1 /* 2 * Copyright (c) 1994, 2019, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26 package java.lang; 27 28 import jdk.internal.HotSpotIntrinsicCandidate; 29 30 /** 31 * Class {@code Object} is the root of the class hierarchy. 32 * Every class has {@code Object} as a superclass. All objects, 33 * including arrays, implement the methods of this class. 34 * 35 * @author unascribed 36 * @see java.lang.Class 37 * @since 1.0 38 */ 39 public class Object { 40 41 /** 42 * Constructs a new object. 43 */ 44 @HotSpotIntrinsicCandidate 45 public Object() {} 46 47 /** 48 * Returns the runtime class of this {@code Object}. The returned 49 * {@code Class} object is the object that is locked by {@code 50 * static synchronized} methods of the represented class. 51 * 52 * <p><b>The actual result type is {@code Class<? extends |X|>} 53 * where {@code |X|} is the erasure of the static type of the 54 * expression on which {@code getClass} is called.</b> For 55 * example, no cast is required in this code fragment:</p> 56 * 57 * <p> 58 * {@code Number n = 0; }<br> 59 * {@code Class<? extends Number> c = n.getClass(); } 60 * </p> 61 * 62 * @return The {@code Class} object that represents the runtime 63 * class of this object. 64 * @jls 15.8.2 Class Literals 65 */ 66 @HotSpotIntrinsicCandidate 67 public final native Class<?> getClass(); 68 69 /** 70 * Returns a hash code value for the object. This method is 71 * supported for the benefit of hash tables such as those provided by 72 * {@link java.util.HashMap}. 73 * <p> 74 * The general contract of {@code hashCode} is: 75 * <ul> 76 * <li>Whenever it is invoked on the same object more than once during 77 * an execution of a Java application, the {@code hashCode} method 78 * must consistently return the same integer, provided no information 79 * used in {@code equals} comparisons on the object is modified. 80 * This integer need not remain consistent from one execution of an 81 * application to another execution of the same application. 82 * <li>If two objects are equal according to the {@code equals(Object)} 83 * method, then calling the {@code hashCode} method on each of 84 * the two objects must produce the same integer result. 85 * <li>It is <em>not</em> required that if two objects are unequal 86 * according to the {@link java.lang.Object#equals(java.lang.Object)} 87 * method, then calling the {@code hashCode} method on each of the 88 * two objects must produce distinct integer results. However, the 89 * programmer should be aware that producing distinct integer results 90 * for unequal objects may improve the performance of hash tables. 91 * </ul> 92 * 93 * @implSpec 94 * As far as is reasonably practical, the {@code hashCode} method defined 95 * by class {@code Object} returns distinct integers for distinct objects. 96 * 97 * @return a hash code value for this object. 98 * @see java.lang.Object#equals(java.lang.Object) 99 * @see java.lang.System#identityHashCode 100 */ 101 @HotSpotIntrinsicCandidate 102 public native int hashCode(); 103 104 /** 105 * Indicates whether some other object is "equal to" this one. 106 * <p> 107 * The {@code equals} method implements an equivalence relation 108 * on non-null object references: 109 * <ul> 110 * <li>It is <i>reflexive</i>: for any non-null reference value 111 * {@code x}, {@code x.equals(x)} should return 112 * {@code true}. 113 * <li>It is <i>symmetric</i>: for any non-null reference values 114 * {@code x} and {@code y}, {@code x.equals(y)} 115 * should return {@code true} if and only if 116 * {@code y.equals(x)} returns {@code true}. 117 * <li>It is <i>transitive</i>: for any non-null reference values 118 * {@code x}, {@code y}, and {@code z}, if 119 * {@code x.equals(y)} returns {@code true} and 120 * {@code y.equals(z)} returns {@code true}, then 121 * {@code x.equals(z)} should return {@code true}. 122 * <li>It is <i>consistent</i>: for any non-null reference values 123 * {@code x} and {@code y}, multiple invocations of 124 * {@code x.equals(y)} consistently return {@code true} 125 * or consistently return {@code false}, provided no 126 * information used in {@code equals} comparisons on the 127 * objects is modified. 128 * <li>For any non-null reference value {@code x}, 129 * {@code x.equals(null)} should return {@code false}. 130 * </ul> 131 * <p> 132 * The {@code equals} method for class {@code Object} implements 133 * the most discriminating possible equivalence relation on objects; 134 * that is, for any non-null reference values {@code x} and 135 * {@code y}, this method returns {@code true} if and only 136 * if {@code x} and {@code y} refer to the same object 137 * ({@code x == y} has the value {@code true}). 138 * <p> 139 * Note that it is generally necessary to override the {@code hashCode} 140 * method whenever this method is overridden, so as to maintain the 141 * general contract for the {@code hashCode} method, which states 142 * that equal objects must have equal hash codes. 143 * 144 * @param obj the reference object with which to compare. 145 * @return {@code true} if this object is the same as the obj 146 * argument; {@code false} otherwise. 147 * @see #hashCode() 148 * @see java.util.HashMap 149 */ 150 public boolean equals(Object obj) { 151 return (this == obj); 152 } 153 154 /** 155 * Creates and returns a copy of this object. The precise meaning 156 * of "copy" may depend on the class of the object. The general 157 * intent is that, for any object {@code x}, the expression: 158 * <blockquote> 159 * <pre> 160 * x.clone() != x</pre></blockquote> 161 * will be true, and that the expression: 162 * <blockquote> 163 * <pre> 164 * x.clone().getClass() == x.getClass()</pre></blockquote> 165 * will be {@code true}, but these are not absolute requirements. 166 * While it is typically the case that: 167 * <blockquote> 168 * <pre> 169 * x.clone().equals(x)</pre></blockquote> 170 * will be {@code true}, this is not an absolute requirement. 171 * <p> 172 * By convention, the returned object should be obtained by calling 173 * {@code super.clone}. If a class and all of its superclasses (except 174 * {@code Object}) obey this convention, it will be the case that 175 * {@code x.clone().getClass() == x.getClass()}. 176 * <p> 177 * By convention, the object returned by this method should be independent 178 * of this object (which is being cloned). To achieve this independence, 179 * it may be necessary to modify one or more fields of the object returned 180 * by {@code super.clone} before returning it. Typically, this means 181 * copying any mutable objects that comprise the internal "deep structure" 182 * of the object being cloned and replacing the references to these 183 * objects with references to the copies. If a class contains only 184 * primitive fields or references to immutable objects, then it is usually 185 * the case that no fields in the object returned by {@code super.clone} 186 * need to be modified. 187 * <p> 188 * The method {@code clone} for class {@code Object} performs a 189 * specific cloning operation. First, if the class of this object does 190 * not implement the interface {@code Cloneable}, then a 191 * {@code CloneNotSupportedException} is thrown. Note that all arrays 192 * are considered to implement the interface {@code Cloneable} and that 193 * the return type of the {@code clone} method of an array type {@code T[]} 194 * is {@code T[]} where T is any reference or primitive type. 195 * Otherwise, this method creates a new instance of the class of this 196 * object and initializes all its fields with exactly the contents of 197 * the corresponding fields of this object, as if by assignment; the 198 * contents of the fields are not themselves cloned. Thus, this method 199 * performs a "shallow copy" of this object, not a "deep copy" operation. 200 * <p> 201 * The class {@code Object} does not itself implement the interface 202 * {@code Cloneable}, so calling the {@code clone} method on an object 203 * whose class is {@code Object} will result in throwing an 204 * exception at run time. 205 * 206 * @return a clone of this instance. 207 * @throws CloneNotSupportedException if the object's class does not 208 * support the {@code Cloneable} interface. Subclasses 209 * that override the {@code clone} method can also 210 * throw this exception to indicate that an instance cannot 211 * be cloned. 212 * @see java.lang.Cloneable 213 */ 214 @HotSpotIntrinsicCandidate 215 protected native Object clone() throws CloneNotSupportedException; 216 217 /** 218 * Returns a string representation of the object. In general, the 219 * {@code toString} method returns a string that 220 * "textually represents" this object. The result should 221 * be a concise but informative representation that is easy for a 222 * person to read. 223 * It is recommended that all subclasses override this method. 224 * <p> 225 * The {@code toString} method for class {@code Object} 226 * returns a string consisting of the name of the class of which the 227 * object is an instance, the at-sign character `{@code @}', and 228 * the unsigned hexadecimal representation of the hash code of the 229 * object. In other words, this method returns a string equal to the 230 * value of: 231 * <blockquote> 232 * <pre> 233 * getClass().getName() + '@' + Integer.toHexString(hashCode()) 234 * </pre></blockquote> 235 * 236 * @return a string representation of the object. 237 */ 238 public String toString() { 239 return getClass().getName() + "@" + Integer.toHexString(hashCode()); 240 } 241 242 /** 243 * Wakes up a single thread that is waiting on this object's 244 * monitor. If any threads are waiting on this object, one of them 245 * is chosen to be awakened. The choice is arbitrary and occurs at 246 * the discretion of the implementation. A thread waits on an object's 247 * monitor by calling one of the {@code wait} methods. 248 * <p> 249 * The awakened thread will not be able to proceed until the current 250 * thread relinquishes the lock on this object. The awakened thread will 251 * compete in the usual manner with any other threads that might be 252 * actively competing to synchronize on this object; for example, the 253 * awakened thread enjoys no reliable privilege or disadvantage in being 254 * the next thread to lock this object. 255 * <p> 256 * This method should only be called by a thread that is the owner 257 * of this object's monitor. A thread becomes the owner of the 258 * object's monitor in one of three ways: 259 * <ul> 260 * <li>By executing a synchronized instance method of that object. 261 * <li>By executing the body of a {@code synchronized} statement 262 * that synchronizes on the object. 263 * <li>For objects of type {@code Class,} by executing a 264 * synchronized static method of that class. 265 * </ul> 266 * <p> 267 * Only one thread at a time can own an object's monitor. 268 * 269 * @throws IllegalMonitorStateException if the current thread is not 270 * the owner of this object's monitor. 271 * @see java.lang.Object#notifyAll() 272 * @see java.lang.Object#wait() 273 */ 274 @HotSpotIntrinsicCandidate 275 public final native void notify(); 276 277 /** 278 * Wakes up all threads that are waiting on this object's monitor. A 279 * thread waits on an object's monitor by calling one of the 280 * {@code wait} methods. 281 * <p> 282 * The awakened threads will not be able to proceed until the current 283 * thread relinquishes the lock on this object. The awakened threads 284 * will compete in the usual manner with any other threads that might 285 * be actively competing to synchronize on this object; for example, 286 * the awakened threads enjoy no reliable privilege or disadvantage in 287 * being the next thread to lock this object. 288 * <p> 289 * This method should only be called by a thread that is the owner 290 * of this object's monitor. See the {@code notify} method for a 291 * description of the ways in which a thread can become the owner of 292 * a monitor. 293 * 294 * @throws IllegalMonitorStateException if the current thread is not 295 * the owner of this object's monitor. 296 * @see java.lang.Object#notify() 297 * @see java.lang.Object#wait() 298 */ 299 @HotSpotIntrinsicCandidate 300 public final native void notifyAll(); 301 302 /** 303 * Causes the current thread to wait until it is awakened, typically 304 * by being <em>notified</em> or <em>interrupted</em>. 305 * <p> 306 * In all respects, this method behaves as if {@code wait(0L, 0)} 307 * had been called. See the specification of the {@link #wait(long, int)} method 308 * for details. 309 * 310 * @throws IllegalMonitorStateException if the current thread is not 311 * the owner of the object's monitor 312 * @throws InterruptedException if any thread interrupted the current thread before or 313 * while the current thread was waiting. The <em>interrupted status</em> of the 314 * current thread is cleared when this exception is thrown. 315 * @see #notify() 316 * @see #notifyAll() 317 * @see #wait(long) 318 * @see #wait(long, int) 319 */ 320 public final void wait() throws InterruptedException { 321 wait(0L); 322 } 323 324 /** 325 * Causes the current thread to wait until it is awakened, typically 326 * by being <em>notified</em> or <em>interrupted</em>, or until a 327 * certain amount of real time has elapsed. 328 * <p> 329 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)} 330 * had been called. See the specification of the {@link #wait(long, int)} method 331 * for details. 332 * 333 * @param timeoutMillis the maximum time to wait, in milliseconds 334 * @throws IllegalArgumentException if {@code timeoutMillis} is negative 335 * @throws IllegalMonitorStateException if the current thread is not 336 * the owner of the object's monitor 337 * @throws InterruptedException if any thread interrupted the current thread before or 338 * while the current thread was waiting. The <em>interrupted status</em> of the 339 * current thread is cleared when this exception is thrown. 340 * @see #notify() 341 * @see #notifyAll() 342 * @see #wait() 343 * @see #wait(long, int) 344 */ 345 public final native void wait(long timeoutMillis) throws InterruptedException; 346 347 /** 348 * Causes the current thread to wait until it is awakened, typically 349 * by being <em>notified</em> or <em>interrupted</em>, or until a 350 * certain amount of real time has elapsed. 351 * <p> 352 * The current thread must own this object's monitor lock. See the 353 * {@link #notify notify} method for a description of the ways in which 354 * a thread can become the owner of a monitor lock. 355 * <p> 356 * This method causes the current thread (referred to here as <var>T</var>) to 357 * place itself in the wait set for this object and then to relinquish any 358 * and all synchronization claims on this object. Note that only the locks 359 * on this object are relinquished; any other objects on which the current 360 * thread may be synchronized remain locked while the thread waits. 361 * <p> 362 * Thread <var>T</var> then becomes disabled for thread scheduling purposes 363 * and lies dormant until one of the following occurs: 364 * <ul> 365 * <li>Some other thread invokes the {@code notify} method for this 366 * object and thread <var>T</var> happens to be arbitrarily chosen as 367 * the thread to be awakened. 368 * <li>Some other thread invokes the {@code notifyAll} method for this 369 * object. 370 * <li>Some other thread {@linkplain Thread#interrupt() interrupts} 371 * thread <var>T</var>. 372 * <li>The specified amount of real time has elapsed, more or less. 373 * The amount of real time, in nanoseconds, is given by the expression 374 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos} 375 * are both zero, then real time is not taken into consideration and the 376 * thread waits until awakened by one of the other causes. 377 * <li>Thread <var>T</var> is awakened spuriously. (See below.) 378 * </ul> 379 * <p> 380 * The thread <var>T</var> is then removed from the wait set for this 381 * object and re-enabled for thread scheduling. It competes in the 382 * usual manner with other threads for the right to synchronize on the 383 * object; once it has regained control of the object, all its 384 * synchronization claims on the object are restored to the status quo 385 * ante - that is, to the situation as of the time that the {@code wait} 386 * method was invoked. Thread <var>T</var> then returns from the 387 * invocation of the {@code wait} method. Thus, on return from the 388 * {@code wait} method, the synchronization state of the object and of 389 * thread {@code T} is exactly as it was when the {@code wait} method 390 * was invoked. 391 * <p> 392 * A thread can wake up without being notified, interrupted, or timing out, a 393 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice, 394 * applications must guard against it by testing for the condition that should 395 * have caused the thread to be awakened, and continuing to wait if the condition 396 * is not satisfied. See the example below. 397 * <p> 398 * For more information on this topic, see section 14.2, 399 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency 400 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua 401 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley, 402 * 2008). 403 * <p> 404 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted} 405 * by any thread before or while it is waiting, then an {@code InterruptedException} 406 * is thrown. The <em>interrupted status</em> of the current thread is cleared when 407 * this exception is thrown. This exception is not thrown until the lock status of 408 * this object has been restored as described above. 409 * 410 * @apiNote 411 * The recommended approach to waiting is to check the condition being awaited in 412 * a {@code while} loop around the call to {@code wait}, as shown in the example 413 * below. Among other things, this approach avoids problems that can be caused 414 * by spurious wakeups. 415 * 416 * <pre>{@code 417 * synchronized (obj) { 418 * while (<condition does not hold> and <timeout not exceeded>) { 419 * long timeoutMillis = ... ; // recompute timeout values 420 * int nanos = ... ; 421 * obj.wait(timeoutMillis, nanos); 422 * } 423 * ... // Perform action appropriate to condition or timeout 424 * } 425 * }</pre> 426 * 427 * @param timeoutMillis the maximum time to wait, in milliseconds 428 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive 429 * @throws IllegalArgumentException if {@code timeoutMillis} is negative, 430 * or if the value of {@code nanos} is out of range 431 * @throws IllegalMonitorStateException if the current thread is not 432 * the owner of the object's monitor 433 * @throws InterruptedException if any thread interrupted the current thread before or 434 * while the current thread was waiting. The <em>interrupted status</em> of the 435 * current thread is cleared when this exception is thrown. 436 * @see #notify() 437 * @see #notifyAll() 438 * @see #wait() 439 * @see #wait(long) 440 */ 441 public final void wait(long timeoutMillis, int nanos) throws InterruptedException { 442 if (timeoutMillis < 0) { 443 throw new IllegalArgumentException("timeoutMillis value is negative"); 444 } 445 446 if (nanos < 0 || nanos > 999999) { 447 throw new IllegalArgumentException( 448 "nanosecond timeout value out of range"); 449 } 450 451 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) { 452 timeoutMillis++; 453 } 454 455 wait(timeoutMillis); 456 } 457 458 /** 459 * Called by the garbage collector on an object when garbage collection 460 * determines that there are no more references to the object. 461 * A subclass overrides the {@code finalize} method to dispose of 462 * system resources or to perform other cleanup. 463 * <p> 464 * The general contract of {@code finalize} is that it is invoked 465 * if and when the Java™ virtual 466 * machine has determined that there is no longer any 467 * means by which this object can be accessed by any thread that has 468 * not yet died, except as a result of an action taken by the 469 * finalization of some other object or class which is ready to be 470 * finalized. The {@code finalize} method may take any action, including 471 * making this object available again to other threads; the usual purpose 472 * of {@code finalize}, however, is to perform cleanup actions before 473 * the object is irrevocably discarded. For example, the finalize method 474 * for an object that represents an input/output connection might perform 475 * explicit I/O transactions to break the connection before the object is 476 * permanently discarded. 477 * <p> 478 * The {@code finalize} method of class {@code Object} performs no 479 * special action; it simply returns normally. Subclasses of 480 * {@code Object} may override this definition. 481 * <p> 482 * The Java programming language does not guarantee which thread will 483 * invoke the {@code finalize} method for any given object. It is 484 * guaranteed, however, that the thread that invokes finalize will not 485 * be holding any user-visible synchronization locks when finalize is 486 * invoked. If an uncaught exception is thrown by the finalize method, 487 * the exception is ignored and finalization of that object terminates. 488 * <p> 489 * After the {@code finalize} method has been invoked for an object, no 490 * further action is taken until the Java virtual machine has again 491 * determined that there is no longer any means by which this object can 492 * be accessed by any thread that has not yet died, including possible 493 * actions by other objects or classes which are ready to be finalized, 494 * at which point the object may be discarded. 495 * <p> 496 * The {@code finalize} method is never invoked more than once by a Java 497 * virtual machine for any given object. 498 * <p> 499 * Any exception thrown by the {@code finalize} method causes 500 * the finalization of this object to be halted, but is otherwise 501 * ignored. 502 * 503 * @apiNote 504 * Classes that embed non-heap resources have many options 505 * for cleanup of those resources. The class must ensure that the 506 * lifetime of each instance is longer than that of any resource it embeds. 507 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that 508 * objects remain reachable while resources embedded in the object are in use. 509 * <p> 510 * A subclass should avoid overriding the {@code finalize} method 511 * unless the subclass embeds non-heap resources that must be cleaned up 512 * before the instance is collected. 513 * Finalizer invocations are not automatically chained, unlike constructors. 514 * If a subclass overrides {@code finalize} it must invoke the superclass 515 * finalizer explicitly. 516 * To guard against exceptions prematurely terminating the finalize chain, 517 * the subclass should use a {@code try-finally} block to ensure 518 * {@code super.finalize()} is always invoked. For example, 519 * <pre>{@code @Override 520 * protected void finalize() throws Throwable { 521 * try { 522 * ... // cleanup subclass state 523 * } finally { 524 * super.finalize(); 525 * } 526 * } 527 * }</pre> 528 * 529 * @deprecated The finalization mechanism is inherently problematic. 530 * Finalization can lead to performance issues, deadlocks, and hangs. 531 * Errors in finalizers can lead to resource leaks; there is no way to cancel 532 * finalization if it is no longer necessary; and no ordering is specified 533 * among calls to {@code finalize} methods of different objects. 534 * Furthermore, there are no guarantees regarding the timing of finalization. 535 * The {@code finalize} method might be called on a finalizable object 536 * only after an indefinite delay, if at all. 537 * 538 * Classes whose instances hold non-heap resources should provide a method 539 * to enable explicit release of those resources, and they should also 540 * implement {@link AutoCloseable} if appropriate. 541 * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference} 542 * provide more flexible and efficient ways to release resources when an object 543 * becomes unreachable. 544 * 545 * @throws Throwable the {@code Exception} raised by this method 546 * @see java.lang.ref.WeakReference 547 * @see java.lang.ref.PhantomReference 548 * @jls 12.6 Finalization of Class Instances 549 */ 550 @Deprecated(since="9") 551 protected void finalize() throws Throwable { } 552 }