1 /* 2 * Copyright (c) 1994, 2021, 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.vm.annotation.IntrinsicCandidate; 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 * @see java.lang.Class 36 * @since 1.0 37 */ 38 public class Object { 39 40 /** 41 * Constructs a new object. 42 */ 43 @IntrinsicCandidate 44 public Object() {} 45 46 /** 47 * Returns the runtime class of this {@code Object}. The returned 48 * {@code Class} object is the object that is locked by {@code 49 * static synchronized} methods of the represented class. 50 * 51 * <p><b>The actual result type is {@code Class<? extends |X|>} 52 * where {@code |X|} is the erasure of the static type of the 53 * expression on which {@code getClass} is called.</b> For 54 * example, no cast is required in this code fragment:</p> 55 * 56 * <p> 57 * {@code Number n = 0; }<br> 58 * {@code Class<? extends Number> c = n.getClass(); } 59 * </p> 60 * 61 * @return The {@code Class} object that represents the runtime 62 * class of this object. 63 * @jls 15.8.2 Class Literals 64 */ 65 @IntrinsicCandidate 66 public final native Class<?> getClass(); 67 68 /** 69 * Returns a hash code value for the object. This method is 70 * supported for the benefit of hash tables such as those provided by 71 * {@link java.util.HashMap}. 72 * <p> 73 * The general contract of {@code hashCode} is: 74 * <ul> 75 * <li>Whenever it is invoked on the same object more than once during 76 * an execution of a Java application, the {@code hashCode} method 77 * must consistently return the same integer, provided no information 78 * used in {@code equals} comparisons on the object is modified. 79 * This integer need not remain consistent from one execution of an 80 * application to another execution of the same application. 81 * <li>If two objects are equal according to the {@link 82 * equals(Object) equals} method, then calling the {@code 83 * hashCode} method on each of the two objects must produce the 84 * same integer result. 85 * <li>It is <em>not</em> required that if two objects are unequal 86 * according to the {@link equals(Object) equals} method, then 87 * calling the {@code hashCode} method on each of the two objects 88 * must produce distinct integer results. However, the programmer 89 * should be aware that producing distinct integer results for 90 * 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 @IntrinsicCandidate 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 * 132 * <p> 133 * An equivalence relation partitions the elements it operates on 134 * into <i>equivalence classes</i>; all the members of an 135 * equivalence class are equal to each other. Members of an 136 * equivalence class are substitutable for each other, at least 137 * for some purposes. 138 * 139 * @implSpec 140 * The {@code equals} method for class {@code Object} implements 141 * the most discriminating possible equivalence relation on objects; 142 * that is, for any non-null reference values {@code x} and 143 * {@code y}, this method returns {@code true} if and only 144 * if {@code x} and {@code y} refer to the same object 145 * ({@code x == y} has the value {@code true}). 146 * 147 * In other words, under the reference equality equivalence 148 * relation, each equivalence class only has a single element. 149 * 150 * @apiNote 151 * It is generally necessary to override the {@link hashCode hashCode} 152 * method whenever this method is overridden, so as to maintain the 153 * general contract for the {@code hashCode} method, which states 154 * that equal objects must have equal hash codes. 155 * 156 * @param obj the reference object with which to compare. 157 * @return {@code true} if this object is the same as the obj 158 * argument; {@code false} otherwise. 159 * @see #hashCode() 160 * @see java.util.HashMap 161 */ 162 public boolean equals(Object obj) { 163 return (this == obj); 164 } 165 166 /** 167 * Creates and returns a copy of this object. The precise meaning 168 * of "copy" may depend on the class of the object. The general 169 * intent is that, for any object {@code x}, the expression: 170 * <blockquote> 171 * <pre> 172 * x.clone() != x</pre></blockquote> 173 * will be true, and that the expression: 174 * <blockquote> 175 * <pre> 176 * x.clone().getClass() == x.getClass()</pre></blockquote> 177 * will be {@code true}, but these are not absolute requirements. 178 * While it is typically the case that: 179 * <blockquote> 180 * <pre> 181 * x.clone().equals(x)</pre></blockquote> 182 * will be {@code true}, this is not an absolute requirement. 183 * <p> 184 * By convention, the returned object should be obtained by calling 185 * {@code super.clone}. If a class and all of its superclasses (except 186 * {@code Object}) obey this convention, it will be the case that 187 * {@code x.clone().getClass() == x.getClass()}. 188 * <p> 189 * By convention, the object returned by this method should be independent 190 * of this object (which is being cloned). To achieve this independence, 191 * it may be necessary to modify one or more fields of the object returned 192 * by {@code super.clone} before returning it. Typically, this means 193 * copying any mutable objects that comprise the internal "deep structure" 194 * of the object being cloned and replacing the references to these 195 * objects with references to the copies. If a class contains only 196 * primitive fields or references to immutable objects, then it is usually 197 * the case that no fields in the object returned by {@code super.clone} 198 * need to be modified. 199 * 200 * @implSpec 201 * The method {@code clone} for class {@code Object} performs a 202 * specific cloning operation. First, if the class of this object does 203 * not implement the interface {@code Cloneable}, then a 204 * {@code CloneNotSupportedException} is thrown. Note that all arrays 205 * are considered to implement the interface {@code Cloneable} and that 206 * the return type of the {@code clone} method of an array type {@code T[]} 207 * is {@code T[]} where T is any reference or primitive type. 208 * Otherwise, this method creates a new instance of the class of this 209 * object and initializes all its fields with exactly the contents of 210 * the corresponding fields of this object, as if by assignment; the 211 * contents of the fields are not themselves cloned. Thus, this method 212 * performs a "shallow copy" of this object, not a "deep copy" operation. 213 * <p> 214 * The class {@code Object} does not itself implement the interface 215 * {@code Cloneable}, so calling the {@code clone} method on an object 216 * whose class is {@code Object} will result in throwing an 217 * exception at run time. 218 * 219 * @return a clone of this instance. 220 * @throws CloneNotSupportedException if the object's class does not 221 * support the {@code Cloneable} interface. Subclasses 222 * that override the {@code clone} method can also 223 * throw this exception to indicate that an instance cannot 224 * be cloned. 225 * @see java.lang.Cloneable 226 */ 227 @IntrinsicCandidate 228 protected native Object clone() throws CloneNotSupportedException; 229 230 /** 231 * Returns a string representation of the object. 232 * @apiNote 233 * In general, the 234 * {@code toString} method returns a string that 235 * "textually represents" this object. The result should 236 * be a concise but informative representation that is easy for a 237 * person to read. 238 * It is recommended that all subclasses override this method. 239 * The string output is not necessarily stable over time or across 240 * JVM invocations. 241 * @implSpec 242 * The {@code toString} method for class {@code Object} 243 * returns a string consisting of the name of the class of which the 244 * object is an instance, the at-sign character `{@code @}', and 245 * the unsigned hexadecimal representation of the hash code of the 246 * object. In other words, this method returns a string equal to the 247 * value of: 248 * <blockquote> 249 * <pre> 250 * getClass().getName() + '@' + Integer.toHexString(hashCode()) 251 * </pre></blockquote> 252 * 253 * @return a string representation of the object. 254 */ 255 public String toString() { 256 return getClass().getName() + "@" + Integer.toHexString(hashCode()); 257 } 258 259 /** 260 * Wakes up a single thread that is waiting on this object's 261 * monitor. If any threads are waiting on this object, one of them 262 * is chosen to be awakened. The choice is arbitrary and occurs at 263 * the discretion of the implementation. A thread waits on an object's 264 * monitor by calling one of the {@code wait} methods. 265 * <p> 266 * The awakened thread will not be able to proceed until the current 267 * thread relinquishes the lock on this object. The awakened thread will 268 * compete in the usual manner with any other threads that might be 269 * actively competing to synchronize on this object; for example, the 270 * awakened thread enjoys no reliable privilege or disadvantage in being 271 * the next thread to lock this object. 272 * <p> 273 * This method should only be called by a thread that is the owner 274 * of this object's monitor. A thread becomes the owner of the 275 * object's monitor in one of three ways: 276 * <ul> 277 * <li>By executing a synchronized instance method of that object. 278 * <li>By executing the body of a {@code synchronized} statement 279 * that synchronizes on the object. 280 * <li>For objects of type {@code Class,} by executing a 281 * synchronized static method of that class. 282 * </ul> 283 * <p> 284 * Only one thread at a time can own an object's monitor. 285 * 286 * @throws IllegalMonitorStateException if the current thread is not 287 * the owner of this object's monitor. 288 * @see java.lang.Object#notifyAll() 289 * @see java.lang.Object#wait() 290 */ 291 @IntrinsicCandidate 292 public final native void notify(); 293 294 /** 295 * Wakes up all threads that are waiting on this object's monitor. A 296 * thread waits on an object's monitor by calling one of the 297 * {@code wait} methods. 298 * <p> 299 * The awakened threads will not be able to proceed until the current 300 * thread relinquishes the lock on this object. The awakened threads 301 * will compete in the usual manner with any other threads that might 302 * be actively competing to synchronize on this object; for example, 303 * the awakened threads enjoy no reliable privilege or disadvantage in 304 * being the next thread to lock this object. 305 * <p> 306 * This method should only be called by a thread that is the owner 307 * of this object's monitor. See the {@code notify} method for a 308 * description of the ways in which a thread can become the owner of 309 * a monitor. 310 * 311 * @throws IllegalMonitorStateException if the current thread is not 312 * the owner of this object's monitor. 313 * @see java.lang.Object#notify() 314 * @see java.lang.Object#wait() 315 */ 316 @IntrinsicCandidate 317 public final native void notifyAll(); 318 319 /** 320 * Causes the current thread to wait until it is awakened, typically 321 * by being <em>notified</em> or <em>interrupted</em>. 322 * <p> 323 * In all respects, this method behaves as if {@code wait(0L, 0)} 324 * had been called. See the specification of the {@link #wait(long, int)} method 325 * for details. 326 * 327 * @throws IllegalMonitorStateException if the current thread is not 328 * the owner of the object's monitor 329 * @throws InterruptedException if any thread interrupted the current thread before or 330 * while the current thread was waiting. The <em>interrupted status</em> of the 331 * current thread is cleared when this exception is thrown. 332 * @see #notify() 333 * @see #notifyAll() 334 * @see #wait(long) 335 * @see #wait(long, int) 336 */ 337 public final void wait() throws InterruptedException { 338 wait(0L); 339 } 340 341 /** 342 * Causes the current thread to wait until it is awakened, typically 343 * by being <em>notified</em> or <em>interrupted</em>, or until a 344 * certain amount of real time has elapsed. 345 * <p> 346 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)} 347 * had been called. See the specification of the {@link #wait(long, int)} method 348 * for details. 349 * 350 * @param timeoutMillis the maximum time to wait, in milliseconds 351 * @throws IllegalArgumentException if {@code timeoutMillis} is negative 352 * @throws IllegalMonitorStateException if the current thread is not 353 * the owner of the object's monitor 354 * @throws InterruptedException if any thread interrupted the current thread before or 355 * while the current thread was waiting. The <em>interrupted status</em> of the 356 * current thread is cleared when this exception is thrown. 357 * @see #notify() 358 * @see #notifyAll() 359 * @see #wait() 360 * @see #wait(long, int) 361 */ 362 public final native void wait(long timeoutMillis) throws InterruptedException; 363 364 /** 365 * Causes the current thread to wait until it is awakened, typically 366 * by being <em>notified</em> or <em>interrupted</em>, or until a 367 * certain amount of real time has elapsed. 368 * <p> 369 * The current thread must own this object's monitor lock. See the 370 * {@link #notify notify} method for a description of the ways in which 371 * a thread can become the owner of a monitor lock. 372 * <p> 373 * This method causes the current thread (referred to here as <var>T</var>) to 374 * place itself in the wait set for this object and then to relinquish any 375 * and all synchronization claims on this object. Note that only the locks 376 * on this object are relinquished; any other objects on which the current 377 * thread may be synchronized remain locked while the thread waits. 378 * <p> 379 * Thread <var>T</var> then becomes disabled for thread scheduling purposes 380 * and lies dormant until one of the following occurs: 381 * <ul> 382 * <li>Some other thread invokes the {@code notify} method for this 383 * object and thread <var>T</var> happens to be arbitrarily chosen as 384 * the thread to be awakened. 385 * <li>Some other thread invokes the {@code notifyAll} method for this 386 * object. 387 * <li>Some other thread {@linkplain Thread#interrupt() interrupts} 388 * thread <var>T</var>. 389 * <li>The specified amount of real time has elapsed, more or less. 390 * The amount of real time, in nanoseconds, is given by the expression 391 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos} 392 * are both zero, then real time is not taken into consideration and the 393 * thread waits until awakened by one of the other causes. 394 * <li>Thread <var>T</var> is awakened spuriously. (See below.) 395 * </ul> 396 * <p> 397 * The thread <var>T</var> is then removed from the wait set for this 398 * object and re-enabled for thread scheduling. It competes in the 399 * usual manner with other threads for the right to synchronize on the 400 * object; once it has regained control of the object, all its 401 * synchronization claims on the object are restored to the status quo 402 * ante - that is, to the situation as of the time that the {@code wait} 403 * method was invoked. Thread <var>T</var> then returns from the 404 * invocation of the {@code wait} method. Thus, on return from the 405 * {@code wait} method, the synchronization state of the object and of 406 * thread {@code T} is exactly as it was when the {@code wait} method 407 * was invoked. 408 * <p> 409 * A thread can wake up without being notified, interrupted, or timing out, a 410 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice, 411 * applications must guard against it by testing for the condition that should 412 * have caused the thread to be awakened, and continuing to wait if the condition 413 * is not satisfied. See the example below. 414 * <p> 415 * For more information on this topic, see section 14.2, 416 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency 417 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua 418 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley, 419 * 2008). 420 * <p> 421 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted} 422 * by any thread before or while it is waiting, then an {@code InterruptedException} 423 * is thrown. The <em>interrupted status</em> of the current thread is cleared when 424 * this exception is thrown. This exception is not thrown until the lock status of 425 * this object has been restored as described above. 426 * 427 * @apiNote 428 * The recommended approach to waiting is to check the condition being awaited in 429 * a {@code while} loop around the call to {@code wait}, as shown in the example 430 * below. Among other things, this approach avoids problems that can be caused 431 * by spurious wakeups. 432 * 433 * <pre>{@code 434 * synchronized (obj) { 435 * while (<condition does not hold> and <timeout not exceeded>) { 436 * long timeoutMillis = ... ; // recompute timeout values 437 * int nanos = ... ; 438 * obj.wait(timeoutMillis, nanos); 439 * } 440 * ... // Perform action appropriate to condition or timeout 441 * } 442 * }</pre> 443 * 444 * @param timeoutMillis the maximum time to wait, in milliseconds 445 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive 446 * @throws IllegalArgumentException if {@code timeoutMillis} is negative, 447 * or if the value of {@code nanos} is out of range 448 * @throws IllegalMonitorStateException if the current thread is not 449 * the owner of the object's monitor 450 * @throws InterruptedException if any thread interrupted the current thread before or 451 * while the current thread was waiting. The <em>interrupted status</em> of the 452 * current thread is cleared when this exception is thrown. 453 * @see #notify() 454 * @see #notifyAll() 455 * @see #wait() 456 * @see #wait(long) 457 */ 458 public final void wait(long timeoutMillis, int nanos) throws InterruptedException { 459 if (timeoutMillis < 0) { 460 throw new IllegalArgumentException("timeoutMillis value is negative"); 461 } 462 463 if (nanos < 0 || nanos > 999999) { 464 throw new IllegalArgumentException( 465 "nanosecond timeout value out of range"); 466 } 467 468 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) { 469 timeoutMillis++; 470 } 471 472 wait(timeoutMillis); 473 } 474 475 /** 476 * Called by the garbage collector on an object when garbage collection 477 * determines that there are no more references to the object. 478 * A subclass overrides the {@code finalize} method to dispose of 479 * system resources or to perform other cleanup. 480 * <p> 481 * The general contract of {@code finalize} is that it is invoked 482 * if and when the Java virtual 483 * machine has determined that there is no longer any 484 * means by which this object can be accessed by any thread that has 485 * not yet died, except as a result of an action taken by the 486 * finalization of some other object or class which is ready to be 487 * finalized. The {@code finalize} method may take any action, including 488 * making this object available again to other threads; the usual purpose 489 * of {@code finalize}, however, is to perform cleanup actions before 490 * the object is irrevocably discarded. For example, the finalize method 491 * for an object that represents an input/output connection might perform 492 * explicit I/O transactions to break the connection before the object is 493 * permanently discarded. 494 * <p> 495 * The {@code finalize} method of class {@code Object} performs no 496 * special action; it simply returns normally. Subclasses of 497 * {@code Object} may override this definition. 498 * <p> 499 * The Java programming language does not guarantee which thread will 500 * invoke the {@code finalize} method for any given object. It is 501 * guaranteed, however, that the thread that invokes finalize will not 502 * be holding any user-visible synchronization locks when finalize is 503 * invoked. If an uncaught exception is thrown by the finalize method, 504 * the exception is ignored and finalization of that object terminates. 505 * <p> 506 * After the {@code finalize} method has been invoked for an object, no 507 * further action is taken until the Java virtual machine has again 508 * determined that there is no longer any means by which this object can 509 * be accessed by any thread that has not yet died, including possible 510 * actions by other objects or classes which are ready to be finalized, 511 * at which point the object may be discarded. 512 * <p> 513 * The {@code finalize} method is never invoked more than once by a Java 514 * virtual machine for any given object. 515 * <p> 516 * Any exception thrown by the {@code finalize} method causes 517 * the finalization of this object to be halted, but is otherwise 518 * ignored. 519 * 520 * @apiNote 521 * Classes that embed non-heap resources have many options 522 * for cleanup of those resources. The class must ensure that the 523 * lifetime of each instance is longer than that of any resource it embeds. 524 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that 525 * objects remain reachable while resources embedded in the object are in use. 526 * <p> 527 * A subclass should avoid overriding the {@code finalize} method 528 * unless the subclass embeds non-heap resources that must be cleaned up 529 * before the instance is collected. 530 * Finalizer invocations are not automatically chained, unlike constructors. 531 * If a subclass overrides {@code finalize} it must invoke the superclass 532 * finalizer explicitly. 533 * To guard against exceptions prematurely terminating the finalize chain, 534 * the subclass should use a {@code try-finally} block to ensure 535 * {@code super.finalize()} is always invoked. For example, 536 * <pre>{@code @Override 537 * protected void finalize() throws Throwable { 538 * try { 539 * ... // cleanup subclass state 540 * } finally { 541 * super.finalize(); 542 * } 543 * } 544 * }</pre> 545 * 546 * @deprecated The finalization mechanism is inherently problematic. 547 * Finalization can lead to performance issues, deadlocks, and hangs. 548 * Errors in finalizers can lead to resource leaks; there is no way to cancel 549 * finalization if it is no longer necessary; and no ordering is specified 550 * among calls to {@code finalize} methods of different objects. 551 * Furthermore, there are no guarantees regarding the timing of finalization. 552 * The {@code finalize} method might be called on a finalizable object 553 * only after an indefinite delay, if at all. 554 * 555 * Classes whose instances hold non-heap resources should provide a method 556 * to enable explicit release of those resources, and they should also 557 * implement {@link AutoCloseable} if appropriate. 558 * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference} 559 * provide more flexible and efficient ways to release resources when an object 560 * becomes unreachable. 561 * 562 * @throws Throwable the {@code Exception} raised by this method 563 * @see java.lang.ref.WeakReference 564 * @see java.lang.ref.PhantomReference 565 * @jls 12.6 Finalization of Class Instances 566 */ 567 @Deprecated(since="9") 568 protected void finalize() throws Throwable { } 569 }