR 2034 .br 2035 Loads the specified debugging/profiling library\&. This option was superseded by the \f3-agentlib\fR option\&. 2036 .TP 2037 -XX:CMSIncrementalDutyCycle=\fIpercent\fR 2038 .br 2039 Sets the percentage of time (0 to 100) between minor collections that the concurrent collector is allowed to run\&. This option was deprecated in JDK 8 with no replacement, following the deprecation of the \f3-XX:+CMSIncrementalMode\fR option\&. 2040 .TP 2041 -XX:CMSIncrementalDutyCycleMin=\fIpercent\fR 2042 .br 2043 Sets the percentage of time (0 to 100) between minor collections that is the lower bound for the duty cycle when \f3-XX:+CMSIncrementalPacing\fR is enabled\&. This option was deprecated in JDK 8 with no replacement, following the deprecation of the \f3-XX:+CMSIncrementalMode\fR option\&. 2044 .TP 2045 -XX:+CMSIncrementalMode 2046 .br 2047 Enables the incremental mode for the CMS collector\&. This option was deprecated in JDK 8 with no replacement, along with other options that start with \f3CMSIncremental\fR\&. 2048 .TP 2049 -XX:CMSIncrementalOffset=\fIpercent\fR 2050 .br 2051 Sets the percentage of time (0 to 100) by which the incremental mode duty cycle is shifted to the right within the period between minor collections\&. This option was deprecated in JDK 8 with no replacement, following the deprecation of the \f3-XX:+CMSIncrementalMode\fR option\&. 2052 .TP 2053 -XX:+CMSIncrementalPacing 2054 .br 2055 Enables automatic adjustment of the incremental mode duty cycle based on statistics collected while the JVM is running\&. This option was deprecated in JDK 8 with no replacement, following the deprecation of the \f3-XX:+CMSIncrementalMode\fR option\&. 2056 .TP 2057 -XX:CMSIncrementalSafetyFactor=\fIpercent\fR 2058 .br 2059 Sets the percentage of time (0 to 100) used to add conservatism when computing the duty cycle\&. This option was deprecated in JDK 8 with no replacement, following the deprecation of the \f3-XX:+CMSIncrementalMode\fR option\&. 2060 .TP 2061 -XX:CMSInitiatingPermOccupancyFraction=\fIpercent\fR 2062 .br 2063 Sets the percentage of the permanent generation occupancy (0 to 100) at which to start a GC\&. This option was deprecated in JDK 8 with no replacement\&. 2064 .TP 2065 -XX:MaxPermSize=\fIsize\fR 2066 .br 2067 Sets the maximum permanent generation space size (in bytes)\&. This option was deprecated in JDK 8, and superseded by the \f3-XX:MaxMetaspaceSize\fR option\&. 2068 .TP 2069 -XX:PermSize=\fIsize\fR 2070 .br 2071 Sets the space (in bytes) allocated to the permanent generation that triggers a garbage collection if it is exceeded\&. This option was deprecated un JDK 8, and superseded by the \f3-XX:MetaspaceSize\fR option\&. 2072 .TP 2073 -XX:+UseSplitVerifier 2074 .br 2075 Enables splitting of the verification process\&. By default, this option was enabled in the previous releases, and verification was split into two phases: type referencing (performed by the compiler) and type checking (performed by the JVM runtime)\&. This option was deprecated in JDK 8, and verification is now split by default without a way to disable it\&. 2076 .TP 2077 -XX:+UseStringCache 2078 .br 2079 Enables caching of commonly allocated strings\&. This option was removed from JDK 8 with no replacement\&. 2080 .SH PERFORMANCE\ TUNING\ EXAMPLES 2081 The following examples show how to use experimental tuning flags to either optimize throughput or to provide lower response time\&. 2082 .PP 2083 \f3Example 1 Tuning for Higher Throughput\fR 2084 .sp 2085 .nf 2086 \f3java \-d64 \-server \-XX:+AggressiveOpts \-XX:+UseLargePages \-Xmn10g \-Xms26g \-Xmx26g\fP 2087 .fi 2088 .nf 2089 \f3\fP 2090 .fi 2091 .sp 2092 \f3Example 2 Tuning for Lower Response Time\fR 2093 .sp 2094 .nf 2095 \f3java \-d64 \-XX:+UseG1GC \-Xms26g Xmx26g \-XX:MaxGCPauseMillis=500 \-XX:+PrintGCTimeStamp\fP 2096 .fi 2097 .nf 2098 \f3\fP 2099 .fi 2100 .sp 2101 .SH LARGE\ PAGES 2102 Also known as huge pages, large pages are memory pages that are significantly larger than the standard memory page size (which varies depending on the processor and operating system)\&. Large pages optimize processor Translation-Lookaside Buffers\&. 2103 .PP 2104 A Translation-Lookaside Buffer (TLB) is a page translation cache that holds the most-recently used virtual-to-physical address translations\&. TLB is a scarce system resource\&. A TLB miss can be costly as the processor must then read from the hierarchical page table, which may require multiple memory accesses\&. By using a larger memory page size, a single TLB entry can represent a larger memory range\&. There will be less pressure on TLB, and memory-intensive applications may have better performance\&. 2105 .PP 2106 However, large pages page memory can negatively affect system performance\&. For example, when a large mount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system\&. Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory\&. When this happens, either the OS or JVM reverts to using regular pages\&. 2107 .SS LARGE\ PAGES\ SUPPORT 2108 Solaris and Linux support large pages\&. 2109 .PP 2110 Solaris 9 and later include Multiple Page Size Support (MPSS); no additional configuration is necessary\&. See http://www\&.oracle\&.com/technetwork/server-storage/solaris10/overview/solaris9-features-scalability-135663\&.html\&. 2111 .PP 2112 The 2\&.6 kernel supports large pages\&. Some vendors have backported the code to their 2\&.4-based releases\&. To check if your system can support large page memory, try the following: 2113 .sp 2114 .nf 2115 \f3# cat /proc/meminfo | grep Huge\fP 2116 .fi 2117 .nf 2118 \f3HugePages_Total: 0\fP 2119 .fi 2120 .nf 2121 \f3HugePages_Free: 0\fP 2122 .fi 2123 .nf 2124 \f3Hugepagesize: 2048 kB\fP 2125 .fi 2126 .nf 2127 \f3\fP 2128 .fi 2129 .sp 2130 If the output shows the three "Huge" variables, then your system can support large page memory but it needs to be configured\&. If the command prints nothing, then your system does not support large pages\&. To configure the system to use large page memory, login as \f3root\fR, and then follow these steps: 2131 .TP 0.4i 2132 1\&. 2133 If you are using the option \f3-XX:+UseSHM\fR (instead of \f3-XX:+UseHugeTLBFS\fR), then increase the \f3SHMMAX\fR value\&. It must be larger than the Java heap size\&. On a system with 4 GB of physical RAM (or less), the following will make all the memory sharable: 2134 .sp 2135 .nf 2136 \f3# echo 4294967295 > /proc/sys/kernel/shmmax\fP 2137 .fi 2138 .nf 2139 \f3\fP 2140 .fi 2141 .sp 2142 2143 .TP 0.4i 2144 2\&. 2145 If you are using the option \f3-XX:+UseSHM\fR or \f3-XX:+UseHugeTLBFS\fR, then specify the number of large pages\&. In the following example, 3 GB of a 4 GB system are reserved for large pages (assuming a large page size of 2048kB, then 3 GB = 3 * 1024 MB = 3072 MB = 3072 * 1024 kB = 3145728 kB and 3145728 kB / 2048 kB = 1536): 2146 .sp 2147 .nf 2148 \f3# echo 1536 > /proc/sys/vm/nr_hugepages\fP 2149 .fi 2150 .nf 2151 \f3\fP 2152 .fi 2153 .sp 2154 2155 .PP 2156 Note 2157 .PP 2158 Note that the values contained in \f3/proc\fR will reset after you reboot your system, so may want to set them in an initialization script (for example, \f3rc\&.local\fR or \f3sysctl\&.conf\fR)\&. 2159 .PP 2160 If you configure (or resize) the OS kernel parameters \f3/proc/sys/kernel/shmmax\fR or \f3/proc/sys/vm/nr_hugepages\fR, Java processes may allocate large pages for areas in addition to the Java heap\&. These steps can allocate large pages for the following areas: 2161 .PP 2162 Java heap 2163 .PP 2164 Permanent generation 2165 .PP 2166 Code cache 2167 .PP 2168 The marking bitmap data structure for the parallel GC 2169 .PP 2170 Consequently, if you configure the \f3nr_hugepages\fR parameter to the size of the Java heap, then the JVM can fail in allocating the permanent generation and code cache areas on large pages because these areas are quite large in size\&. 2171 .SH EXIT\ STATUS 2172 The following exit values are typically returned by the launcher when the launcher is called with the wrong arguments, serious errors, or exceptions thrown by the JVM\&. However, a Java application may choose to return any value by using the API call \f3System\&.exit(exitValue)\fR\&. The values are: 2173 .TP 0.2i 2174 \(bu 2175 \f30\fR: Successful completion 2176 .TP 0.2i 2177 \(bu 2178 \f3>0\fR: An error occurred 2179 .SH SEE\ ALSO 2180 .TP 0.2i 2181 \(bu 2182 javac(1) 2183 .TP 0.2i 2184 \(bu 2185 jdb(1) 2186 .TP 0.2i 2187 \(bu 2188 javah(1) 2189 .TP 0.2i 2190 \(bu 2191 jar(1) 2192 .TP 0.2i 2193 \(bu 2194 jstat(1) 2195 .RE 2196 .br 2197 'pl 8.5i 2198 'bp