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.misc.Blocker;
29 import jdk.internal.vm.annotation.IntrinsicCandidate;
30
31 /**
32 * Class {@code Object} is the root of the class hierarchy.
33 * Every class has {@code Object} as a superclass. All objects,
34 * including arrays, implement the methods of this class.
35 * <p>
36 * Subclasses of {@code java.lang.Object} can be either an {@linkplain Class#isIdentity identity class}
37 * or a {@linkplain Class#isValue value class}.
38 * See {@jls The Java Language Specification 8.1.1.5 Value Classes}.
39 *
40 * @see java.lang.Class
41 * @since 1.0
42 */
43 public class Object {
44
45 /**
46 * Constructs a new object.
47 */
48 @IntrinsicCandidate
49 public Object() {}
50
51 /**
52 * Returns the runtime class of this {@code Object}. The returned
53 * {@code Class} object is the object that is locked by {@code
54 * static synchronized} methods of the represented class.
55 *
56 * <p><b>The actual result type is {@code Class<? extends |X|>}
57 * where {@code |X|} is the erasure of the static type of the
58 * expression on which {@code getClass} is called.</b> For
59 * example, no cast is required in this code fragment:</p>
60 *
61 * <p>
62 * {@code Number n = 0; }<br>
63 * {@code Class<? extends Number> c = n.getClass(); }
64 * </p>
65 *
66 * @return The {@code Class} object that represents the runtime
67 * class of this object.
68 * @jls 15.8.2 Class Literals
69 */
70 @IntrinsicCandidate
71 public final native Class<?> getClass();
72
73 /**
74 * {@return a hash code value for this object} This method is
75 * supported for the benefit of hash tables such as those provided by
76 * {@link java.util.HashMap}.
77 * <p>
78 * The general contract of {@code hashCode} is:
79 * <ul>
80 * <li>Whenever it is invoked on the same object more than once during
81 * an execution of a Java application, the {@code hashCode} method
82 * must consistently return the same integer, provided no information
83 * used in {@code equals} comparisons on the object is modified.
84 * This integer need not remain consistent from one execution of an
85 * application to another execution of the same application.
86 * <li>If two objects are equal according to the {@link
87 * #equals(Object) equals} method, then calling the {@code
88 * hashCode} method on each of the two objects must produce the
89 * same integer result.
90 * <li>It is <em>not</em> required that if two objects are unequal
91 * according to the {@link #equals(Object) equals} method, then
92 * calling the {@code hashCode} method on each of the two objects
93 * must produce distinct integer results. However, the programmer
94 * should be aware that producing distinct integer results for
95 * 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 * @apiNote
103 * The {@link java.util.Objects#hash(Object...) hash} and {@link
104 * java.util.Objects#hashCode(Object) hashCode} methods of {@link
105 * java.util.Objects} can be used to help construct simple hash codes.
106 *
107 * @see java.lang.Object#equals(java.lang.Object)
108 * @see java.lang.System#identityHashCode
109 */
110 @IntrinsicCandidate
111 public native int hashCode();
112
113 /**
114 * Indicates whether some other object is "equal to" this one.
115 * <p>
116 * The {@code equals} method implements an <dfn>{@index "equivalence relation"}</dfn>
117 * on non-null object references:
118 * <ul>
119 * <li>It is <i>reflexive</i>: for any non-null reference value
120 * {@code x}, {@code x.equals(x)} should return
121 * {@code true}.
122 * <li>It is <i>symmetric</i>: for any non-null reference values
123 * {@code x} and {@code y}, {@code x.equals(y)}
124 * should return {@code true} if and only if
125 * {@code y.equals(x)} returns {@code true}.
126 * <li>It is <i>transitive</i>: for any non-null reference values
127 * {@code x}, {@code y}, and {@code z}, if
128 * {@code x.equals(y)} returns {@code true} and
129 * {@code y.equals(z)} returns {@code true}, then
130 * {@code x.equals(z)} should return {@code true}.
131 * <li>It is <i>consistent</i>: for any non-null reference values
132 * {@code x} and {@code y}, multiple invocations of
133 * {@code x.equals(y)} consistently return {@code true}
134 * or consistently return {@code false}, provided no
135 * information used in {@code equals} comparisons on the
136 * objects is modified.
137 * <li>For any non-null reference value {@code x},
138 * {@code x.equals(null)} should return {@code false}.
139 * </ul>
140 *
141 * <p>
142 * An equivalence relation partitions the elements it operates on
143 * into <i>equivalence classes</i>; all the members of an
144 * equivalence class are equal to each other. Members of an
145 * equivalence class are substitutable for each other, at least
146 * for some purposes.
147 *
148 * @implSpec
149 * The {@code equals} method for class {@code Object} implements
150 * the most discriminating possible equivalence relation on objects;
151 * that is, for any non-null reference values {@code x} and
152 * {@code y}, this method returns {@code true} if and only
153 * if {@code x} and {@code y} refer to the same object
154 * ({@code x == y} has the value {@code true}).
155 *
156 * In other words, under the reference equality equivalence
157 * relation, each equivalence class only has a single element.
158 *
159 * @apiNote
160 * It is generally necessary to override the {@link #hashCode() hashCode}
161 * method whenever this method is overridden, so as to maintain the
162 * general contract for the {@code hashCode} method, which states
163 * that equal objects must have equal hash codes.
164 * <p>The two-argument {@link java.util.Objects#equals(Object,
165 * Object) Objects.equals} method implements an equivalence relation
166 * on two possibly-null object references.
167 *
168 * @param obj the reference object with which to compare.
169 * @return {@code true} if this object is the same as the obj
170 * argument; {@code false} otherwise.
171 * @see #hashCode()
172 * @see java.util.HashMap
173 */
174 public boolean equals(Object obj) {
175 return (this == obj);
176 }
177
178 /**
179 * Creates and returns a copy of this object. The precise meaning
180 * of "copy" may depend on the class of the object. The general
181 * intent is that, for any object {@code x}, the expression:
182 * <blockquote>
183 * <pre>
184 * x.clone() != x</pre></blockquote>
185 * will be true, and that the expression:
186 * <blockquote>
187 * <pre>
188 * x.clone().getClass() == x.getClass()</pre></blockquote>
189 * will be {@code true}, but these are not absolute requirements.
190 * While it is typically the case that:
191 * <blockquote>
192 * <pre>
193 * x.clone().equals(x)</pre></blockquote>
194 * will be {@code true}, this is not an absolute requirement.
195 * <p>
196 * By convention, the returned object should be obtained by calling
197 * {@code super.clone}. If a class and all of its superclasses (except
198 * {@code Object}) obey this convention, it will be the case that
199 * {@code x.clone().getClass() == x.getClass()}.
200 * <p>
201 * By convention, the object returned by this method should be independent
202 * of this object (which is being cloned). To achieve this independence,
203 * it may be necessary to modify one or more fields of the object returned
204 * by {@code super.clone} before returning it. Typically, this means
205 * copying any mutable objects that comprise the internal "deep structure"
206 * of the object being cloned and replacing the references to these
207 * objects with references to the copies. If a class contains only
208 * primitive fields or references to immutable objects, then it is usually
209 * the case that no fields in the object returned by {@code super.clone}
210 * need to be modified.
211 *
212 * @implSpec
213 * The method {@code clone} for class {@code Object} performs a
214 * specific cloning operation. First, if the class of this object does
215 * not implement the interface {@code Cloneable}, then a
216 * {@code CloneNotSupportedException} is thrown. Note that all arrays
217 * are considered to implement the interface {@code Cloneable} and that
218 * the return type of the {@code clone} method of an array type {@code T[]}
219 * is {@code T[]} where T is any reference or primitive type.
220 * Otherwise, this method creates a new instance of the class of this
221 * object and initializes all its fields with exactly the contents of
222 * the corresponding fields of this object, as if by assignment; the
223 * contents of the fields are not themselves cloned. Thus, this method
224 * performs a "shallow copy" of this object, not a "deep copy" operation.
225 * <p>
226 * The class {@code Object} does not itself implement the interface
227 * {@code Cloneable}, so calling the {@code clone} method on an object
228 * whose class is {@code Object} will result in throwing an
229 * exception at run time.
230 *
231 * @return a clone of this instance.
232 * @throws CloneNotSupportedException if the object's class does not
233 * support the {@code Cloneable} interface. Subclasses
234 * that override the {@code clone} method can also
235 * throw this exception to indicate that an instance cannot
236 * be cloned.
237 * @see java.lang.Cloneable
238 */
239 @IntrinsicCandidate
240 protected native Object clone() throws CloneNotSupportedException;
241
242 /**
243 * {@return a string representation of the object}
244 *
245 * Satisfying this method's contract implies a non-{@code null}
246 * result must be returned.
247 *
248 * @apiNote
249 * In general, the
250 * {@code toString} method returns a string that
251 * "textually represents" this object. The result should
252 * be a concise but informative representation that is easy for a
253 * person to read.
254 * It is recommended that all subclasses override this method.
255 * The string output is not necessarily stable over time or across
256 * JVM invocations.
257 * @implSpec
258 * The {@code toString} method for class {@code Object}
259 * returns a string consisting of the name of the class of which the
260 * object is an instance, the at-sign character `{@code @}', and
261 * the unsigned hexadecimal representation of the hash code of the
262 * object. In other words, this method returns a string equal to the
263 * value of:
264 * {@snippet lang=java :
265 * getClass().getName() + '@' + Integer.toHexString(hashCode())
266 * }
267 * The {@link java.util.Objects#toIdentityString(Object)
268 * Objects.toIdentityString} method returns the string for an
269 * object equal to the string that would be returned if neither
270 * the {@code toString} nor {@code hashCode} methods were
271 * overridden by the object's class.
272 */
273 public String toString() {
274 return getClass().getName() + "@" + Integer.toHexString(hashCode());
275 }
276
277 /**
278 * Wakes up a single thread that is waiting on this object's
279 * monitor. If any threads are waiting on this object, one of them
280 * is chosen to be awakened. The choice is arbitrary and occurs at
281 * the discretion of the implementation. A thread waits on an object's
282 * monitor by calling one of the {@code wait} methods.
283 * <p>
284 * The awakened thread will not be able to proceed until the current
285 * thread relinquishes the lock on this object. The awakened thread will
286 * compete in the usual manner with any other threads that might be
287 * actively competing to synchronize on this object; for example, the
288 * awakened thread enjoys no reliable privilege or disadvantage in being
289 * 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. A thread becomes the owner of the
293 * object's monitor in one of three ways:
294 * <ul>
295 * <li>By executing a synchronized instance method of that object.
296 * <li>By executing the body of a {@code synchronized} statement
297 * that synchronizes on the object.
298 * <li>For objects of type {@code Class,} by executing a
299 * static synchronized method of that class.
300 * </ul>
301 * <p>
302 * Only one thread at a time can own an object's monitor.
303 * <div class="preview-block">
304 * <div class="preview-comment">
305 * If this object is a {@linkplain Class#isValue() value object},
306 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
307 * </div>
308 * </div>
309 *
310 * @throws IllegalMonitorStateException if the current thread is not
311 * the owner of this object's monitor or
312 * if this object is a {@linkplain Class#isValue() value object}.
313 * @see java.lang.Object#notifyAll()
314 * @see java.lang.Object#wait()
315 */
316 @IntrinsicCandidate
317 public final native void notify();
318
319 /**
320 * Wakes up all threads that are waiting on this object's monitor. A
321 * thread waits on an object's monitor by calling one of the
322 * {@code wait} methods.
323 * <p>
324 * The awakened threads will not be able to proceed until the current
325 * thread relinquishes the lock on this object. The awakened threads
326 * will compete in the usual manner with any other threads that might
327 * be actively competing to synchronize on this object; for example,
328 * the awakened threads enjoy no reliable privilege or disadvantage in
329 * being the next thread to lock this object.
330 * <p>
331 * This method should only be called by a thread that is the owner
332 * of this object's monitor. See the {@code notify} method for a
333 * description of the ways in which a thread can become the owner of
334 * a monitor.
335 *
336 * <div class="preview-block">
337 * <div class="preview-comment">
338 * If this object is a {@linkplain Class#isValue() value object},
339 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
340 * </div>
341 * </div>
342 *
343 * @throws IllegalMonitorStateException if the current thread is not
344 * the owner of this object's monitor or
345 * if this object is a {@linkplain Class#isValue() value object}.
346 * @see java.lang.Object#notify()
347 * @see java.lang.Object#wait()
348 */
349 @IntrinsicCandidate
350 public final native void notifyAll();
351
352 /**
353 * Causes the current thread to wait until it is awakened, typically
354 * by being <em>notified</em> or <em>interrupted</em>.
355 * <p>
356 * In all respects, this method behaves as if {@code wait(0L, 0)}
357 * had been called. See the specification of the {@link #wait(long, int)} method
358 * for details.
359 *
360 * <div class="preview-block">
361 * <div class="preview-comment">
362 * If this object is a {@linkplain Class#isValue() value object},
363 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
364 * </div>
365 * </div>
366 *
367 * @throws IllegalMonitorStateException if the current thread is not
368 * the owner of the object's monitor or
369 * if this object is a {@linkplain Class#isValue() value object}.
370 * @throws InterruptedException if any thread interrupted the current thread before or
371 * while the current thread was waiting. The <em>interrupted status</em> of the
372 * current thread is cleared when this exception is thrown.
373 * @see #notify()
374 * @see #notifyAll()
375 * @see #wait(long)
376 * @see #wait(long, int)
377 */
378 public final void wait() throws InterruptedException {
379 wait(0L);
380 }
381
382 /**
383 * Causes the current thread to wait until it is awakened, typically
384 * by being <em>notified</em> or <em>interrupted</em>, or until a
385 * certain amount of real time has elapsed.
386 * <p>
387 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
388 * had been called. See the specification of the {@link #wait(long, int)} method
389 * for details.
390 *
391 * <div class="preview-block">
392 * <div class="preview-comment">
393 * If this object is a {@linkplain Class#isValue() value object},
394 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
395 * </div>
396 * </div>
397 *
398 * @param timeoutMillis the maximum time to wait, in milliseconds
399 * @throws IllegalArgumentException if {@code timeoutMillis} is negative
400 * @throws IllegalMonitorStateException if the current thread is not
401 * the owner of the object's monitor or
402 * if this object is a {@linkplain Class#isValue() value object}.
403 * @throws InterruptedException if any thread interrupted the current thread before or
404 * while the current thread was waiting. The <em>interrupted status</em> of the
405 * current thread is cleared when this exception is thrown.
406 * @see #notify()
407 * @see #notifyAll()
408 * @see #wait()
409 * @see #wait(long, int)
410 */
411 public final void wait(long timeoutMillis) throws InterruptedException {
412 if (!Thread.currentThread().isVirtual()) {
413 wait0(timeoutMillis);
414 return;
415 }
416
417 // virtual thread waiting
418 boolean attempted = Blocker.begin();
419 try {
420 wait0(timeoutMillis);
421 } catch (InterruptedException e) {
422 // virtual thread's interrupt status needs to be cleared
423 Thread.currentThread().getAndClearInterrupt();
424 throw e;
425 } finally {
426 Blocker.end(attempted);
427 }
428 }
429
430 // final modifier so method not in vtable
431 private final native void wait0(long timeoutMillis) throws InterruptedException;
432
433 /**
434 * Causes the current thread to wait until it is awakened, typically
435 * by being <em>notified</em> or <em>interrupted</em>, or until a
436 * certain amount of real time has elapsed.
437 * <p>
438 * The current thread must own this object's monitor lock. See the
439 * {@link #notify notify} method for a description of the ways in which
440 * a thread can become the owner of a monitor lock.
441 * <p>
442 * This method causes the current thread (referred to here as <var>T</var>) to
443 * place itself in the wait set for this object and then to relinquish any
444 * and all synchronization claims on this object. Note that only the locks
445 * on this object are relinquished; any other objects on which the current
446 * thread may be synchronized remain locked while the thread waits.
447 * <p>
448 * Thread <var>T</var> then becomes disabled for thread scheduling purposes
449 * and lies dormant until one of the following occurs:
450 * <ul>
451 * <li>Some other thread invokes the {@code notify} method for this
452 * object and thread <var>T</var> happens to be arbitrarily chosen as
453 * the thread to be awakened.
454 * <li>Some other thread invokes the {@code notifyAll} method for this
455 * object.
456 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
457 * thread <var>T</var>.
458 * <li>The specified amount of real time has elapsed, more or less.
459 * The amount of real time, in nanoseconds, is given by the expression
460 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
461 * are both zero, then real time is not taken into consideration and the
462 * thread waits until awakened by one of the other causes.
463 * <li>Thread <var>T</var> is awakened spuriously. (See below.)
464 * </ul>
465 * <p>
466 * The thread <var>T</var> is then removed from the wait set for this
467 * object and re-enabled for thread scheduling. It competes in the
468 * usual manner with other threads for the right to synchronize on the
469 * object; once it has regained control of the object, all its
470 * synchronization claims on the object are restored to the status quo
471 * ante - that is, to the situation as of the time that the {@code wait}
472 * method was invoked. Thread <var>T</var> then returns from the
473 * invocation of the {@code wait} method. Thus, on return from the
474 * {@code wait} method, the synchronization state of the object and of
475 * thread {@code T} is exactly as it was when the {@code wait} method
476 * was invoked.
477 * <p>
478 * A thread can wake up without being notified, interrupted, or timing out, a
479 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice,
480 * applications must guard against it by testing for the condition that should
481 * have caused the thread to be awakened, and continuing to wait if the condition
482 * is not satisfied. See the example below.
483 * <p>
484 * For more information on this topic, see section 14.2,
485 * "Condition Queues," in Brian Goetz and others' <cite>Java Concurrency
486 * in Practice</cite> (Addison-Wesley, 2006) or Item 81 in Joshua
487 * Bloch's <cite>Effective Java, Third Edition</cite> (Addison-Wesley,
488 * 2018).
489 * <p>
490 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
491 * by any thread before or while it is waiting, then an {@code InterruptedException}
492 * is thrown. The <em>interrupted status</em> of the current thread is cleared when
493 * this exception is thrown. This exception is not thrown until the lock status of
494 * this object has been restored as described above.
495 *
496 * @apiNote
497 * The recommended approach to waiting is to check the condition being awaited in
498 * a {@code while} loop around the call to {@code wait}, as shown in the example
499 * below. Among other things, this approach avoids problems that can be caused
500 * by spurious wakeups.
501 *
502 * {@snippet lang=java :
503 * synchronized (obj) {
504 * while ( <condition does not hold and timeout not exceeded> ) {
505 * long timeoutMillis = ... ; // recompute timeout values
506 * int nanos = ... ;
507 * obj.wait(timeoutMillis, nanos);
508 * }
509 * ... // Perform action appropriate to condition or timeout
510 * }
511 * }
512 *
513 * <div class="preview-block">
514 * <div class="preview-comment">
515 * If this object is a {@linkplain Class#isValue() value object},
516 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
517 * </div>
518 * </div>
519 * @param timeoutMillis the maximum time to wait, in milliseconds
520 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive
521 * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
522 * or if the value of {@code nanos} is out of range
523 * @throws IllegalMonitorStateException if the current thread is not
524 * the owner of the object's monitor or
525 * if this object is a {@linkplain Class#isValue() value object}.
526 * @throws InterruptedException if any thread interrupted the current thread before or
527 * while the current thread was waiting. The <em>interrupted status</em> of the
528 * current thread is cleared when this exception is thrown.
529 * @see #notify()
530 * @see #notifyAll()
531 * @see #wait()
532 * @see #wait(long)
533 */
534 public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
535 if (timeoutMillis < 0) {
536 throw new IllegalArgumentException("timeoutMillis value is negative");
537 }
538
539 if (nanos < 0 || nanos > 999999) {
540 throw new IllegalArgumentException(
541 "nanosecond timeout value out of range");
542 }
543
544 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
545 timeoutMillis++;
546 }
547
548 wait(timeoutMillis);
549 }
550
551 /**
552 * Called by the garbage collector on an object when garbage collection
553 * determines that there are no more references to the object.
554 * A subclass overrides the {@code finalize} method to dispose of
555 * system resources or to perform other cleanup.
556 * <p>
557 * <b>When running in a Java virtual machine in which finalization has been
558 * disabled or removed, the garbage collector will never call
559 * {@code finalize()}. In a Java virtual machine in which finalization is
560 * enabled, the garbage collector might call {@code finalize} only after an
561 * indefinite delay.</b>
562 * <p>
563 * The general contract of {@code finalize} is that it is invoked
564 * if and when the Java virtual
565 * machine has determined that there is no longer any
566 * means by which this object can be accessed by any thread that has
567 * not yet died, except as a result of an action taken by the
568 * finalization of some other object or class which is ready to be
569 * finalized. The {@code finalize} method may take any action, including
570 * making this object available again to other threads; the usual purpose
571 * of {@code finalize}, however, is to perform cleanup actions before
572 * the object is irrevocably discarded. For example, the finalize method
573 * for an object that represents an input/output connection might perform
574 * explicit I/O transactions to break the connection before the object is
575 * permanently discarded.
576 * <p>
577 * The {@code finalize} method of class {@code Object} performs no
578 * special action; it simply returns normally. Subclasses of
579 * {@code Object} may override this definition.
580 * <p>
581 * The Java programming language does not guarantee which thread will
582 * invoke the {@code finalize} method for any given object. It is
583 * guaranteed, however, that the thread that invokes finalize will not
584 * be holding any user-visible synchronization locks when finalize is
585 * invoked. If an uncaught exception is thrown by the finalize method,
586 * the exception is ignored and finalization of that object terminates.
587 * <p>
588 * After the {@code finalize} method has been invoked for an object, no
589 * further action is taken until the Java virtual machine has again
590 * determined that there is no longer any means by which this object can
591 * be accessed by any thread that has not yet died, including possible
592 * actions by other objects or classes which are ready to be finalized,
593 * at which point the object may be discarded.
594 * <p>
595 * The {@code finalize} method is never invoked more than once by a Java
596 * virtual machine for any given object.
597 * <p>
598 * Any exception thrown by the {@code finalize} method causes
599 * the finalization of this object to be halted, but is otherwise
600 * ignored.
601 *
602 * @apiNote
603 * Classes that embed non-heap resources have many options
604 * for cleanup of those resources. The class must ensure that the
605 * lifetime of each instance is longer than that of any resource it embeds.
606 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
607 * objects remain reachable while resources embedded in the object are in use.
608 * <p>
609 * A subclass should avoid overriding the {@code finalize} method
610 * unless the subclass embeds non-heap resources that must be cleaned up
611 * before the instance is collected.
612 * Finalizer invocations are not automatically chained, unlike constructors.
613 * If a subclass overrides {@code finalize} it must invoke the superclass
614 * finalizer explicitly.
615 * To guard against exceptions prematurely terminating the finalize chain,
616 * the subclass should use a {@code try-finally} block to ensure
617 * {@code super.finalize()} is always invoked. For example,
618 * {@snippet lang="java":
619 * @Override
620 * protected void finalize() throws Throwable {
621 * try {
622 * ... // cleanup subclass state
623 * } finally {
624 * super.finalize();
625 * }
626 * }
627 * }
628 *
629 * @deprecated Finalization is deprecated and subject to removal in a future
630 * release. The use of finalization can lead to problems with security,
631 * performance, and reliability.
632 * See <a href="https://openjdk.org/jeps/421">JEP 421</a> for
633 * discussion and alternatives.
634 * <p>
635 * Subclasses that override {@code finalize} to perform cleanup should use
636 * alternative cleanup mechanisms and remove the {@code finalize} method.
637 * Use {@link java.lang.ref.Cleaner} and
638 * {@link java.lang.ref.PhantomReference} as safer ways to release resources
639 * when an object becomes unreachable. Alternatively, add a {@code close}
640 * method to explicitly release resources, and implement
641 * {@code AutoCloseable} to enable use of the {@code try}-with-resources
642 * statement.
643 * <p>
644 * This method will remain in place until finalizers have been removed from
645 * most existing code.
646 *
647 * @throws Throwable the {@code Exception} raised by this method
648 * @see java.lang.ref.WeakReference
649 * @see java.lang.ref.PhantomReference
650 * @jls 12.6 Finalization of Class Instances
651 */
652 @Deprecated(since="9", forRemoval=true)
653 protected void finalize() throws Throwable { }
654 }