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