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 * <b>When running in a Java virtual machine in which finalization has been
482 * disabled or removed, the garbage collector will never call
483 * {@code finalize()}. In a Java virtual machine in which finalization is
484 * enabled, the garbage collector might call {@code finalize} only after an
485 * indefinite delay.</b>
486 * <p>
487 * The general contract of {@code finalize} is that it is invoked
488 * if and when the Java virtual
489 * machine has determined that there is no longer any
490 * means by which this object can be accessed by any thread that has
491 * not yet died, except as a result of an action taken by the
492 * finalization of some other object or class which is ready to be
493 * finalized. The {@code finalize} method may take any action, including
494 * making this object available again to other threads; the usual purpose
495 * of {@code finalize}, however, is to perform cleanup actions before
496 * the object is irrevocably discarded. For example, the finalize method
497 * for an object that represents an input/output connection might perform
498 * explicit I/O transactions to break the connection before the object is
499 * permanently discarded.
500 * <p>
501 * The {@code finalize} method of class {@code Object} performs no
502 * special action; it simply returns normally. Subclasses of
503 * {@code Object} may override this definition.
504 * <p>
505 * The Java programming language does not guarantee which thread will
506 * invoke the {@code finalize} method for any given object. It is
507 * guaranteed, however, that the thread that invokes finalize will not
508 * be holding any user-visible synchronization locks when finalize is
509 * invoked. If an uncaught exception is thrown by the finalize method,
510 * the exception is ignored and finalization of that object terminates.
511 * <p>
512 * After the {@code finalize} method has been invoked for an object, no
513 * further action is taken until the Java virtual machine has again
514 * determined that there is no longer any means by which this object can
515 * be accessed by any thread that has not yet died, including possible
516 * actions by other objects or classes which are ready to be finalized,
517 * at which point the object may be discarded.
518 * <p>
519 * The {@code finalize} method is never invoked more than once by a Java
520 * virtual machine for any given object.
521 * <p>
522 * Any exception thrown by the {@code finalize} method causes
523 * the finalization of this object to be halted, but is otherwise
524 * ignored.
525 *
526 * @apiNote
527 * Classes that embed non-heap resources have many options
528 * for cleanup of those resources. The class must ensure that the
529 * lifetime of each instance is longer than that of any resource it embeds.
530 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
531 * objects remain reachable while resources embedded in the object are in use.
532 * <p>
533 * A subclass should avoid overriding the {@code finalize} method
534 * unless the subclass embeds non-heap resources that must be cleaned up
535 * before the instance is collected.
536 * Finalizer invocations are not automatically chained, unlike constructors.
537 * If a subclass overrides {@code finalize} it must invoke the superclass
538 * finalizer explicitly.
539 * To guard against exceptions prematurely terminating the finalize chain,
540 * the subclass should use a {@code try-finally} block to ensure
541 * {@code super.finalize()} is always invoked. For example,
542 * <pre>{@code @Override
543 * protected void finalize() throws Throwable {
544 * try {
545 * ... // cleanup subclass state
546 * } finally {
547 * super.finalize();
548 * }
549 * }
550 * }</pre>
551 *
552 * @deprecated Finalization is deprecated and subject to removal in a future
553 * release. The use of finalization can lead to problems with security,
554 * performance, and reliability.
555 * See <a href="https://openjdk.java.net/jeps/421">JEP 421</a> for
556 * discussion and alternatives.
557 * <p>
558 * Subclasses that override {@code finalize} to perform cleanup should use
559 * alternative cleanup mechanisms and remove the {@code finalize} method.
560 * Use {@link java.lang.ref.Cleaner} and
561 * {@link java.lang.ref.PhantomReference} as safer ways to release resources
562 * when an object becomes unreachable. Alternatively, add a {@code close}
563 * method to explicitly release resources, and implement
564 * {@code AutoCloseable} to enable use of the {@code try}-with-resources
565 * statement.
566 * <p>
567 * This method will remain in place until finalizers have been removed from
568 * most existing code.
569 *
570 * @throws Throwable the {@code Exception} raised by this method
571 * @see java.lang.ref.WeakReference
572 * @see java.lang.ref.PhantomReference
573 * @jls 12.6 Finalization of Class Instances
574 */
575 @Deprecated(since="9", forRemoval=true)
576 protected void finalize() throws Throwable { }
577 }