1 /*
2 * Copyright (c) 1994, 2024, 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 * <div class="preview-block">
36 * <div class="preview-comment">
37 * When preview features are enabled, subclasses of {@code java.lang.Object} can be either
38 * an {@linkplain Class#isIdentity identity class} or a {@linkplain Class#isValue value class}.
39 * See {@jls The Java Language Specification 8.1.1.5 Value Classes}.
40 * Use of value class instances for synchronization, mutexes, or with
41 * {@linkplain java.lang.ref.Reference object references} result in
42 * {@link IdentityException}.
43 * </div>
44 * </div>
45 *
46 * @see java.lang.Class
47 * @since 1.0
48 */
49 public class Object {
50
51 /**
52 * Constructs a new object.
53 */
54 @IntrinsicCandidate
55 public Object() {}
56
57 /**
58 * Returns the runtime class of this {@code Object}. The returned
59 * {@code Class} object is the object that is locked by {@code
60 * static synchronized} methods of the represented class.
61 *
62 * <p><b>The actual result type is {@code Class<? extends |X|>}
63 * where {@code |X|} is the erasure of the static type of the
64 * expression on which {@code getClass} is called.</b> For
65 * example, no cast is required in this code fragment:</p>
66 *
67 * <p>
68 * {@code Number n = 0; }<br>
69 * {@code Class<? extends Number> c = n.getClass(); }
70 * </p>
71 *
72 * @return The {@code Class} object that represents the runtime
73 * class of this object.
74 * @jls 15.8.2 Class Literals
75 */
76 @IntrinsicCandidate
77 public final native Class<?> getClass();
78
79 /**
80 * {@return a hash code value for this object} This method is
81 * supported for the benefit of hash tables such as those provided by
82 * {@link java.util.HashMap}.
83 * <p>
84 * The general contract of {@code hashCode} is:
85 * <ul>
86 * <li>Whenever it is invoked on the same object more than once during
87 * an execution of a Java application, the {@code hashCode} method
88 * must consistently return the same integer, provided no information
89 * used in {@code equals} comparisons on the object is modified.
90 * This integer need not remain consistent from one execution of an
91 * application to another execution of the same application.
92 * <li>If two objects are equal according to the {@link
93 * #equals(Object) equals} method, then calling the {@code
94 * hashCode} method on each of the two objects must produce the
95 * same integer result.
96 * <li>It is <em>not</em> required that if two objects are unequal
97 * according to the {@link #equals(Object) equals} method, then
98 * calling the {@code hashCode} method on each of the two objects
99 * must produce distinct integer results. However, the programmer
100 * should be aware that producing distinct integer results for
101 * unequal objects may improve the performance of hash tables.
102 * </ul>
103 *
104 * @implSpec
105 * As far as is reasonably practical, the {@code hashCode} method defined
106 * by class {@code Object} returns distinct integers for distinct objects.
107 *
108 * @apiNote
109 * The {@link java.util.Objects#hash(Object...) hash} and {@link
110 * java.util.Objects#hashCode(Object) hashCode} methods of {@link
111 * java.util.Objects} can be used to help construct simple hash codes.
112 *
113 * @see java.lang.Object#equals(java.lang.Object)
114 * @see java.lang.System#identityHashCode
115 */
116 @IntrinsicCandidate
117 public native int hashCode();
118
119 /**
120 * Indicates whether some other object is "equal to" this one.
121 * <p>
122 * The {@code equals} method implements an <dfn>{@index "equivalence relation"}</dfn>
123 * on non-null object references:
124 * <ul>
125 * <li>It is <i>reflexive</i>: for any non-null reference value
126 * {@code x}, {@code x.equals(x)} should return
127 * {@code true}.
128 * <li>It is <i>symmetric</i>: for any non-null reference values
129 * {@code x} and {@code y}, {@code x.equals(y)}
130 * should return {@code true} if and only if
131 * {@code y.equals(x)} returns {@code true}.
132 * <li>It is <i>transitive</i>: for any non-null reference values
133 * {@code x}, {@code y}, and {@code z}, if
134 * {@code x.equals(y)} returns {@code true} and
135 * {@code y.equals(z)} returns {@code true}, then
136 * {@code x.equals(z)} should return {@code true}.
137 * <li>It is <i>consistent</i>: for any non-null reference values
138 * {@code x} and {@code y}, multiple invocations of
139 * {@code x.equals(y)} consistently return {@code true}
140 * or consistently return {@code false}, provided no
141 * information used in {@code equals} comparisons on the
142 * objects is modified.
143 * <li>For any non-null reference value {@code x},
144 * {@code x.equals(null)} should return {@code false}.
145 * </ul>
146 *
147 * <p>
148 * An equivalence relation partitions the elements it operates on
149 * into <i>equivalence classes</i>; all the members of an
150 * equivalence class are equal to each other. Members of an
151 * equivalence class are substitutable for each other, at least
152 * for some purposes.
153 *
154 * @implSpec
155 * The {@code equals} method for class {@code Object} implements
156 * the most discriminating possible equivalence relation on objects;
157 * that is, for any non-null reference values {@code x} and
158 * {@code y}, this method returns {@code true} if and only
159 * if {@code x} and {@code y} refer to the same object
160 * ({@code x == y} has the value {@code true}).
161 *
162 * In other words, under the reference equality equivalence
163 * relation, each equivalence class only has a single element.
164 *
165 * @apiNote
166 * It is generally necessary to override the {@link #hashCode() hashCode}
167 * method whenever this method is overridden, so as to maintain the
168 * general contract for the {@code hashCode} method, which states
169 * that equal objects must have equal hash codes.
170 * <p>The two-argument {@link java.util.Objects#equals(Object,
171 * Object) Objects.equals} method implements an equivalence relation
172 * on two possibly-null object references.
173 *
174 * @param obj the reference object with which to compare.
175 * @return {@code true} if this object is the same as the obj
176 * argument; {@code false} otherwise.
177 * @see #hashCode()
178 * @see java.util.HashMap
179 */
180 public boolean equals(Object obj) {
181 return (this == obj);
182 }
183
184 /**
185 * Creates and returns a copy of this object. The precise meaning
186 * of "copy" may depend on the class of the object. The general
187 * intent is that, for any object {@code x}, the expression:
188 * <blockquote>
189 * <pre>
190 * x.clone() != x</pre></blockquote>
191 * will be true, and that the expression:
192 * <blockquote>
193 * <pre>
194 * x.clone().getClass() == x.getClass()</pre></blockquote>
195 * will be {@code true}, but these are not absolute requirements.
196 * While it is typically the case that:
197 * <blockquote>
198 * <pre>
199 * x.clone().equals(x)</pre></blockquote>
200 * will be {@code true}, this is not an absolute requirement.
201 * <p>
202 * By convention, the returned object should be obtained by calling
203 * {@code super.clone}. If a class and all of its superclasses (except
204 * {@code Object}) obey this convention, it will be the case that
205 * {@code x.clone().getClass() == x.getClass()}.
206 * <p>
207 * By convention, the object returned by this method should be independent
208 * of this object (which is being cloned). To achieve this independence,
209 * it may be necessary to modify one or more fields of the object returned
210 * by {@code super.clone} before returning it. Typically, this means
211 * copying any mutable objects that comprise the internal "deep structure"
212 * of the object being cloned and replacing the references to these
213 * objects with references to the copies. If a class contains only
214 * primitive fields or references to immutable objects, then it is usually
215 * the case that no fields in the object returned by {@code super.clone}
216 * need to be modified.
217 *
218 * @implSpec
219 * The method {@code clone} for class {@code Object} performs a
220 * specific cloning operation. First, if the class of this object does
221 * not implement the interface {@code Cloneable}, then a
222 * {@code CloneNotSupportedException} is thrown. Note that all arrays
223 * are considered to implement the interface {@code Cloneable} and that
224 * the return type of the {@code clone} method of an array type {@code T[]}
225 * is {@code T[]} where T is any reference or primitive type.
226 * Otherwise, this method creates a new instance of the class of this
227 * object and initializes all its fields with exactly the contents of
228 * the corresponding fields of this object, as if by assignment; the
229 * contents of the fields are not themselves cloned. Thus, this method
230 * performs a "shallow copy" of this object, not a "deep copy" operation.
231 * <p>
232 * The class {@code Object} does not itself implement the interface
233 * {@code Cloneable}, so calling the {@code clone} method on an object
234 * whose class is {@code Object} will result in throwing an
235 * exception at run time.
236 *
237 * @return a clone of this instance.
238 * @throws CloneNotSupportedException if the object's class does not
239 * support the {@code Cloneable} interface. Subclasses
240 * that override the {@code clone} method can also
241 * throw this exception to indicate that an instance cannot
242 * be cloned.
243 * @see java.lang.Cloneable
244 */
245 @IntrinsicCandidate
246 protected native Object clone() throws CloneNotSupportedException;
247
248 /**
249 * {@return a string representation of the object}
250 *
251 * Satisfying this method's contract implies a non-{@code null}
252 * result must be returned.
253 *
254 * @apiNote
255 * In general, the
256 * {@code toString} method returns a string that
257 * "textually represents" this object. The result should
258 * be a concise but informative representation that is easy for a
259 * person to read.
260 * It is recommended that all subclasses override this method.
261 * The string output is not necessarily stable over time or across
262 * JVM invocations.
263 * @implSpec
264 * The {@code toString} method for class {@code Object}
265 * returns a string consisting of the name of the class of which the
266 * object is an instance, the at-sign character `{@code @}', and
267 * the unsigned hexadecimal representation of the hash code of the
268 * object. In other words, this method returns a string equal to the
269 * value of:
270 * {@snippet lang=java :
271 * getClass().getName() + '@' + Integer.toHexString(hashCode())
272 * }
273 * The {@link java.util.Objects#toIdentityString(Object)
274 * Objects.toIdentityString} method returns the string for an
275 * object equal to the string that would be returned if neither
276 * the {@code toString} nor {@code hashCode} methods were
277 * overridden by the object's class.
278 */
279 public String toString() {
280 return getClass().getName() + "@" + Integer.toHexString(hashCode());
281 }
282
283 /**
284 * Wakes up a single thread that is waiting on this object's
285 * monitor. If any threads are waiting on this object, one of them
286 * is chosen to be awakened. The choice is arbitrary and occurs at
287 * the discretion of the implementation. A thread waits on an object's
288 * monitor by calling one of the {@code wait} methods.
289 * <p>
290 * The awakened thread will not be able to proceed until the current
291 * thread relinquishes the lock on this object. The awakened thread will
292 * compete in the usual manner with any other threads that might be
293 * actively competing to synchronize on this object; for example, the
294 * awakened thread enjoys no reliable privilege or disadvantage in being
295 * the next thread to lock this object.
296 * <p>
297 * This method should only be called by a thread that is the owner
298 * of this object's monitor. A thread becomes the owner of the
299 * object's monitor in one of three ways:
300 * <ul>
301 * <li>By executing a synchronized instance method of that object.
302 * <li>By executing the body of a {@code synchronized} statement
303 * that synchronizes on the object.
304 * <li>For objects of type {@code Class,} by executing a
305 * static synchronized method of that class.
306 * </ul>
307 * <p>
308 * Only one thread at a time can own an object's monitor.
309 * <div class="preview-block">
310 * <div class="preview-comment">
311 * If this object is a {@linkplain Class#isValue() value object},
312 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
313 * </div>
314 * </div>
315 *
316 * @throws IllegalMonitorStateException if the current thread is not
317 * the owner of this object's monitor or
318 * if this object is a {@linkplain Class#isValue() value object}.
319 * @see java.lang.Object#notifyAll()
320 * @see java.lang.Object#wait()
321 */
322 @IntrinsicCandidate
323 public final native void notify();
324
325 /**
326 * Wakes up all threads that are waiting on this object's monitor. A
327 * thread waits on an object's monitor by calling one of the
328 * {@code wait} methods.
329 * <p>
330 * The awakened threads will not be able to proceed until the current
331 * thread relinquishes the lock on this object. The awakened threads
332 * will compete in the usual manner with any other threads that might
333 * be actively competing to synchronize on this object; for example,
334 * the awakened threads enjoy no reliable privilege or disadvantage in
335 * being the next thread to lock this object.
336 * <p>
337 * This method should only be called by a thread that is the owner
338 * of this object's monitor. See the {@code notify} method for a
339 * description of the ways in which a thread can become the owner of
340 * a monitor.
341 *
342 * <div class="preview-block">
343 * <div class="preview-comment">
344 * If this object is a {@linkplain Class#isValue() value object},
345 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
346 * </div>
347 * </div>
348 *
349 * @throws IllegalMonitorStateException if the current thread is not
350 * the owner of this object's monitor or
351 * if this object is a {@linkplain Class#isValue() value object}.
352 * @see java.lang.Object#notify()
353 * @see java.lang.Object#wait()
354 */
355 @IntrinsicCandidate
356 public final native void notifyAll();
357
358 /**
359 * Causes the current thread to wait until it is awakened, typically
360 * by being <em>notified</em> or <em>interrupted</em>.
361 * <p>
362 * In all respects, this method behaves as if {@code wait(0L, 0)}
363 * had been called. See the specification of the {@link #wait(long, int)} method
364 * for details.
365 *
366 * <div class="preview-block">
367 * <div class="preview-comment">
368 * If this object is a {@linkplain Class#isValue() value object},
369 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
370 * </div>
371 * </div>
372 *
373 * @throws IllegalMonitorStateException if the current thread is not
374 * the owner of the object's monitor or
375 * if this object is a {@linkplain Class#isValue() value object}.
376 * @throws InterruptedException if any thread interrupted the current thread before or
377 * while the current thread was waiting. The <em>interrupted status</em> of the
378 * current thread is cleared when this exception is thrown.
379 * @see #notify()
380 * @see #notifyAll()
381 * @see #wait(long)
382 * @see #wait(long, int)
383 */
384 public final void wait() throws InterruptedException {
385 wait(0L);
386 }
387
388 /**
389 * Causes the current thread to wait until it is awakened, typically
390 * by being <em>notified</em> or <em>interrupted</em>, or until a
391 * certain amount of real time has elapsed.
392 * <p>
393 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
394 * had been called. See the specification of the {@link #wait(long, int)} method
395 * for details.
396 *
397 * <div class="preview-block">
398 * <div class="preview-comment">
399 * If this object is a {@linkplain Class#isValue() value object},
400 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
401 * </div>
402 * </div>
403 *
404 * @param timeoutMillis the maximum time to wait, in milliseconds
405 * @throws IllegalArgumentException if {@code timeoutMillis} is negative
406 * @throws IllegalMonitorStateException if the current thread is not
407 * the owner of the object's monitor or
408 * if this object is a {@linkplain Class#isValue() value object}.
409 * @throws InterruptedException if any thread interrupted the current thread before or
410 * while the current thread was waiting. The <em>interrupted status</em> of the
411 * current thread is cleared when this exception is thrown.
412 * @see #notify()
413 * @see #notifyAll()
414 * @see #wait()
415 * @see #wait(long, int)
416 */
417 public final void wait(long timeoutMillis) throws InterruptedException {
418 if (timeoutMillis < 0) {
419 throw new IllegalArgumentException("timeout value is negative");
420 }
421
422 if (Thread.currentThread() instanceof VirtualThread vthread) {
423 try {
424 wait0(timeoutMillis);
425 } catch (InterruptedException e) {
426 // virtual thread's interrupt status needs to be cleared
427 vthread.getAndClearInterrupt();
428 throw e;
429 }
430 } else {
431 wait0(timeoutMillis);
432 }
433 }
434
435 // final modifier so method not in vtable
436 private final native void wait0(long timeoutMillis) throws InterruptedException;
437
438 /**
439 * Causes the current thread to wait until it is awakened, typically
440 * by being <em>notified</em> or <em>interrupted</em>, or until a
441 * certain amount of real time has elapsed.
442 * <p>
443 * The current thread must own this object's monitor lock. See the
444 * {@link #notify notify} method for a description of the ways in which
445 * a thread can become the owner of a monitor lock.
446 * <p>
447 * This method causes the current thread (referred to here as <var>T</var>) to
448 * place itself in the wait set for this object and then to relinquish any
449 * and all synchronization claims on this object. Note that only the locks
450 * on this object are relinquished; any other objects on which the current
451 * thread may be synchronized remain locked while the thread waits.
452 * <p>
453 * Thread <var>T</var> then becomes disabled for thread scheduling purposes
454 * and lies dormant until one of the following occurs:
455 * <ul>
456 * <li>Some other thread invokes the {@code notify} method for this
457 * object and thread <var>T</var> happens to be arbitrarily chosen as
458 * the thread to be awakened.
459 * <li>Some other thread invokes the {@code notifyAll} method for this
460 * object.
461 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
462 * thread <var>T</var>.
463 * <li>The specified amount of real time has elapsed, more or less.
464 * The amount of real time, in nanoseconds, is given by the expression
465 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
466 * are both zero, then real time is not taken into consideration and the
467 * thread waits until awakened by one of the other causes.
468 * <li>Thread <var>T</var> is awakened spuriously. (See below.)
469 * </ul>
470 * <p>
471 * The thread <var>T</var> is then removed from the wait set for this
472 * object and re-enabled for thread scheduling. It competes in the
473 * usual manner with other threads for the right to synchronize on the
474 * object; once it has regained control of the object, all its
475 * synchronization claims on the object are restored to the status quo
476 * ante - that is, to the situation as of the time that the {@code wait}
477 * method was invoked. Thread <var>T</var> then returns from the
478 * invocation of the {@code wait} method. Thus, on return from the
479 * {@code wait} method, the synchronization state of the object and of
480 * thread {@code T} is exactly as it was when the {@code wait} method
481 * was invoked.
482 * <p>
483 * A thread can wake up without being notified, interrupted, or timing out, a
484 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice,
485 * applications must guard against it by testing for the condition that should
486 * have caused the thread to be awakened, and continuing to wait if the condition
487 * is not satisfied. See the example below.
488 * <p>
489 * For more information on this topic, see section 14.2,
490 * "Condition Queues," in Brian Goetz and others' <cite>Java Concurrency
491 * in Practice</cite> (Addison-Wesley, 2006) or Item 81 in Joshua
492 * Bloch's <cite>Effective Java, Third Edition</cite> (Addison-Wesley,
493 * 2018).
494 * <p>
495 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
496 * by any thread before or while it is waiting, then an {@code InterruptedException}
497 * is thrown. The <em>interrupted status</em> of the current thread is cleared when
498 * this exception is thrown. This exception is not thrown until the lock status of
499 * this object has been restored as described above.
500 *
501 * @apiNote
502 * The recommended approach to waiting is to check the condition being awaited in
503 * a {@code while} loop around the call to {@code wait}, as shown in the example
504 * below. Among other things, this approach avoids problems that can be caused
505 * by spurious wakeups.
506 *
507 * {@snippet lang=java :
508 * synchronized (obj) {
509 * while ( <condition does not hold and timeout not exceeded> ) {
510 * long timeoutMillis = ... ; // recompute timeout values
511 * int nanos = ... ;
512 * obj.wait(timeoutMillis, nanos);
513 * }
514 * ... // Perform action appropriate to condition or timeout
515 * }
516 * }
517 *
518 * <div class="preview-block">
519 * <div class="preview-comment">
520 * If this object is a {@linkplain Class#isValue() value object},
521 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
522 * </div>
523 * </div>
524 * @param timeoutMillis the maximum time to wait, in milliseconds
525 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive
526 * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
527 * or if the value of {@code nanos} is out of range
528 * @throws IllegalMonitorStateException if the current thread is not
529 * the owner of the object's monitor or
530 * if this object is a {@linkplain Class#isValue() value object}.
531 * @throws InterruptedException if any thread interrupted the current thread before or
532 * while the current thread was waiting. The <em>interrupted status</em> of the
533 * current thread is cleared when this exception is thrown.
534 * @see #notify()
535 * @see #notifyAll()
536 * @see #wait()
537 * @see #wait(long)
538 */
539 public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
540 if (timeoutMillis < 0) {
541 throw new IllegalArgumentException("timeoutMillis value is negative");
542 }
543
544 if (nanos < 0 || nanos > 999999) {
545 throw new IllegalArgumentException(
546 "nanosecond timeout value out of range");
547 }
548
549 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
550 timeoutMillis++;
551 }
552
553 wait(timeoutMillis);
554 }
555
556 /**
557 * Called by the garbage collector on an object when garbage collection
558 * determines that there are no more references to the object.
559 * A subclass overrides the {@code finalize} method to dispose of
560 * system resources or to perform other cleanup.
561 * <p>
562 * <b>When running in a Java virtual machine in which finalization has been
563 * disabled or removed, the garbage collector will never call
564 * {@code finalize()}. In a Java virtual machine in which finalization is
565 * enabled, the garbage collector might call {@code finalize} only after an
566 * indefinite delay.</b>
567 * <p>
568 * The general contract of {@code finalize} is that it is invoked
569 * if and when the Java virtual
570 * machine has determined that there is no longer any
571 * means by which this object can be accessed by any thread that has
572 * not yet died, except as a result of an action taken by the
573 * finalization of some other object or class which is ready to be
574 * finalized. The {@code finalize} method may take any action, including
575 * making this object available again to other threads; the usual purpose
576 * of {@code finalize}, however, is to perform cleanup actions before
577 * the object is irrevocably discarded. For example, the finalize method
578 * for an object that represents an input/output connection might perform
579 * explicit I/O transactions to break the connection before the object is
580 * permanently discarded.
581 * <p>
582 * The {@code finalize} method of class {@code Object} performs no
583 * special action; it simply returns normally. Subclasses of
584 * {@code Object} may override this definition.
585 * <p>
586 * The Java programming language does not guarantee which thread will
587 * invoke the {@code finalize} method for any given object. It is
588 * guaranteed, however, that the thread that invokes finalize will not
589 * be holding any user-visible synchronization locks when finalize is
590 * invoked. If an uncaught exception is thrown by the finalize method,
591 * the exception is ignored and finalization of that object terminates.
592 * <p>
593 * After the {@code finalize} method has been invoked for an object, no
594 * further action is taken until the Java virtual machine has again
595 * determined that there is no longer any means by which this object can
596 * be accessed by any thread that has not yet died, including possible
597 * actions by other objects or classes which are ready to be finalized,
598 * at which point the object may be discarded.
599 * <p>
600 * The {@code finalize} method is never invoked more than once by a Java
601 * virtual machine for any given object.
602 * <p>
603 * Any exception thrown by the {@code finalize} method causes
604 * the finalization of this object to be halted, but is otherwise
605 * ignored.
606 *
607 * @apiNote
608 * Classes that embed non-heap resources have many options
609 * for cleanup of those resources. The class must ensure that the
610 * lifetime of each instance is longer than that of any resource it embeds.
611 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
612 * objects remain reachable while resources embedded in the object are in use.
613 * <p>
614 * A subclass should avoid overriding the {@code finalize} method
615 * unless the subclass embeds non-heap resources that must be cleaned up
616 * before the instance is collected.
617 * Finalizer invocations are not automatically chained, unlike constructors.
618 * If a subclass overrides {@code finalize} it must invoke the superclass
619 * finalizer explicitly.
620 * To guard against exceptions prematurely terminating the finalize chain,
621 * the subclass should use a {@code try-finally} block to ensure
622 * {@code super.finalize()} is always invoked. For example,
623 * {@snippet lang="java":
624 * @Override
625 * protected void finalize() throws Throwable {
626 * try {
627 * ... // cleanup subclass state
628 * } finally {
629 * super.finalize();
630 * }
631 * }
632 * }
633 *
634 * @deprecated Finalization is deprecated and subject to removal in a future
635 * release. The use of finalization can lead to problems with security,
636 * performance, and reliability.
637 * See <a href="https://openjdk.org/jeps/421">JEP 421</a> for
638 * discussion and alternatives.
639 * <p>
640 * Subclasses that override {@code finalize} to perform cleanup should use
641 * alternative cleanup mechanisms and remove the {@code finalize} method.
642 * Use {@link java.lang.ref.Cleaner} and
643 * {@link java.lang.ref.PhantomReference} as safer ways to release resources
644 * when an object becomes unreachable. Alternatively, add a {@code close}
645 * method to explicitly release resources, and implement
646 * {@code AutoCloseable} to enable use of the {@code try}-with-resources
647 * statement.
648 * <p>
649 * This method will remain in place until finalizers have been removed from
650 * most existing code.
651 *
652 * @throws Throwable the {@code Exception} raised by this method
653 * @see java.lang.ref.WeakReference
654 * @see java.lang.ref.PhantomReference
655 * @jls 12.6 Finalization of Class Instances
656 */
657 @Deprecated(since="9", forRemoval=true)
658 protected void finalize() throws Throwable { }
659 }