1 /*
2 * Copyright (c) 1994, 2020, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.lang;
27
28 import jdk.internal.HotSpotIntrinsicCandidate;
29
30 /**
31 * Class {@code Object} is the root of the class hierarchy.
32 * Every class has {@code Object} as a superclass. All objects,
33 * including arrays, implement the methods of this class.
34 *
35 * @author unascribed
36 * @see java.lang.Class
37 * @since 1.0
38 */
39 public class Object {
40
41 /**
42 * Constructs a new object.
43 * @apiNote {@link IdentityObject#newIdentity new IdentityObject.newIdentity()}
44 * should be used instead of {@code new Object()}.
45 */
46 @HotSpotIntrinsicCandidate
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 @HotSpotIntrinsicCandidate
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 {@code equals(Object)}
85 * method, then calling the {@code hashCode} method on each of
86 * the two objects must produce the same integer result.
87 * <li>It is <em>not</em> required that if two objects are unequal
88 * according to the {@link java.lang.Object#equals(java.lang.Object)}
89 * method, then calling the {@code hashCode} method on each of the
90 * two objects must produce distinct integer results. However, the
91 * programmer should be aware that producing distinct integer results
92 * for unequal objects may improve the performance of hash tables.
93 * </ul>
94 *
95 * @implSpec
96 * As far as is reasonably practical, the {@code hashCode} method defined
97 * by class {@code Object} returns distinct integers for distinct objects.
98 *
99 * @return a hash code value for this object.
100 * @see java.lang.Object#equals(java.lang.Object)
101 * @see java.lang.System#identityHashCode
102 */
103 @HotSpotIntrinsicCandidate
104 public native int hashCode();
105
106 /**
107 * Indicates whether some other object is "equal to" this one.
108 * <p>
109 * The {@code equals} method implements an equivalence relation
110 * on non-null object references:
111 * <ul>
112 * <li>It is <i>reflexive</i>: for any non-null reference value
113 * {@code x}, {@code x.equals(x)} should return
114 * {@code true}.
115 * <li>It is <i>symmetric</i>: for any non-null reference values
116 * {@code x} and {@code y}, {@code x.equals(y)}
117 * should return {@code true} if and only if
118 * {@code y.equals(x)} returns {@code true}.
119 * <li>It is <i>transitive</i>: for any non-null reference values
120 * {@code x}, {@code y}, and {@code z}, if
121 * {@code x.equals(y)} returns {@code true} and
122 * {@code y.equals(z)} returns {@code true}, then
123 * {@code x.equals(z)} should return {@code true}.
124 * <li>It is <i>consistent</i>: for any non-null reference values
125 * {@code x} and {@code y}, multiple invocations of
126 * {@code x.equals(y)} consistently return {@code true}
127 * or consistently return {@code false}, provided no
128 * information used in {@code equals} comparisons on the
129 * objects is modified.
130 * <li>For any non-null reference value {@code x},
131 * {@code x.equals(null)} should return {@code false}.
132 * </ul>
133 * <p>
134 * The {@code equals} method for class {@code Object} implements
135 * the most discriminating possible equivalence relation on objects;
136 * that is, for any non-null reference values {@code x} and
137 * {@code y}, this method returns {@code true} if and only
138 * if {@code x} and {@code y} refer to the same object
139 * ({@code x == y} has the value {@code true}).
140 * <p>
141 * Note that it is generally necessary to override the {@code hashCode}
142 * method whenever this method is overridden, so as to maintain the
143 * general contract for the {@code hashCode} method, which states
144 * that equal objects must have equal hash codes.
145 *
146 * @param obj the reference object with which to compare.
147 * @return {@code true} if this object is the same as the obj
148 * argument; {@code false} otherwise.
149 * @see #hashCode()
150 * @see java.util.HashMap
151 */
152 public boolean equals(Object obj) {
153 return (this == obj);
154 }
155
156 /**
157 * Creates and returns a copy of this object. The precise meaning
158 * of "copy" may depend on the class of the object. The general
159 * intent is that, for any object {@code x}, the expression:
160 * <blockquote>
161 * <pre>
162 * x.clone() != x</pre></blockquote>
163 * will be true, and that the expression:
164 * <blockquote>
165 * <pre>
166 * x.clone().getClass() == x.getClass()</pre></blockquote>
167 * will be {@code true}, but these are not absolute requirements.
168 * While it is typically the case that:
169 * <blockquote>
170 * <pre>
171 * x.clone().equals(x)</pre></blockquote>
172 * will be {@code true}, this is not an absolute requirement.
173 * <p>
174 * By convention, the returned object should be obtained by calling
175 * {@code super.clone}. If a class and all of its superclasses (except
176 * {@code Object}) obey this convention, it will be the case that
177 * {@code x.clone().getClass() == x.getClass()}.
178 * <p>
179 * By convention, the object returned by this method should be independent
180 * of this object (which is being cloned). To achieve this independence,
181 * it may be necessary to modify one or more fields of the object returned
182 * by {@code super.clone} before returning it. Typically, this means
183 * copying any mutable objects that comprise the internal "deep structure"
184 * of the object being cloned and replacing the references to these
185 * objects with references to the copies. If a class contains only
186 * primitive fields or references to immutable objects, then it is usually
187 * the case that no fields in the object returned by {@code super.clone}
188 * need to be modified.
189 * <p>
190 * The method {@code clone} for class {@code Object} performs a
191 * specific cloning operation. First, if the class of this object does
192 * not implement the interface {@code Cloneable}, then a
193 * {@code CloneNotSupportedException} is thrown. Note that all arrays
194 * are considered to implement the interface {@code Cloneable} and that
195 * the return type of the {@code clone} method of an array type {@code T[]}
196 * is {@code T[]} where T is any reference or primitive type.
197 * Otherwise, this method creates a new instance of the class of this
198 * object and initializes all its fields with exactly the contents of
199 * the corresponding fields of this object, as if by assignment; the
200 * contents of the fields are not themselves cloned. Thus, this method
201 * performs a "shallow copy" of this object, not a "deep copy" operation.
202 * <p>
203 * The class {@code Object} does not itself implement the interface
204 * {@code Cloneable}, so calling the {@code clone} method on an object
205 * whose class is {@code Object} will result in throwing an
206 * exception at run time.
207 *
208 * @return a clone of this instance.
209 * @throws CloneNotSupportedException if the object's class does not
210 * support the {@code Cloneable} interface. Subclasses
211 * that override the {@code clone} method can also
212 * throw this exception to indicate that an instance cannot
213 * be cloned.
214 * @see java.lang.Cloneable
215 */
216 @HotSpotIntrinsicCandidate
217 protected native Object clone() throws CloneNotSupportedException;
218
219 /**
220 * Returns a string representation of the object. In general, the
221 * {@code toString} method returns a string that
222 * "textually represents" this object. The result should
223 * be a concise but informative representation that is easy for a
224 * person to read.
225 * It is recommended that all subclasses override this method.
226 * <p>
227 * The {@code toString} method for class {@code Object}
228 * returns a string consisting of the name of the class of which the
229 * object is an instance, the at-sign character `{@code @}', and
230 * the unsigned hexadecimal representation of the hash code of the
231 * object. In other words, this method returns a string equal to the
232 * value of:
233 * <blockquote>
234 * <pre>
235 * getClass().getName() + '@' + Integer.toHexString(hashCode())
236 * </pre></blockquote>
237 *
238 * @return a string representation of the object.
239 */
240 public String toString() {
241 return getClass().getName() + "@" + Integer.toHexString(hashCode());
242 }
243
244 /**
245 * Wakes up a single thread that is waiting on this object's
246 * monitor. If any threads are waiting on this object, one of them
247 * is chosen to be awakened. The choice is arbitrary and occurs at
248 * the discretion of the implementation. A thread waits on an object's
249 * monitor by calling one of the {@code wait} methods.
250 * <p>
251 * The awakened thread will not be able to proceed until the current
252 * thread relinquishes the lock on this object. The awakened thread will
253 * compete in the usual manner with any other threads that might be
254 * actively competing to synchronize on this object; for example, the
255 * awakened thread enjoys no reliable privilege or disadvantage in being
256 * the next thread to lock this object.
257 * <p>
258 * This method should only be called by a thread that is the owner
259 * of this object's monitor. A thread becomes the owner of the
260 * object's monitor in one of three ways:
261 * <ul>
262 * <li>By executing a synchronized instance method of that object.
263 * <li>By executing the body of a {@code synchronized} statement
264 * that synchronizes on the object.
265 * <li>For objects of type {@code Class,} by executing a
266 * synchronized static method of that class.
267 * </ul>
268 * <p>
269 * Only one thread at a time can own an object's monitor.
270 *
271 * @throws IllegalMonitorStateException if the current thread is not
272 * the owner of this object's monitor.
273 * @see java.lang.Object#notifyAll()
274 * @see java.lang.Object#wait()
275 */
276 @HotSpotIntrinsicCandidate
277 public final native void notify();
278
279 /**
280 * Wakes up all threads that are waiting on this object's monitor. A
281 * thread waits on an object's monitor by calling one of the
282 * {@code wait} methods.
283 * <p>
284 * The awakened threads will not be able to proceed until the current
285 * thread relinquishes the lock on this object. The awakened threads
286 * will compete in the usual manner with any other threads that might
287 * be actively competing to synchronize on this object; for example,
288 * the awakened threads enjoy no reliable privilege or disadvantage in
289 * being the next thread to lock this object.
290 * <p>
291 * This method should only be called by a thread that is the owner
292 * of this object's monitor. See the {@code notify} method for a
293 * description of the ways in which a thread can become the owner of
294 * a monitor.
295 *
296 * @throws IllegalMonitorStateException if the current thread is not
297 * the owner of this object's monitor.
298 * @see java.lang.Object#notify()
299 * @see java.lang.Object#wait()
300 */
301 @HotSpotIntrinsicCandidate
302 public final native void notifyAll();
303
304 /**
305 * Causes the current thread to wait until it is awakened, typically
306 * by being <em>notified</em> or <em>interrupted</em>.
307 * <p>
308 * In all respects, this method behaves as if {@code wait(0L, 0)}
309 * had been called. See the specification of the {@link #wait(long, int)} method
310 * for details.
311 *
312 * @throws IllegalMonitorStateException if the current thread is not
313 * the owner of the object's monitor
314 * @throws InterruptedException if any thread interrupted the current thread before or
315 * while the current thread was waiting. The <em>interrupted status</em> of the
316 * current thread is cleared when this exception is thrown.
317 * @see #notify()
318 * @see #notifyAll()
319 * @see #wait(long)
320 * @see #wait(long, int)
321 */
322 public final void wait() throws InterruptedException {
323 wait(0L);
324 }
325
326 /**
327 * Causes the current thread to wait until it is awakened, typically
328 * by being <em>notified</em> or <em>interrupted</em>, or until a
329 * certain amount of real time has elapsed.
330 * <p>
331 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
332 * had been called. See the specification of the {@link #wait(long, int)} method
333 * for details.
334 *
335 * @param timeoutMillis the maximum time to wait, in milliseconds
336 * @throws IllegalArgumentException if {@code timeoutMillis} is negative
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()
345 * @see #wait(long, int)
346 */
347 public final native void wait(long timeoutMillis) throws InterruptedException;
348
349 /**
350 * Causes the current thread to wait until it is awakened, typically
351 * by being <em>notified</em> or <em>interrupted</em>, or until a
352 * certain amount of real time has elapsed.
353 * <p>
354 * The current thread must own this object's monitor lock. See the
355 * {@link #notify notify} method for a description of the ways in which
356 * a thread can become the owner of a monitor lock.
357 * <p>
358 * This method causes the current thread (referred to here as <var>T</var>) to
359 * place itself in the wait set for this object and then to relinquish any
360 * and all synchronization claims on this object. Note that only the locks
361 * on this object are relinquished; any other objects on which the current
362 * thread may be synchronized remain locked while the thread waits.
363 * <p>
364 * Thread <var>T</var> then becomes disabled for thread scheduling purposes
365 * and lies dormant until one of the following occurs:
366 * <ul>
367 * <li>Some other thread invokes the {@code notify} method for this
368 * object and thread <var>T</var> happens to be arbitrarily chosen as
369 * the thread to be awakened.
370 * <li>Some other thread invokes the {@code notifyAll} method for this
371 * object.
372 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
373 * thread <var>T</var>.
374 * <li>The specified amount of real time has elapsed, more or less.
375 * The amount of real time, in nanoseconds, is given by the expression
376 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
377 * are both zero, then real time is not taken into consideration and the
378 * thread waits until awakened by one of the other causes.
379 * <li>Thread <var>T</var> is awakened spuriously. (See below.)
380 * </ul>
381 * <p>
382 * The thread <var>T</var> is then removed from the wait set for this
383 * object and re-enabled for thread scheduling. It competes in the
384 * usual manner with other threads for the right to synchronize on the
385 * object; once it has regained control of the object, all its
386 * synchronization claims on the object are restored to the status quo
387 * ante - that is, to the situation as of the time that the {@code wait}
388 * method was invoked. Thread <var>T</var> then returns from the
389 * invocation of the {@code wait} method. Thus, on return from the
390 * {@code wait} method, the synchronization state of the object and of
391 * thread {@code T} is exactly as it was when the {@code wait} method
392 * was invoked.
393 * <p>
394 * A thread can wake up without being notified, interrupted, or timing out, a
395 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice,
396 * applications must guard against it by testing for the condition that should
397 * have caused the thread to be awakened, and continuing to wait if the condition
398 * is not satisfied. See the example below.
399 * <p>
400 * For more information on this topic, see section 14.2,
401 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency
402 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua
403 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley,
404 * 2008).
405 * <p>
406 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
407 * by any thread before or while it is waiting, then an {@code InterruptedException}
408 * is thrown. The <em>interrupted status</em> of the current thread is cleared when
409 * this exception is thrown. This exception is not thrown until the lock status of
410 * this object has been restored as described above.
411 *
412 * @apiNote
413 * The recommended approach to waiting is to check the condition being awaited in
414 * a {@code while} loop around the call to {@code wait}, as shown in the example
415 * below. Among other things, this approach avoids problems that can be caused
416 * by spurious wakeups.
417 *
418 * <pre>{@code
419 * synchronized (obj) {
420 * while (<condition does not hold> and <timeout not exceeded>) {
421 * long timeoutMillis = ... ; // recompute timeout values
422 * int nanos = ... ;
423 * obj.wait(timeoutMillis, nanos);
424 * }
425 * ... // Perform action appropriate to condition or timeout
426 * }
427 * }</pre>
428 *
429 * @param timeoutMillis the maximum time to wait, in milliseconds
430 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive
431 * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
432 * or if the value of {@code nanos} is out of range
433 * @throws IllegalMonitorStateException if the current thread is not
434 * the owner of the object's monitor
435 * @throws InterruptedException if any thread interrupted the current thread before or
436 * while the current thread was waiting. The <em>interrupted status</em> of the
437 * current thread is cleared when this exception is thrown.
438 * @see #notify()
439 * @see #notifyAll()
440 * @see #wait()
441 * @see #wait(long)
442 */
443 public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
444 if (timeoutMillis < 0) {
445 throw new IllegalArgumentException("timeoutMillis value is negative");
446 }
447
448 if (nanos < 0 || nanos > 999999) {
449 throw new IllegalArgumentException(
450 "nanosecond timeout value out of range");
451 }
452
453 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
454 timeoutMillis++;
455 }
456
457 wait(timeoutMillis);
458 }
459
460 /**
461 * Called by the garbage collector on an object when garbage collection
462 * determines that there are no more references to the object.
463 * A subclass overrides the {@code finalize} method to dispose of
464 * system resources or to perform other cleanup.
465 * <p>
466 * The general contract of {@code finalize} is that it is invoked
467 * if and when the Java™ virtual
468 * machine has determined that there is no longer any
469 * means by which this object can be accessed by any thread that has
470 * not yet died, except as a result of an action taken by the
471 * finalization of some other object or class which is ready to be
472 * finalized. The {@code finalize} method may take any action, including
473 * making this object available again to other threads; the usual purpose
474 * of {@code finalize}, however, is to perform cleanup actions before
475 * the object is irrevocably discarded. For example, the finalize method
476 * for an object that represents an input/output connection might perform
477 * explicit I/O transactions to break the connection before the object is
478 * permanently discarded.
479 * <p>
480 * The {@code finalize} method of class {@code Object} performs no
481 * special action; it simply returns normally. Subclasses of
482 * {@code Object} may override this definition.
483 * <p>
484 * The Java programming language does not guarantee which thread will
485 * invoke the {@code finalize} method for any given object. It is
486 * guaranteed, however, that the thread that invokes finalize will not
487 * be holding any user-visible synchronization locks when finalize is
488 * invoked. If an uncaught exception is thrown by the finalize method,
489 * the exception is ignored and finalization of that object terminates.
490 * <p>
491 * After the {@code finalize} method has been invoked for an object, no
492 * further action is taken until the Java virtual machine has again
493 * determined that there is no longer any means by which this object can
494 * be accessed by any thread that has not yet died, including possible
495 * actions by other objects or classes which are ready to be finalized,
496 * at which point the object may be discarded.
497 * <p>
498 * The {@code finalize} method is never invoked more than once by a Java
499 * virtual machine for any given object.
500 * <p>
501 * Any exception thrown by the {@code finalize} method causes
502 * the finalization of this object to be halted, but is otherwise
503 * ignored.
504 *
505 * @apiNote
506 * Classes that embed non-heap resources have many options
507 * for cleanup of those resources. The class must ensure that the
508 * lifetime of each instance is longer than that of any resource it embeds.
509 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
510 * objects remain reachable while resources embedded in the object are in use.
511 * <p>
512 * A subclass should avoid overriding the {@code finalize} method
513 * unless the subclass embeds non-heap resources that must be cleaned up
514 * before the instance is collected.
515 * Finalizer invocations are not automatically chained, unlike constructors.
516 * If a subclass overrides {@code finalize} it must invoke the superclass
517 * finalizer explicitly.
518 * To guard against exceptions prematurely terminating the finalize chain,
519 * the subclass should use a {@code try-finally} block to ensure
520 * {@code super.finalize()} is always invoked. For example,
521 * <pre>{@code @Override
522 * protected void finalize() throws Throwable {
523 * try {
524 * ... // cleanup subclass state
525 * } finally {
526 * super.finalize();
527 * }
528 * }
529 * }</pre>
530 *
531 * @deprecated The finalization mechanism is inherently problematic.
532 * Finalization can lead to performance issues, deadlocks, and hangs.
533 * Errors in finalizers can lead to resource leaks; there is no way to cancel
534 * finalization if it is no longer necessary; and no ordering is specified
535 * among calls to {@code finalize} methods of different objects.
536 * Furthermore, there are no guarantees regarding the timing of finalization.
537 * The {@code finalize} method might be called on a finalizable object
538 * only after an indefinite delay, if at all.
539 *
540 * Classes whose instances hold non-heap resources should provide a method
541 * to enable explicit release of those resources, and they should also
542 * implement {@link AutoCloseable} if appropriate.
543 * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference}
544 * provide more flexible and efficient ways to release resources when an object
545 * becomes unreachable.
546 *
547 * @throws Throwable the {@code Exception} raised by this method
548 * @see java.lang.ref.WeakReference
549 * @see java.lang.ref.PhantomReference
550 * @jls 12.6 Finalization of Class Instances
551 */
552 @Deprecated(since="9")
553 protected void finalize() throws Throwable { }
554 }