1 /*
2 * Copyright (c) 1994, 2022, 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
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20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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24 */
25
26 package java.lang;
27
28 import jdk.internal.vm.annotation.IntrinsicCandidate;
29 import jdk.internal.access.SharedSecrets;
30
31 import java.util.Objects;
32
33 /**
34 * Class {@code Object} is the root of the class hierarchy.
35 * Every class has {@code Object} as a superclass. All objects,
36 * including arrays, implement the methods of this class.
37 *
38 * @see java.lang.Class
39 * @since 1.0
40 */
41 public class Object {
42
43 /**
44 * Constructs a new object.
45 */
46 @IntrinsicCandidate
47 public Object() {}
48
49 /**
50 * Returns the runtime class of this {@code Object}. The returned
51 * {@code Class} object is the object that is locked by {@code
52 * static synchronized} methods of the represented class.
53 *
54 * <p><b>The actual result type is {@code Class<? extends |X|>}
55 * where {@code |X|} is the erasure of the static type of the
56 * expression on which {@code getClass} is called.</b> For
57 * example, no cast is required in this code fragment:</p>
58 *
59 * <p>
60 * {@code Number n = 0; }<br>
61 * {@code Class<? extends Number> c = n.getClass(); }
62 * </p>
63 *
64 * @return The {@code Class} object that represents the runtime
65 * class of this object.
66 * @jls 15.8.2 Class Literals
67 */
68 @IntrinsicCandidate
69 public final native Class<?> getClass();
70
71 /**
72 * Returns a hash code value for the object. This method is
73 * supported for the benefit of hash tables such as those provided by
74 * {@link java.util.HashMap}.
75 * <p>
76 * The general contract of {@code hashCode} is:
77 * <ul>
78 * <li>Whenever it is invoked on the same object more than once during
79 * an execution of a Java application, the {@code hashCode} method
80 * must consistently return the same integer, provided no information
81 * used in {@code equals} comparisons on the object is modified.
82 * This integer need not remain consistent from one execution of an
83 * application to another execution of the same application.
84 * <li>If two objects are equal according to the {@link
85 * equals(Object) equals} method, then calling the {@code
86 * hashCode} method on each of the two objects must produce the
87 * same integer result.
88 * <li>It is <em>not</em> required that if two objects are unequal
89 * according to the {@link equals(Object) equals} method, then
90 * calling the {@code hashCode} method on each of the two objects
91 * must produce distinct integer results. However, the programmer
92 * should be aware that producing distinct integer results for
93 * unequal objects may improve the performance of hash tables.
94 * </ul>
95 *
96 * @implSpec
97 * As far as is reasonably practical, the {@code hashCode} method defined
98 * by class {@code Object} returns distinct integers for distinct objects.
99 *
100 * @return a hash code value for this object.
101 * @see java.lang.Object#equals(java.lang.Object)
102 * @see java.lang.System#identityHashCode
103 */
104 @IntrinsicCandidate
105 public native int hashCode();
106
107 /**
108 * Indicates whether some other object is "equal to" this one.
109 * <p>
110 * The {@code equals} method implements an equivalence relation
111 * on non-null object references:
112 * <ul>
113 * <li>It is <i>reflexive</i>: for any non-null reference value
114 * {@code x}, {@code x.equals(x)} should return
115 * {@code true}.
116 * <li>It is <i>symmetric</i>: for any non-null reference values
117 * {@code x} and {@code y}, {@code x.equals(y)}
118 * should return {@code true} if and only if
119 * {@code y.equals(x)} returns {@code true}.
120 * <li>It is <i>transitive</i>: for any non-null reference values
121 * {@code x}, {@code y}, and {@code z}, if
122 * {@code x.equals(y)} returns {@code true} and
123 * {@code y.equals(z)} returns {@code true}, then
124 * {@code x.equals(z)} should return {@code true}.
125 * <li>It is <i>consistent</i>: for any non-null reference values
126 * {@code x} and {@code y}, multiple invocations of
127 * {@code x.equals(y)} consistently return {@code true}
128 * or consistently return {@code false}, provided no
129 * information used in {@code equals} comparisons on the
130 * objects is modified.
131 * <li>For any non-null reference value {@code x},
132 * {@code x.equals(null)} should return {@code false}.
133 * </ul>
134 *
135 * <p>
136 * An equivalence relation partitions the elements it operates on
137 * into <i>equivalence classes</i>; all the members of an
138 * equivalence class are equal to each other. Members of an
139 * equivalence class are substitutable for each other, at least
140 * for some purposes.
141 *
142 * @implSpec
143 * The {@code equals} method for class {@code Object} implements
144 * the most discriminating possible equivalence relation on objects;
145 * that is, for any non-null reference values {@code x} and
146 * {@code y}, this method returns {@code true} if and only
147 * if {@code x} and {@code y} refer to the same object
148 * ({@code x == y} has the value {@code true}).
149 *
150 * In other words, under the reference equality equivalence
151 * relation, each equivalence class only has a single element.
152 *
153 * @apiNote
154 * It is generally necessary to override the {@link hashCode hashCode}
155 * method whenever this method is overridden, so as to maintain the
156 * general contract for the {@code hashCode} method, which states
157 * that equal objects must have equal hash codes.
158 *
159 * @param obj the reference object with which to compare.
160 * @return {@code true} if this object is the same as the obj
161 * argument; {@code false} otherwise.
162 * @see #hashCode()
163 * @see java.util.HashMap
164 */
165 public boolean equals(Object obj) {
166 return (this == obj);
167 }
168
169 /**
170 * Creates and returns a copy of this object. The precise meaning
171 * of "copy" may depend on the class of the object. The general
172 * intent is that, for any object {@code x}, the expression:
173 * <blockquote>
174 * <pre>
175 * x.clone() != x</pre></blockquote>
176 * will be true, and that the expression:
177 * <blockquote>
178 * <pre>
179 * x.clone().getClass() == x.getClass()</pre></blockquote>
180 * will be {@code true}, but these are not absolute requirements.
181 * While it is typically the case that:
182 * <blockquote>
183 * <pre>
184 * x.clone().equals(x)</pre></blockquote>
185 * will be {@code true}, this is not an absolute requirement.
186 * <p>
187 * By convention, the returned object should be obtained by calling
188 * {@code super.clone}. If a class and all of its superclasses (except
189 * {@code Object}) obey this convention, it will be the case that
190 * {@code x.clone().getClass() == x.getClass()}.
191 * <p>
192 * By convention, the object returned by this method should be independent
193 * of this object (which is being cloned). To achieve this independence,
194 * it may be necessary to modify one or more fields of the object returned
195 * by {@code super.clone} before returning it. Typically, this means
196 * copying any mutable objects that comprise the internal "deep structure"
197 * of the object being cloned and replacing the references to these
198 * objects with references to the copies. If a class contains only
199 * primitive fields or references to immutable objects, then it is usually
200 * the case that no fields in the object returned by {@code super.clone}
201 * need to be modified.
202 *
203 * @implSpec
204 * The method {@code clone} for class {@code Object} performs a
205 * specific cloning operation. First, if the class of this object does
206 * not implement the interface {@code Cloneable}, then a
207 * {@code CloneNotSupportedException} is thrown. Note that all arrays
208 * are considered to implement the interface {@code Cloneable} and that
209 * the return type of the {@code clone} method of an array type {@code T[]}
210 * is {@code T[]} where T is any reference or primitive type.
211 * Otherwise, this method creates a new instance of the class of this
212 * object and initializes all its fields with exactly the contents of
213 * the corresponding fields of this object, as if by assignment; the
214 * contents of the fields are not themselves cloned. Thus, this method
215 * performs a "shallow copy" of this object, not a "deep copy" operation.
216 * <p>
217 * The class {@code Object} does not itself implement the interface
218 * {@code Cloneable}, so calling the {@code clone} method on an object
219 * whose class is {@code Object} will result in throwing an
220 * exception at run time.
221 *
222 * @return a clone of this instance.
223 * @throws CloneNotSupportedException if the object's class does not
224 * support the {@code Cloneable} interface. Subclasses
225 * that override the {@code clone} method can also
226 * throw this exception to indicate that an instance cannot
227 * be cloned.
228 * @see java.lang.Cloneable
229 */
230 @IntrinsicCandidate
231 protected native Object clone() throws CloneNotSupportedException;
232
233 /**
234 * Returns a string representation of the object.
235 * @apiNote
236 * In general, the
237 * {@code toString} method returns a string that
238 * "textually represents" this object. The result should
239 * be a concise but informative representation that is easy for a
240 * person to read.
241 * It is recommended that all subclasses override this method.
242 * The string output is not necessarily stable over time or across
243 * JVM invocations.
244 * @implSpec
245 * If this object is an instance of an identity class, then
246 * the {@code toString} method for class {@code Object}
247 * returns a string consisting of the name of the class of which the
248 * object is an instance, the at-sign character `{@code @}', and
249 * the unsigned hexadecimal representation of the hash code of the
250 * object. In other words, this method returns a string equal to the
251 * value of:
252 * <blockquote>
253 * <pre>
254 * getClass().getName() + '@' + Integer.toHexString(hashCode())
255 * </pre></blockquote>
256 * <p>
257 * If this object is an instance of a primitive class, then
258 * the {@code toString} method returns a string which contains
259 * the name of the primitive class, and string representations of
260 * all its fields. The precise format produced by this method
261 * is unspecified and subject to change.
262 *
263 * @return a string representation of the object.
264 */
265 public String toString() {
266 return getClass().getName() + "@" + Integer.toHexString(hashCode());
267 }
268
269 /**
270 * Wakes up a single thread that is waiting on this object's
271 * monitor. If any threads are waiting on this object, one of them
272 * is chosen to be awakened. The choice is arbitrary and occurs at
273 * the discretion of the implementation. A thread waits on an object's
274 * monitor by calling one of the {@code wait} methods.
275 * <p>
276 * The awakened thread will not be able to proceed until the current
277 * thread relinquishes the lock on this object. The awakened thread will
278 * compete in the usual manner with any other threads that might be
279 * actively competing to synchronize on this object; for example, the
280 * awakened thread enjoys no reliable privilege or disadvantage in being
281 * the next thread to lock this object.
282 * <p>
283 * This method should only be called by a thread that is the owner
284 * of this object's monitor. A thread becomes the owner of the
285 * object's monitor in one of three ways:
286 * <ul>
287 * <li>By executing a synchronized instance method of that object.
288 * <li>By executing the body of a {@code synchronized} statement
289 * that synchronizes on the object.
290 * <li>For objects of type {@code Class,} by executing a
291 * static synchronized method of that class.
292 * </ul>
293 * <p>
294 * Only one thread at a time can own an object's monitor.
295 *
296 * @throws IllegalMonitorStateException if the current thread is not
297 * the owner of this object's monitor.
298 * @see java.lang.Object#notifyAll()
299 * @see java.lang.Object#wait()
300 */
301 @IntrinsicCandidate
302 public final native void notify();
303
304 /**
305 * Wakes up all threads that are waiting on this object's monitor. A
306 * thread waits on an object's monitor by calling one of the
307 * {@code wait} methods.
308 * <p>
309 * The awakened threads will not be able to proceed until the current
310 * thread relinquishes the lock on this object. The awakened threads
311 * will compete in the usual manner with any other threads that might
312 * be actively competing to synchronize on this object; for example,
313 * the awakened threads enjoy no reliable privilege or disadvantage in
314 * being the next thread to lock this object.
315 * <p>
316 * This method should only be called by a thread that is the owner
317 * of this object's monitor. See the {@code notify} method for a
318 * description of the ways in which a thread can become the owner of
319 * a monitor.
320 *
321 * @throws IllegalMonitorStateException if the current thread is not
322 * the owner of this object's monitor.
323 * @see java.lang.Object#notify()
324 * @see java.lang.Object#wait()
325 */
326 @IntrinsicCandidate
327 public final native void notifyAll();
328
329 /**
330 * Causes the current thread to wait until it is awakened, typically
331 * by being <em>notified</em> or <em>interrupted</em>.
332 * <p>
333 * In all respects, this method behaves as if {@code wait(0L, 0)}
334 * had been called. See the specification of the {@link #wait(long, int)} method
335 * for details.
336 *
337 * @throws IllegalMonitorStateException if the current thread is not
338 * the owner of the object's monitor
339 * @throws InterruptedException if any thread interrupted the current thread before or
340 * while the current thread was waiting. The <em>interrupted status</em> of the
341 * current thread is cleared when this exception is thrown.
342 * @see #notify()
343 * @see #notifyAll()
344 * @see #wait(long)
345 * @see #wait(long, int)
346 */
347 public final void wait() throws InterruptedException {
348 wait(0L);
349 }
350
351 /**
352 * Causes the current thread to wait until it is awakened, typically
353 * by being <em>notified</em> or <em>interrupted</em>, or until a
354 * certain amount of real time has elapsed.
355 * <p>
356 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
357 * had been called. See the specification of the {@link #wait(long, int)} method
358 * for details.
359 *
360 * @param timeoutMillis the maximum time to wait, in milliseconds
361 * @throws IllegalArgumentException if {@code timeoutMillis} is negative
362 * @throws IllegalMonitorStateException if the current thread is not
363 * the owner of the object's monitor
364 * @throws InterruptedException if any thread interrupted the current thread before or
365 * while the current thread was waiting. The <em>interrupted status</em> of the
366 * current thread is cleared when this exception is thrown.
367 * @see #notify()
368 * @see #notifyAll()
369 * @see #wait()
370 * @see #wait(long, int)
371 */
372 public final native void wait(long timeoutMillis) throws InterruptedException;
373
374 /**
375 * Causes the current thread to wait until it is awakened, typically
376 * by being <em>notified</em> or <em>interrupted</em>, or until a
377 * certain amount of real time has elapsed.
378 * <p>
379 * The current thread must own this object's monitor lock. See the
380 * {@link #notify notify} method for a description of the ways in which
381 * a thread can become the owner of a monitor lock.
382 * <p>
383 * This method causes the current thread (referred to here as <var>T</var>) to
384 * place itself in the wait set for this object and then to relinquish any
385 * and all synchronization claims on this object. Note that only the locks
386 * on this object are relinquished; any other objects on which the current
387 * thread may be synchronized remain locked while the thread waits.
388 * <p>
389 * Thread <var>T</var> then becomes disabled for thread scheduling purposes
390 * and lies dormant until one of the following occurs:
391 * <ul>
392 * <li>Some other thread invokes the {@code notify} method for this
393 * object and thread <var>T</var> happens to be arbitrarily chosen as
394 * the thread to be awakened.
395 * <li>Some other thread invokes the {@code notifyAll} method for this
396 * object.
397 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
398 * thread <var>T</var>.
399 * <li>The specified amount of real time has elapsed, more or less.
400 * The amount of real time, in nanoseconds, is given by the expression
401 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
402 * are both zero, then real time is not taken into consideration and the
403 * thread waits until awakened by one of the other causes.
404 * <li>Thread <var>T</var> is awakened spuriously. (See below.)
405 * </ul>
406 * <p>
407 * The thread <var>T</var> is then removed from the wait set for this
408 * object and re-enabled for thread scheduling. It competes in the
409 * usual manner with other threads for the right to synchronize on the
410 * object; once it has regained control of the object, all its
411 * synchronization claims on the object are restored to the status quo
412 * ante - that is, to the situation as of the time that the {@code wait}
413 * method was invoked. Thread <var>T</var> then returns from the
414 * invocation of the {@code wait} method. Thus, on return from the
415 * {@code wait} method, the synchronization state of the object and of
416 * thread {@code T} is exactly as it was when the {@code wait} method
417 * was invoked.
418 * <p>
419 * A thread can wake up without being notified, interrupted, or timing out, a
420 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice,
421 * applications must guard against it by testing for the condition that should
422 * have caused the thread to be awakened, and continuing to wait if the condition
423 * is not satisfied. See the example below.
424 * <p>
425 * For more information on this topic, see section 14.2,
426 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency
427 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua
428 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley,
429 * 2008).
430 * <p>
431 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
432 * by any thread before or while it is waiting, then an {@code InterruptedException}
433 * is thrown. The <em>interrupted status</em> of the current thread is cleared when
434 * this exception is thrown. This exception is not thrown until the lock status of
435 * this object has been restored as described above.
436 *
437 * @apiNote
438 * The recommended approach to waiting is to check the condition being awaited in
439 * a {@code while} loop around the call to {@code wait}, as shown in the example
440 * below. Among other things, this approach avoids problems that can be caused
441 * by spurious wakeups.
442 *
443 * <pre>{@code
444 * synchronized (obj) {
445 * while (<condition does not hold> and <timeout not exceeded>) {
446 * long timeoutMillis = ... ; // recompute timeout values
447 * int nanos = ... ;
448 * obj.wait(timeoutMillis, nanos);
449 * }
450 * ... // Perform action appropriate to condition or timeout
451 * }
452 * }</pre>
453 *
454 * @param timeoutMillis the maximum time to wait, in milliseconds
455 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive
456 * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
457 * or if the value of {@code nanos} is out of range
458 * @throws IllegalMonitorStateException if the current thread is not
459 * the owner of the object's monitor
460 * @throws InterruptedException if any thread interrupted the current thread before or
461 * while the current thread was waiting. The <em>interrupted status</em> of the
462 * current thread is cleared when this exception is thrown.
463 * @see #notify()
464 * @see #notifyAll()
465 * @see #wait()
466 * @see #wait(long)
467 */
468 public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
469 if (timeoutMillis < 0) {
470 throw new IllegalArgumentException("timeoutMillis value is negative");
471 }
472
473 if (nanos < 0 || nanos > 999999) {
474 throw new IllegalArgumentException(
475 "nanosecond timeout value out of range");
476 }
477
478 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
479 timeoutMillis++;
480 }
481
482 wait(timeoutMillis);
483 }
484
485 /**
486 * Called by the garbage collector on an object when garbage collection
487 * determines that there are no more references to the object.
488 * A subclass overrides the {@code finalize} method to dispose of
489 * system resources or to perform other cleanup.
490 * <p>
491 * <b>When running in a Java virtual machine in which finalization has been
492 * disabled or removed, the garbage collector will never call
493 * {@code finalize()}. In a Java virtual machine in which finalization is
494 * enabled, the garbage collector might call {@code finalize} only after an
495 * indefinite delay.</b>
496 * <p>
497 * The general contract of {@code finalize} is that it is invoked
498 * if and when the Java virtual
499 * machine has determined that there is no longer any
500 * means by which this object can be accessed by any thread that has
501 * not yet died, except as a result of an action taken by the
502 * finalization of some other object or class which is ready to be
503 * finalized. The {@code finalize} method may take any action, including
504 * making this object available again to other threads; the usual purpose
505 * of {@code finalize}, however, is to perform cleanup actions before
506 * the object is irrevocably discarded. For example, the finalize method
507 * for an object that represents an input/output connection might perform
508 * explicit I/O transactions to break the connection before the object is
509 * permanently discarded.
510 * <p>
511 * The {@code finalize} method of class {@code Object} performs no
512 * special action; it simply returns normally. Subclasses of
513 * {@code Object} may override this definition.
514 * <p>
515 * The Java programming language does not guarantee which thread will
516 * invoke the {@code finalize} method for any given object. It is
517 * guaranteed, however, that the thread that invokes finalize will not
518 * be holding any user-visible synchronization locks when finalize is
519 * invoked. If an uncaught exception is thrown by the finalize method,
520 * the exception is ignored and finalization of that object terminates.
521 * <p>
522 * After the {@code finalize} method has been invoked for an object, no
523 * further action is taken until the Java virtual machine has again
524 * determined that there is no longer any means by which this object can
525 * be accessed by any thread that has not yet died, including possible
526 * actions by other objects or classes which are ready to be finalized,
527 * at which point the object may be discarded.
528 * <p>
529 * The {@code finalize} method is never invoked more than once by a Java
530 * virtual machine for any given object.
531 * <p>
532 * Any exception thrown by the {@code finalize} method causes
533 * the finalization of this object to be halted, but is otherwise
534 * ignored.
535 *
536 * @apiNote
537 * Classes that embed non-heap resources have many options
538 * for cleanup of those resources. The class must ensure that the
539 * lifetime of each instance is longer than that of any resource it embeds.
540 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
541 * objects remain reachable while resources embedded in the object are in use.
542 * <p>
543 * A subclass should avoid overriding the {@code finalize} method
544 * unless the subclass embeds non-heap resources that must be cleaned up
545 * before the instance is collected.
546 * Finalizer invocations are not automatically chained, unlike constructors.
547 * If a subclass overrides {@code finalize} it must invoke the superclass
548 * finalizer explicitly.
549 * To guard against exceptions prematurely terminating the finalize chain,
550 * the subclass should use a {@code try-finally} block to ensure
551 * {@code super.finalize()} is always invoked. For example,
552 * <pre>{@code @Override
553 * protected void finalize() throws Throwable {
554 * try {
555 * ... // cleanup subclass state
556 * } finally {
557 * super.finalize();
558 * }
559 * }
560 * }</pre>
561 *
562 * @deprecated Finalization is deprecated and subject to removal in a future
563 * release. The use of finalization can lead to problems with security,
564 * performance, and reliability.
565 * See <a href="https://openjdk.java.net/jeps/421">JEP 421</a> for
566 * discussion and alternatives.
567 * <p>
568 * Subclasses that override {@code finalize} to perform cleanup should use
569 * alternative cleanup mechanisms and remove the {@code finalize} method.
570 * Use {@link java.lang.ref.Cleaner} and
571 * {@link java.lang.ref.PhantomReference} as safer ways to release resources
572 * when an object becomes unreachable. Alternatively, add a {@code close}
573 * method to explicitly release resources, and implement
574 * {@code AutoCloseable} to enable use of the {@code try}-with-resources
575 * statement.
576 * <p>
577 * This method will remain in place until finalizers have been removed from
578 * most existing code.
579 *
580 * @throws Throwable the {@code Exception} raised by this method
581 * @see java.lang.ref.WeakReference
582 * @see java.lang.ref.PhantomReference
583 * @jls 12.6 Finalization of Class Instances
584 */
585 @Deprecated(since="9", forRemoval=true)
586 protected void finalize() throws Throwable { }
587 }