1 /*
2 * Copyright (c) 1994, 2017, 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 private static native void registerNatives();
42 static {
43 registerNatives();
44 }
45
46 /**
47 * Constructs a new object.
48 */
49 @HotSpotIntrinsicCandidate
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 @HotSpotIntrinsicCandidate
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 {@code equals(Object)}
88 * method, then calling the {@code hashCode} method on each of
89 * the two objects must produce the same integer result.
90 * <li>It is <em>not</em> required that if two objects are unequal
91 * according to the {@link java.lang.Object#equals(java.lang.Object)}
92 * method, then calling the {@code hashCode} method on each of the
93 * two objects must produce distinct integer results. However, the
94 * programmer should be aware that producing distinct integer results
95 * for unequal objects may improve the performance of hash tables.
96 * </ul>
97 * <p>
98 * As much as is reasonably practical, the hashCode method defined
99 * by class {@code Object} does return distinct integers for
100 * distinct objects. (The hashCode may or may not be implemented
101 * as some function of an object's memory address at some point
102 * in time.)
103 *
104 * @return a hash code value for this object.
105 * @see java.lang.Object#equals(java.lang.Object)
106 * @see java.lang.System#identityHashCode
107 */
108 @HotSpotIntrinsicCandidate
109 public native int hashCode();
110
111 /**
112 * Indicates whether some other object is "equal to" this one.
113 * <p>
114 * The {@code equals} method implements an equivalence relation
115 * on non-null object references:
116 * <ul>
117 * <li>It is <i>reflexive</i>: for any non-null reference value
118 * {@code x}, {@code x.equals(x)} should return
119 * {@code true}.
120 * <li>It is <i>symmetric</i>: for any non-null reference values
121 * {@code x} and {@code y}, {@code x.equals(y)}
122 * should return {@code true} if and only if
123 * {@code y.equals(x)} returns {@code true}.
124 * <li>It is <i>transitive</i>: for any non-null reference values
125 * {@code x}, {@code y}, and {@code z}, if
126 * {@code x.equals(y)} returns {@code true} and
127 * {@code y.equals(z)} returns {@code true}, then
128 * {@code x.equals(z)} should return {@code true}.
129 * <li>It is <i>consistent</i>: for any non-null reference values
130 * {@code x} and {@code y}, multiple invocations of
131 * {@code x.equals(y)} consistently return {@code true}
132 * or consistently return {@code false}, provided no
133 * information used in {@code equals} comparisons on the
134 * objects is modified.
135 * <li>For any non-null reference value {@code x},
136 * {@code x.equals(null)} should return {@code false}.
137 * </ul>
138 * <p>
139 * The {@code equals} method for class {@code Object} implements
140 * the most discriminating possible equivalence relation on objects;
141 * that is, for any non-null reference values {@code x} and
142 * {@code y}, this method returns {@code true} if and only
143 * if {@code x} and {@code y} refer to the same object
144 * ({@code x == y} has the value {@code true}).
145 * <p>
146 * Note that it is generally necessary to override the {@code hashCode}
147 * method whenever this method is overridden, so as to maintain the
148 * general contract for the {@code hashCode} method, which states
149 * that equal objects must have equal hash codes.
150 *
151 * @param obj the reference object with which to compare.
152 * @return {@code true} if this object is the same as the obj
153 * argument; {@code false} otherwise.
154 * @see #hashCode()
155 * @see java.util.HashMap
156 */
157 public boolean equals(Object obj) {
158 return (this == obj);
159 }
160
161 /**
162 * Creates and returns a copy of this object. The precise meaning
163 * of "copy" may depend on the class of the object. The general
164 * intent is that, for any object {@code x}, the expression:
165 * <blockquote>
166 * <pre>
167 * x.clone() != x</pre></blockquote>
168 * will be true, and that the expression:
169 * <blockquote>
170 * <pre>
171 * x.clone().getClass() == x.getClass()</pre></blockquote>
172 * will be {@code true}, but these are not absolute requirements.
173 * While it is typically the case that:
174 * <blockquote>
175 * <pre>
176 * x.clone().equals(x)</pre></blockquote>
177 * will be {@code true}, this is not an absolute requirement.
178 * <p>
179 * By convention, the returned object should be obtained by calling
180 * {@code super.clone}. If a class and all of its superclasses (except
181 * {@code Object}) obey this convention, it will be the case that
182 * {@code x.clone().getClass() == x.getClass()}.
183 * <p>
184 * By convention, the object returned by this method should be independent
185 * of this object (which is being cloned). To achieve this independence,
186 * it may be necessary to modify one or more fields of the object returned
187 * by {@code super.clone} before returning it. Typically, this means
188 * copying any mutable objects that comprise the internal "deep structure"
189 * of the object being cloned and replacing the references to these
190 * objects with references to the copies. If a class contains only
191 * primitive fields or references to immutable objects, then it is usually
192 * the case that no fields in the object returned by {@code super.clone}
193 * need to be modified.
194 * <p>
195 * The method {@code clone} for class {@code Object} performs a
196 * specific cloning operation. First, if the class of this object does
197 * not implement the interface {@code Cloneable}, then a
198 * {@code CloneNotSupportedException} is thrown. Note that all arrays
199 * are considered to implement the interface {@code Cloneable} and that
200 * the return type of the {@code clone} method of an array type {@code T[]}
201 * is {@code T[]} where T is any reference or primitive type.
202 * Otherwise, this method creates a new instance of the class of this
203 * object and initializes all its fields with exactly the contents of
204 * the corresponding fields of this object, as if by assignment; the
205 * contents of the fields are not themselves cloned. Thus, this method
206 * performs a "shallow copy" of this object, not a "deep copy" operation.
207 * <p>
208 * The class {@code Object} does not itself implement the interface
209 * {@code Cloneable}, so calling the {@code clone} method on an object
210 * whose class is {@code Object} will result in throwing an
211 * exception at run time.
212 *
213 * @return a clone of this instance.
214 * @throws CloneNotSupportedException if the object's class does not
215 * support the {@code Cloneable} interface. Subclasses
216 * that override the {@code clone} method can also
217 * throw this exception to indicate that an instance cannot
218 * be cloned.
219 * @see java.lang.Cloneable
220 */
221 @HotSpotIntrinsicCandidate
222 protected native Object clone() throws CloneNotSupportedException;
223
224 /**
225 * Returns a string representation of the object. In general, the
226 * {@code toString} method returns a string that
227 * "textually represents" this object. The result should
228 * be a concise but informative representation that is easy for a
229 * person to read.
230 * It is recommended that all subclasses override this method.
231 * <p>
232 * The {@code toString} method for class {@code Object}
233 * returns a string consisting of the name of the class of which the
234 * object is an instance, the at-sign character `{@code @}', and
235 * the unsigned hexadecimal representation of the hash code of the
236 * object. In other words, this method returns a string equal to the
237 * value of:
238 * <blockquote>
239 * <pre>
240 * getClass().getName() + '@' + Integer.toHexString(hashCode())
241 * </pre></blockquote>
242 *
243 * @return a string representation of the object.
244 */
245 public String toString() {
246 return getClass().getName() + "@" + Integer.toHexString(hashCode());
247 }
248
249 /**
250 * Wakes up a single thread that is waiting on this object's
251 * monitor. If any threads are waiting on this object, one of them
252 * is chosen to be awakened. The choice is arbitrary and occurs at
253 * the discretion of the implementation. A thread waits on an object's
254 * monitor by calling one of the {@code wait} methods.
255 * <p>
256 * The awakened thread will not be able to proceed until the current
257 * thread relinquishes the lock on this object. The awakened thread will
258 * compete in the usual manner with any other threads that might be
259 * actively competing to synchronize on this object; for example, the
260 * awakened thread enjoys no reliable privilege or disadvantage in being
261 * the next thread to lock this object.
262 * <p>
263 * This method should only be called by a thread that is the owner
264 * of this object's monitor. A thread becomes the owner of the
265 * object's monitor in one of three ways:
266 * <ul>
267 * <li>By executing a synchronized instance method of that object.
268 * <li>By executing the body of a {@code synchronized} statement
269 * that synchronizes on the object.
270 * <li>For objects of type {@code Class,} by executing a
271 * synchronized static method of that class.
272 * </ul>
273 * <p>
274 * Only one thread at a time can own an object's monitor.
275 *
276 * @throws IllegalMonitorStateException if the current thread is not
277 * the owner of this object's monitor.
278 * @see java.lang.Object#notifyAll()
279 * @see java.lang.Object#wait()
280 */
281 @HotSpotIntrinsicCandidate
282 public final native void notify();
283
284 /**
285 * Wakes up all threads that are waiting on this object's monitor. A
286 * thread waits on an object's monitor by calling one of the
287 * {@code wait} methods.
288 * <p>
289 * The awakened threads will not be able to proceed until the current
290 * thread relinquishes the lock on this object. The awakened threads
291 * will compete in the usual manner with any other threads that might
292 * be actively competing to synchronize on this object; for example,
293 * the awakened threads enjoy no reliable privilege or disadvantage in
294 * being the next thread to lock this object.
295 * <p>
296 * This method should only be called by a thread that is the owner
297 * of this object's monitor. See the {@code notify} method for a
298 * description of the ways in which a thread can become the owner of
299 * a monitor.
300 *
301 * @throws IllegalMonitorStateException if the current thread is not
302 * the owner of this object's monitor.
303 * @see java.lang.Object#notify()
304 * @see java.lang.Object#wait()
305 */
306 @HotSpotIntrinsicCandidate
307 public final native void notifyAll();
308
309 /**
310 * Causes the current thread to wait until it is awakened, typically
311 * by being <em>notified</em> or <em>interrupted</em>.
312 * <p>
313 * In all respects, this method behaves as if {@code wait(0L, 0)}
314 * had been called. See the specification of the {@link #wait(long, int)} method
315 * for details.
316 *
317 * @throws IllegalMonitorStateException if the current thread is not
318 * the owner of the object's monitor
319 * @throws InterruptedException if any thread interrupted the current thread before or
320 * while the current thread was waiting. The <em>interrupted status</em> of the
321 * current thread is cleared when this exception is thrown.
322 * @see #notify()
323 * @see #notifyAll()
324 * @see #wait(long)
325 * @see #wait(long, int)
326 */
327 public final void wait() throws InterruptedException {
328 wait(0L);
329 }
330
331 /**
332 * Causes the current thread to wait until it is awakened, typically
333 * by being <em>notified</em> or <em>interrupted</em>, or until a
334 * certain amount of real time has elapsed.
335 * <p>
336 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
337 * had been called. See the specification of the {@link #wait(long, int)} method
338 * for details.
339 *
340 * @param timeoutMillis the maximum time to wait, in milliseconds
341 * @throws IllegalArgumentException if {@code timeoutMillis} is negative
342 * @throws IllegalMonitorStateException if the current thread is not
343 * the owner of the object's monitor
344 * @throws InterruptedException if any thread interrupted the current thread before or
345 * while the current thread was waiting. The <em>interrupted status</em> of the
346 * current thread is cleared when this exception is thrown.
347 * @see #notify()
348 * @see #notifyAll()
349 * @see #wait()
350 * @see #wait(long, int)
351 */
352 public final native void wait(long timeoutMillis) throws InterruptedException;
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 * The current thread must own this object's monitor lock. See the
360 * {@link #notify notify} method for a description of the ways in which
361 * a thread can become the owner of a monitor lock.
362 * <p>
363 * This method causes the current thread (referred to here as <var>T</var>) to
364 * place itself in the wait set for this object and then to relinquish any
365 * and all synchronization claims on this object. Note that only the locks
366 * on this object are relinquished; any other objects on which the current
367 * thread may be synchronized remain locked while the thread waits.
368 * <p>
369 * Thread <var>T</var> then becomes disabled for thread scheduling purposes
370 * and lies dormant until one of the following occurs:
371 * <ul>
372 * <li>Some other thread invokes the {@code notify} method for this
373 * object and thread <var>T</var> happens to be arbitrarily chosen as
374 * the thread to be awakened.
375 * <li>Some other thread invokes the {@code notifyAll} method for this
376 * object.
377 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
378 * thread <var>T</var>.
379 * <li>The specified amount of real time has elapsed, more or less.
380 * The amount of real time, in nanoseconds, is given by the expression
381 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
382 * are both zero, then real time is not taken into consideration and the
383 * thread waits until awakened by one of the other causes.
384 * <li>Thread <var>T</var> is awakened spuriously. (See below.)
385 * </ul>
386 * <p>
387 * The thread <var>T</var> is then removed from the wait set for this
388 * object and re-enabled for thread scheduling. It competes in the
389 * usual manner with other threads for the right to synchronize on the
390 * object; once it has regained control of the object, all its
391 * synchronization claims on the object are restored to the status quo
392 * ante - that is, to the situation as of the time that the {@code wait}
393 * method was invoked. Thread <var>T</var> then returns from the
394 * invocation of the {@code wait} method. Thus, on return from the
395 * {@code wait} method, the synchronization state of the object and of
396 * thread {@code T} is exactly as it was when the {@code wait} method
397 * was invoked.
398 * <p>
399 * A thread can wake up without being notified, interrupted, or timing out, a
400 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice,
401 * applications must guard against it by testing for the condition that should
402 * have caused the thread to be awakened, and continuing to wait if the condition
403 * is not satisfied. See the example below.
404 * <p>
405 * For more information on this topic, see section 14.2,
406 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency
407 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua
408 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley,
409 * 2008).
410 * <p>
411 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
412 * by any thread before or while it is waiting, then an {@code InterruptedException}
413 * is thrown. The <em>interrupted status</em> of the current thread is cleared when
414 * this exception is thrown. This exception is not thrown until the lock status of
415 * this object has been restored as described above.
416 *
417 * @apiNote
418 * The recommended approach to waiting is to check the condition being awaited in
419 * a {@code while} loop around the call to {@code wait}, as shown in the example
420 * below. Among other things, this approach avoids problems that can be caused
421 * by spurious wakeups.
422 *
423 * <pre>{@code
424 * synchronized (obj) {
425 * while (<condition does not hold> and <timeout not exceeded>) {
426 * long timeoutMillis = ... ; // recompute timeout values
427 * int nanos = ... ;
428 * obj.wait(timeoutMillis, nanos);
429 * }
430 * ... // Perform action appropriate to condition or timeout
431 * }
432 * }</pre>
433 *
434 * @param timeoutMillis the maximum time to wait, in milliseconds
435 * @param nanos additional time, in nanoseconds, in the range range 0-999999 inclusive
436 * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
437 * or if the value of {@code nanos} is out of range
438 * @throws IllegalMonitorStateException if the current thread is not
439 * the owner of the object's monitor
440 * @throws InterruptedException if any thread interrupted the current thread before or
441 * while the current thread was waiting. The <em>interrupted status</em> of the
442 * current thread is cleared when this exception is thrown.
443 * @see #notify()
444 * @see #notifyAll()
445 * @see #wait()
446 * @see #wait(long)
447 */
448 public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
449 if (timeoutMillis < 0) {
450 throw new IllegalArgumentException("timeoutMillis value is negative");
451 }
452
453 if (nanos < 0 || nanos > 999999) {
454 throw new IllegalArgumentException(
455 "nanosecond timeout value out of range");
456 }
457
458 if (nanos > 0) {
459 timeoutMillis++;
460 }
461
462 wait(timeoutMillis);
463 }
464
465 /**
466 * Called by the garbage collector on an object when garbage collection
467 * determines that there are no more references to the object.
468 * A subclass overrides the {@code finalize} method to dispose of
469 * system resources or to perform other cleanup.
470 * <p>
471 * The general contract of {@code finalize} is that it is invoked
472 * if and when the Java™ virtual
473 * machine has determined that there is no longer any
474 * means by which this object can be accessed by any thread that has
475 * not yet died, except as a result of an action taken by the
476 * finalization of some other object or class which is ready to be
477 * finalized. The {@code finalize} method may take any action, including
478 * making this object available again to other threads; the usual purpose
479 * of {@code finalize}, however, is to perform cleanup actions before
480 * the object is irrevocably discarded. For example, the finalize method
481 * for an object that represents an input/output connection might perform
482 * explicit I/O transactions to break the connection before the object is
483 * permanently discarded.
484 * <p>
485 * The {@code finalize} method of class {@code Object} performs no
486 * special action; it simply returns normally. Subclasses of
487 * {@code Object} may override this definition.
488 * <p>
489 * The Java programming language does not guarantee which thread will
490 * invoke the {@code finalize} method for any given object. It is
491 * guaranteed, however, that the thread that invokes finalize will not
492 * be holding any user-visible synchronization locks when finalize is
493 * invoked. If an uncaught exception is thrown by the finalize method,
494 * the exception is ignored and finalization of that object terminates.
495 * <p>
496 * After the {@code finalize} method has been invoked for an object, no
497 * further action is taken until the Java virtual machine has again
498 * determined that there is no longer any means by which this object can
499 * be accessed by any thread that has not yet died, including possible
500 * actions by other objects or classes which are ready to be finalized,
501 * at which point the object may be discarded.
502 * <p>
503 * The {@code finalize} method is never invoked more than once by a Java
504 * virtual machine for any given object.
505 * <p>
506 * Any exception thrown by the {@code finalize} method causes
507 * the finalization of this object to be halted, but is otherwise
508 * ignored.
509 *
510 * @apiNote
511 * Classes that embed non-heap resources have many options
512 * for cleanup of those resources. The class must ensure that the
513 * lifetime of each instance is longer than that of any resource it embeds.
514 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
515 * objects remain reachable while resources embedded in the object are in use.
516 * <p>
517 * A subclass should avoid overriding the {@code finalize} method
518 * unless the subclass embeds non-heap resources that must be cleaned up
519 * before the instance is collected.
520 * Finalizer invocations are not automatically chained, unlike constructors.
521 * If a subclass overrides {@code finalize} it must invoke the superclass
522 * finalizer explicitly.
523 * To guard against exceptions prematurely terminating the finalize chain,
524 * the subclass should use a {@code try-finally} block to ensure
525 * {@code super.finalize()} is always invoked. For example,
526 * <pre>{@code @Override
527 * protected void finalize() throws Throwable {
528 * try {
529 * ... // cleanup subclass state
530 * } finally {
531 * super.finalize();
532 * }
533 * }
534 * }</pre>
535 *
536 * @deprecated The finalization mechanism is inherently problematic.
537 * Finalization can lead to performance issues, deadlocks, and hangs.
538 * Errors in finalizers can lead to resource leaks; there is no way to cancel
539 * finalization if it is no longer necessary; and no ordering is specified
540 * among calls to {@code finalize} methods of different objects.
541 * Furthermore, there are no guarantees regarding the timing of finalization.
542 * The {@code finalize} method might be called on a finalizable object
543 * only after an indefinite delay, if at all.
544 *
545 * Classes whose instances hold non-heap resources should provide a method
546 * to enable explicit release of those resources, and they should also
547 * implement {@link AutoCloseable} if appropriate.
548 * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference}
549 * provide more flexible and efficient ways to release resources when an object
550 * becomes unreachable.
551 *
552 * @throws Throwable the {@code Exception} raised by this method
553 * @see java.lang.ref.WeakReference
554 * @see java.lang.ref.PhantomReference
555 * @jls 12.6 Finalization of Class Instances
556 */
557 @Deprecated(since="9")
558 protected void finalize() throws Throwable { }
559 }
560