1 /*
2 * Copyright (c) 1994, 2019, 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 *
98 * @implSpec
99 * As far as is reasonably practical, the {@code hashCode} method defined
100 * by class {@code Object} returns distinct integers for distinct objects.
101 *
102 * @return a hash code value for this object.
103 * @see java.lang.Object#equals(java.lang.Object)
104 * @see java.lang.System#identityHashCode
105 */
106 @HotSpotIntrinsicCandidate
107 public native int hashCode();
108
109 /**
110 * Indicates whether some other object is "equal to" this one.
111 * <p>
112 * The {@code equals} method implements an equivalence relation
113 * on non-null object references:
114 * <ul>
115 * <li>It is <i>reflexive</i>: for any non-null reference value
116 * {@code x}, {@code x.equals(x)} should return
117 * {@code true}.
118 * <li>It is <i>symmetric</i>: for any non-null reference values
119 * {@code x} and {@code y}, {@code x.equals(y)}
120 * should return {@code true} if and only if
121 * {@code y.equals(x)} returns {@code true}.
122 * <li>It is <i>transitive</i>: for any non-null reference values
123 * {@code x}, {@code y}, and {@code z}, if
124 * {@code x.equals(y)} returns {@code true} and
125 * {@code y.equals(z)} returns {@code true}, then
126 * {@code x.equals(z)} should return {@code true}.
127 * <li>It is <i>consistent</i>: for any non-null reference values
128 * {@code x} and {@code y}, multiple invocations of
129 * {@code x.equals(y)} consistently return {@code true}
130 * or consistently return {@code false}, provided no
131 * information used in {@code equals} comparisons on the
132 * objects is modified.
133 * <li>For any non-null reference value {@code x},
134 * {@code x.equals(null)} should return {@code false}.
135 * </ul>
136 * <p>
137 * The {@code equals} method for class {@code Object} implements
138 * the most discriminating possible equivalence relation on objects;
139 * that is, for any non-null reference values {@code x} and
140 * {@code y}, this method returns {@code true} if and only
141 * if {@code x} and {@code y} refer to the same object
142 * ({@code x == y} has the value {@code true}).
143 * <p>
144 * Note that it is generally necessary to override the {@code hashCode}
145 * method whenever this method is overridden, so as to maintain the
146 * general contract for the {@code hashCode} method, which states
147 * that equal objects must have equal hash codes.
148 *
149 * @param obj the reference object with which to compare.
150 * @return {@code true} if this object is the same as the obj
151 * argument; {@code false} otherwise.
152 * @see #hashCode()
153 * @see java.util.HashMap
154 */
155 public boolean equals(Object obj) {
156 return (this == obj);
157 }
158
159 /**
160 * Creates and returns a copy of this object. The precise meaning
161 * of "copy" may depend on the class of the object. The general
162 * intent is that, for any object {@code x}, the expression:
163 * <blockquote>
164 * <pre>
165 * x.clone() != x</pre></blockquote>
166 * will be true, and that the expression:
167 * <blockquote>
168 * <pre>
169 * x.clone().getClass() == x.getClass()</pre></blockquote>
170 * will be {@code true}, but these are not absolute requirements.
171 * While it is typically the case that:
172 * <blockquote>
173 * <pre>
174 * x.clone().equals(x)</pre></blockquote>
175 * will be {@code true}, this is not an absolute requirement.
176 * <p>
177 * By convention, the returned object should be obtained by calling
178 * {@code super.clone}. If a class and all of its superclasses (except
179 * {@code Object}) obey this convention, it will be the case that
180 * {@code x.clone().getClass() == x.getClass()}.
181 * <p>
182 * By convention, the object returned by this method should be independent
183 * of this object (which is being cloned). To achieve this independence,
184 * it may be necessary to modify one or more fields of the object returned
185 * by {@code super.clone} before returning it. Typically, this means
186 * copying any mutable objects that comprise the internal "deep structure"
187 * of the object being cloned and replacing the references to these
188 * objects with references to the copies. If a class contains only
189 * primitive fields or references to immutable objects, then it is usually
190 * the case that no fields in the object returned by {@code super.clone}
191 * need to be modified.
192 * <p>
193 * The method {@code clone} for class {@code Object} performs a
194 * specific cloning operation. First, if the class of this object does
195 * not implement the interface {@code Cloneable}, then a
196 * {@code CloneNotSupportedException} is thrown. Note that all arrays
197 * are considered to implement the interface {@code Cloneable} and that
198 * the return type of the {@code clone} method of an array type {@code T[]}
199 * is {@code T[]} where T is any reference or primitive type.
200 * Otherwise, this method creates a new instance of the class of this
201 * object and initializes all its fields with exactly the contents of
202 * the corresponding fields of this object, as if by assignment; the
203 * contents of the fields are not themselves cloned. Thus, this method
204 * performs a "shallow copy" of this object, not a "deep copy" operation.
205 * <p>
206 * The class {@code Object} does not itself implement the interface
207 * {@code Cloneable}, so calling the {@code clone} method on an object
208 * whose class is {@code Object} will result in throwing an
209 * exception at run time.
210 *
211 * @return a clone of this instance.
212 * @throws CloneNotSupportedException if the object's class does not
213 * support the {@code Cloneable} interface. Subclasses
214 * that override the {@code clone} method can also
215 * throw this exception to indicate that an instance cannot
216 * be cloned.
217 * @see java.lang.Cloneable
218 */
219 @HotSpotIntrinsicCandidate
220 protected native Object clone() throws CloneNotSupportedException;
221
222 /**
223 * Returns a string representation of the object. In general, the
224 * {@code toString} method returns a string that
225 * "textually represents" this object. The result should
226 * be a concise but informative representation that is easy for a
227 * person to read.
228 * It is recommended that all subclasses override this method.
229 * <p>
230 * The {@code toString} method for class {@code Object}
231 * returns a string consisting of the name of the class of which the
232 * object is an instance, the at-sign character `{@code @}', and
233 * the unsigned hexadecimal representation of the hash code of the
234 * object. In other words, this method returns a string equal to the
235 * value of:
236 * <blockquote>
237 * <pre>
238 * getClass().getName() + '@' + Integer.toHexString(hashCode())
239 * </pre></blockquote>
240 *
241 * @return a string representation of the object.
242 */
243 public String toString() {
244 return getClass().getName() + "@" + Integer.toHexString(hashCode());
245 }
246
247 /**
248 * Wakes up a single thread that is waiting on this object's
249 * monitor. If any threads are waiting on this object, one of them
250 * is chosen to be awakened. The choice is arbitrary and occurs at
251 * the discretion of the implementation. A thread waits on an object's
252 * monitor by calling one of the {@code wait} methods.
253 * <p>
254 * The awakened thread will not be able to proceed until the current
255 * thread relinquishes the lock on this object. The awakened thread will
256 * compete in the usual manner with any other threads that might be
257 * actively competing to synchronize on this object; for example, the
258 * awakened thread enjoys no reliable privilege or disadvantage in being
259 * the next thread to lock this object.
260 * <p>
261 * This method should only be called by a thread that is the owner
262 * of this object's monitor. A thread becomes the owner of the
263 * object's monitor in one of three ways:
264 * <ul>
265 * <li>By executing a synchronized instance method of that object.
266 * <li>By executing the body of a {@code synchronized} statement
267 * that synchronizes on the object.
268 * <li>For objects of type {@code Class,} by executing a
269 * synchronized static method of that class.
270 * </ul>
271 * <p>
272 * Only one thread at a time can own an object's monitor.
273 *
274 * @throws IllegalMonitorStateException if the current thread is not
275 * the owner of this object's monitor.
276 * @see java.lang.Object#notifyAll()
277 * @see java.lang.Object#wait()
278 */
279 @HotSpotIntrinsicCandidate
280 public final native void notify();
281
282 /**
283 * Wakes up all threads that are waiting on this object's monitor. A
284 * thread waits on an object's monitor by calling one of the
285 * {@code wait} methods.
286 * <p>
287 * The awakened threads will not be able to proceed until the current
288 * thread relinquishes the lock on this object. The awakened threads
289 * will compete in the usual manner with any other threads that might
290 * be actively competing to synchronize on this object; for example,
291 * the awakened threads enjoy no reliable privilege or disadvantage in
292 * being the next thread to lock this object.
293 * <p>
294 * This method should only be called by a thread that is the owner
295 * of this object's monitor. See the {@code notify} method for a
296 * description of the ways in which a thread can become the owner of
297 * a monitor.
298 *
299 * @throws IllegalMonitorStateException if the current thread is not
300 * the owner of this object's monitor.
301 * @see java.lang.Object#notify()
302 * @see java.lang.Object#wait()
303 */
304 @HotSpotIntrinsicCandidate
305 public final native void notifyAll();
306
307 /**
308 * Causes the current thread to wait until it is awakened, typically
309 * by being <em>notified</em> or <em>interrupted</em>.
310 * <p>
311 * In all respects, this method behaves as if {@code wait(0L, 0)}
312 * had been called. See the specification of the {@link #wait(long, int)} method
313 * for details.
314 *
315 * @throws IllegalMonitorStateException if the current thread is not
316 * the owner of the object's monitor
317 * @throws InterruptedException if any thread interrupted the current thread before or
318 * while the current thread was waiting. The <em>interrupted status</em> of the
319 * current thread is cleared when this exception is thrown.
320 * @see #notify()
321 * @see #notifyAll()
322 * @see #wait(long)
323 * @see #wait(long, int)
324 */
325 public final void wait() throws InterruptedException {
326 wait(0L);
327 }
328
329 /**
330 * Causes the current thread to wait until it is awakened, typically
331 * by being <em>notified</em> or <em>interrupted</em>, or until a
332 * certain amount of real time has elapsed.
333 * <p>
334 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
335 * had been called. See the specification of the {@link #wait(long, int)} method
336 * for details.
337 *
338 * @param timeoutMillis the maximum time to wait, in milliseconds
339 * @throws IllegalArgumentException if {@code timeoutMillis} is negative
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()
348 * @see #wait(long, int)
349 */
350 public final native void wait(long timeoutMillis) throws InterruptedException;
351
352 /**
353 * Causes the current thread to wait until it is awakened, typically
354 * by being <em>notified</em> or <em>interrupted</em>, or until a
355 * certain amount of real time has elapsed.
356 * <p>
357 * The current thread must own this object's monitor lock. See the
358 * {@link #notify notify} method for a description of the ways in which
359 * a thread can become the owner of a monitor lock.
360 * <p>
361 * This method causes the current thread (referred to here as <var>T</var>) to
362 * place itself in the wait set for this object and then to relinquish any
363 * and all synchronization claims on this object. Note that only the locks
364 * on this object are relinquished; any other objects on which the current
365 * thread may be synchronized remain locked while the thread waits.
366 * <p>
367 * Thread <var>T</var> then becomes disabled for thread scheduling purposes
368 * and lies dormant until one of the following occurs:
369 * <ul>
370 * <li>Some other thread invokes the {@code notify} method for this
371 * object and thread <var>T</var> happens to be arbitrarily chosen as
372 * the thread to be awakened.
373 * <li>Some other thread invokes the {@code notifyAll} method for this
374 * object.
375 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
376 * thread <var>T</var>.
377 * <li>The specified amount of real time has elapsed, more or less.
378 * The amount of real time, in nanoseconds, is given by the expression
379 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
380 * are both zero, then real time is not taken into consideration and the
381 * thread waits until awakened by one of the other causes.
382 * <li>Thread <var>T</var> is awakened spuriously. (See below.)
383 * </ul>
384 * <p>
385 * The thread <var>T</var> is then removed from the wait set for this
386 * object and re-enabled for thread scheduling. It competes in the
387 * usual manner with other threads for the right to synchronize on the
388 * object; once it has regained control of the object, all its
389 * synchronization claims on the object are restored to the status quo
390 * ante - that is, to the situation as of the time that the {@code wait}
391 * method was invoked. Thread <var>T</var> then returns from the
392 * invocation of the {@code wait} method. Thus, on return from the
393 * {@code wait} method, the synchronization state of the object and of
394 * thread {@code T} is exactly as it was when the {@code wait} method
395 * was invoked.
396 * <p>
397 * A thread can wake up without being notified, interrupted, or timing out, a
398 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice,
399 * applications must guard against it by testing for the condition that should
400 * have caused the thread to be awakened, and continuing to wait if the condition
401 * is not satisfied. See the example below.
402 * <p>
403 * For more information on this topic, see section 14.2,
404 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency
405 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua
406 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley,
407 * 2008).
408 * <p>
409 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
410 * by any thread before or while it is waiting, then an {@code InterruptedException}
411 * is thrown. The <em>interrupted status</em> of the current thread is cleared when
412 * this exception is thrown. This exception is not thrown until the lock status of
413 * this object has been restored as described above.
414 *
415 * @apiNote
416 * The recommended approach to waiting is to check the condition being awaited in
417 * a {@code while} loop around the call to {@code wait}, as shown in the example
418 * below. Among other things, this approach avoids problems that can be caused
419 * by spurious wakeups.
420 *
421 * <pre>{@code
422 * synchronized (obj) {
423 * while (<condition does not hold> and <timeout not exceeded>) {
424 * long timeoutMillis = ... ; // recompute timeout values
425 * int nanos = ... ;
426 * obj.wait(timeoutMillis, nanos);
427 * }
428 * ... // Perform action appropriate to condition or timeout
429 * }
430 * }</pre>
431 *
432 * @param timeoutMillis the maximum time to wait, in milliseconds
433 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive
434 * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
435 * or if the value of {@code nanos} is out of range
436 * @throws IllegalMonitorStateException if the current thread is not
437 * the owner of the object's monitor
438 * @throws InterruptedException if any thread interrupted the current thread before or
439 * while the current thread was waiting. The <em>interrupted status</em> of the
440 * current thread is cleared when this exception is thrown.
441 * @see #notify()
442 * @see #notifyAll()
443 * @see #wait()
444 * @see #wait(long)
445 */
446 public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
447 if (timeoutMillis < 0) {
448 throw new IllegalArgumentException("timeoutMillis value is negative");
449 }
450
451 if (nanos < 0 || nanos > 999999) {
452 throw new IllegalArgumentException(
453 "nanosecond timeout value out of range");
454 }
455
456 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
457 timeoutMillis++;
458 }
459
460 wait(timeoutMillis);
461 }
462
463 /**
464 * Called by the garbage collector on an object when garbage collection
465 * determines that there are no more references to the object.
466 * A subclass overrides the {@code finalize} method to dispose of
467 * system resources or to perform other cleanup.
468 * <p>
469 * The general contract of {@code finalize} is that it is invoked
470 * if and when the Java™ virtual
471 * machine has determined that there is no longer any
472 * means by which this object can be accessed by any thread that has
473 * not yet died, except as a result of an action taken by the
474 * finalization of some other object or class which is ready to be
475 * finalized. The {@code finalize} method may take any action, including
476 * making this object available again to other threads; the usual purpose
477 * of {@code finalize}, however, is to perform cleanup actions before
478 * the object is irrevocably discarded. For example, the finalize method
479 * for an object that represents an input/output connection might perform
480 * explicit I/O transactions to break the connection before the object is
481 * permanently discarded.
482 * <p>
483 * The {@code finalize} method of class {@code Object} performs no
484 * special action; it simply returns normally. Subclasses of
485 * {@code Object} may override this definition.
486 * <p>
487 * The Java programming language does not guarantee which thread will
488 * invoke the {@code finalize} method for any given object. It is
489 * guaranteed, however, that the thread that invokes finalize will not
490 * be holding any user-visible synchronization locks when finalize is
491 * invoked. If an uncaught exception is thrown by the finalize method,
492 * the exception is ignored and finalization of that object terminates.
493 * <p>
494 * After the {@code finalize} method has been invoked for an object, no
495 * further action is taken until the Java virtual machine has again
496 * determined that there is no longer any means by which this object can
497 * be accessed by any thread that has not yet died, including possible
498 * actions by other objects or classes which are ready to be finalized,
499 * at which point the object may be discarded.
500 * <p>
501 * The {@code finalize} method is never invoked more than once by a Java
502 * virtual machine for any given object.
503 * <p>
504 * Any exception thrown by the {@code finalize} method causes
505 * the finalization of this object to be halted, but is otherwise
506 * ignored.
507 *
508 * @apiNote
509 * Classes that embed non-heap resources have many options
510 * for cleanup of those resources. The class must ensure that the
511 * lifetime of each instance is longer than that of any resource it embeds.
512 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
513 * objects remain reachable while resources embedded in the object are in use.
514 * <p>
515 * A subclass should avoid overriding the {@code finalize} method
516 * unless the subclass embeds non-heap resources that must be cleaned up
517 * before the instance is collected.
518 * Finalizer invocations are not automatically chained, unlike constructors.
519 * If a subclass overrides {@code finalize} it must invoke the superclass
520 * finalizer explicitly.
521 * To guard against exceptions prematurely terminating the finalize chain,
522 * the subclass should use a {@code try-finally} block to ensure
523 * {@code super.finalize()} is always invoked. For example,
524 * <pre>{@code @Override
525 * protected void finalize() throws Throwable {
526 * try {
527 * ... // cleanup subclass state
528 * } finally {
529 * super.finalize();
530 * }
531 * }
532 * }</pre>
533 *
534 * @deprecated The finalization mechanism is inherently problematic.
535 * Finalization can lead to performance issues, deadlocks, and hangs.
536 * Errors in finalizers can lead to resource leaks; there is no way to cancel
537 * finalization if it is no longer necessary; and no ordering is specified
538 * among calls to {@code finalize} methods of different objects.
539 * Furthermore, there are no guarantees regarding the timing of finalization.
540 * The {@code finalize} method might be called on a finalizable object
541 * only after an indefinite delay, if at all.
542 *
543 * Classes whose instances hold non-heap resources should provide a method
544 * to enable explicit release of those resources, and they should also
545 * implement {@link AutoCloseable} if appropriate.
546 * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference}
547 * provide more flexible and efficient ways to release resources when an object
548 * becomes unreachable.
549 *
550 * @throws Throwable the {@code Exception} raised by this method
551 * @see java.lang.ref.WeakReference
552 * @see java.lang.ref.PhantomReference
553 * @jls 12.6 Finalization of Class Instances
554 */
555 @Deprecated(since="9")
556 protected void finalize() throws Throwable { }
557 }