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.AOTSafeClassInitializer;
 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  *
 36  * <div class="preview-block">
 37  *      <div class="preview-comment">
 38  *          When preview features are enabled, subclasses of {@code java.lang.Object} can be either
 39  *          an {@linkplain Class#isIdentity identity class} or a {@linkplain Class#isValue value class}.
 40  *          See {@jls The Java Language Specification 8.1.1.5 Value Classes}.
 41  *          Use of value class instances for synchronization, mutexes, or with
 42  *          {@linkplain java.lang.ref.Reference object references} result in
 43  *          {@link IdentityException}.
 44  *      </div>
 45  * </div>
 46  *
 47  * @see     java.lang.Class
 48  * @since   1.0
 49  */
 50 @AOTSafeClassInitializer // for hierarchy checks
 51 public class Object {
 52 
 53     /**
 54      * Constructs a new object.
 55      */
 56     @IntrinsicCandidate
 57     public Object() {}
 58 
 59     /**
 60      * Returns the runtime class of this {@code Object}. The returned
 61      * {@code Class} object is the object that is locked by {@code
 62      * static synchronized} methods of the represented class.
 63      *
 64      * <p><b>The actual result type is {@code Class<? extends |X|>}
 65      * where {@code |X|} is the erasure of the static type of the
 66      * expression on which {@code getClass} is called.</b> For
 67      * example, no cast is required in this code fragment:</p>
 68      *
 69      * <p>
 70      * {@code Number n = 0;                             }<br>
 71      * {@code Class<? extends Number> c = n.getClass(); }
 72      * </p>
 73      *
 74      * @return The {@code Class} object that represents the runtime
 75      *         class of this object.
 76      * @jls 15.8.2 Class Literals
 77      */
 78     @IntrinsicCandidate
 79     public final native Class<?> getClass();
 80 
 81     /**
 82      * {@return a hash code value for this object} This method is
 83      * supported for the benefit of hash tables such as those provided by
 84      * {@link java.util.HashMap}.
 85      * <p>
 86      * The general contract of {@code hashCode} is:
 87      * <ul>
 88      * <li>Whenever it is invoked on the same object more than once during
 89      *     an execution of a Java application, the {@code hashCode} method
 90      *     must consistently return the same integer, provided no information
 91      *     used in {@code equals} comparisons on the object is modified.
 92      *     This integer need not remain consistent from one execution of an
 93      *     application to another execution of the same application.
 94      * <li>If two objects are equal according to the {@link
 95      *     #equals(Object) equals} method, then calling the {@code
 96      *     hashCode} method on each of the two objects must produce the
 97      *     same integer result.
 98      * <li>It is <em>not</em> required that if two objects are unequal
 99      *     according to the {@link #equals(Object) equals} method, then
100      *     calling the {@code hashCode} method on each of the two objects
101      *     must produce distinct integer results.  However, the programmer
102      *     should be aware that producing distinct integer results for
103      *     unequal objects may improve the performance of hash tables.
104      * </ul>
105      *
106      * @implSpec
107      * As far as is reasonably practical, the {@code hashCode} method defined
108      * by class {@code Object} returns distinct integers for distinct objects.
109      *
110      * @apiNote
111      * The {@link java.util.Objects#hash(Object...) hash} and {@link
112      * java.util.Objects#hashCode(Object) hashCode} methods of {@link
113      * java.util.Objects} can be used to help construct simple hash codes.
114      *
115      * @see     java.lang.Object#equals(java.lang.Object)
116      * @see     java.lang.System#identityHashCode
117      */
118     @IntrinsicCandidate
119     public native int hashCode();
120 
121     /**
122      * Indicates whether some other object is "equal to" this one.
123      * <p>
124      * The {@code equals} method implements an <dfn>{@index "equivalence relation"}</dfn>
125      * on non-null object references:
126      * <ul>
127      * <li>It is <i>reflexive</i>: for any non-null reference value
128      *     {@code x}, {@code x.equals(x)} should return
129      *     {@code true}.
130      * <li>It is <i>symmetric</i>: for any non-null reference values
131      *     {@code x} and {@code y}, {@code x.equals(y)}
132      *     should return {@code true} if and only if
133      *     {@code y.equals(x)} returns {@code true}.
134      * <li>It is <i>transitive</i>: for any non-null reference values
135      *     {@code x}, {@code y}, and {@code z}, if
136      *     {@code x.equals(y)} returns {@code true} and
137      *     {@code y.equals(z)} returns {@code true}, then
138      *     {@code x.equals(z)} should return {@code true}.
139      * <li>It is <i>consistent</i>: for any non-null reference values
140      *     {@code x} and {@code y}, multiple invocations of
141      *     {@code x.equals(y)} consistently return {@code true}
142      *     or consistently return {@code false}, provided no
143      *     information used in {@code equals} comparisons on the
144      *     objects is modified.
145      * <li>For any non-null reference value {@code x},
146      *     {@code x.equals(null)} should return {@code false}.
147      * </ul>
148      *
149      * <p>
150      * An equivalence relation partitions the elements it operates on
151      * into <i>equivalence classes</i>; all the members of an
152      * equivalence class are equal to each other. Members of an
153      * equivalence class are substitutable for each other, at least
154      * for some purposes.
155      *
156      * @implSpec
157      * The {@code equals} method for class {@code Object} implements
158      * the most discriminating possible equivalence relation on objects;
159      * that is, for any non-null reference values {@code x} and
160      * {@code y}, this method returns {@code true} if and only
161      * if {@code x} and {@code y} refer to the same object
162      * ({@code x == y} has the value {@code true}).
163      *
164      * In other words, under the reference equality equivalence
165      * relation, each equivalence class only has a single element.
166      *
167      * @apiNote
168      * It is generally necessary to override the {@link #hashCode() hashCode}
169      * method whenever this method is overridden, so as to maintain the
170      * general contract for the {@code hashCode} method, which states
171      * that equal objects must have equal hash codes.
172      * <p>The two-argument {@link java.util.Objects#equals(Object,
173      * Object) Objects.equals} method implements an equivalence relation
174      * on two possibly-null object references.
175      *
176      * @param   obj   the reference object with which to compare.
177      * @return  {@code true} if this object is the same as the obj
178      *          argument; {@code false} otherwise.
179      * @see     #hashCode()
180      * @see     java.util.HashMap
181      */
182     public boolean equals(Object obj) {
183         return (this == obj);
184     }
185 
186     /**
187      * Creates and returns a copy of this object.  The precise meaning
188      * of "copy" may depend on the class of the object. The general
189      * intent is that, for any object {@code x}, the expression:
190      * <blockquote>
191      * <pre>
192      * x.clone() != x</pre></blockquote>
193      * will be true, and that the expression:
194      * <blockquote>
195      * <pre>
196      * x.clone().getClass() == x.getClass()</pre></blockquote>
197      * will be {@code true}, but these are not absolute requirements.
198      * While it is typically the case that:
199      * <blockquote>
200      * <pre>
201      * x.clone().equals(x)</pre></blockquote>
202      * will be {@code true}, this is not an absolute requirement.
203      * <p>
204      * By convention, the returned object should be obtained by calling
205      * {@code super.clone}.  If a class and all of its superclasses (except
206      * {@code Object}) obey this convention, it will be the case that
207      * {@code x.clone().getClass() == x.getClass()}.
208      * <p>
209      * By convention, the object returned by this method should be independent
210      * of this object (which is being cloned).  To achieve this independence,
211      * it may be necessary to modify one or more fields of the object returned
212      * by {@code super.clone} before returning it.  Typically, this means
213      * copying any mutable objects that comprise the internal "deep structure"
214      * of the object being cloned and replacing the references to these
215      * objects with references to the copies.  If a class contains only
216      * primitive fields or references to immutable objects, then it is usually
217      * the case that no fields in the object returned by {@code super.clone}
218      * need to be modified.
219      *
220      * @implSpec
221      * The method {@code clone} for class {@code Object} performs a
222      * specific cloning operation. First, if the class of this object does
223      * not implement the interface {@code Cloneable}, then a
224      * {@code CloneNotSupportedException} is thrown. Note that all arrays
225      * are considered to implement the interface {@code Cloneable} and that
226      * the return type of the {@code clone} method of an array type {@code T[]}
227      * is {@code T[]} where T is any reference or primitive type.
228      * Otherwise, this method creates a new instance of the class of this
229      * object and initializes all its fields with exactly the contents of
230      * the corresponding fields of this object, as if by assignment; the
231      * contents of the fields are not themselves cloned. Thus, this method
232      * performs a "shallow copy" of this object, not a "deep copy" operation.
233      * <p>
234      * The class {@code Object} does not itself implement the interface
235      * {@code Cloneable}, so calling the {@code clone} method on an object
236      * whose class is {@code Object} will result in throwing an
237      * exception at run time.
238      *
239      * @return     a clone of this instance.
240      * @throws  CloneNotSupportedException  if the object's class does not
241      *               support the {@code Cloneable} interface. Subclasses
242      *               that override the {@code clone} method can also
243      *               throw this exception to indicate that an instance cannot
244      *               be cloned.
245      * @see java.lang.Cloneable
246      */
247     @IntrinsicCandidate
248     protected native Object clone() throws CloneNotSupportedException;
249 
250     /**
251      * {@return a string representation of the object}
252      *
253      * Satisfying this method's contract implies a non-{@code null}
254      * result must be returned.
255      *
256      * @apiNote
257      * In general, the
258      * {@code toString} method returns a string that
259      * "textually represents" this object. The result should
260      * be a concise but informative representation that is easy for a
261      * person to read.
262      * It is recommended that all subclasses override this method.
263      * The string output is not necessarily stable over time or across
264      * JVM invocations.
265      * @implSpec
266      * The {@code toString} method for class {@code Object}
267      * returns a string consisting of the name of the class of which the
268      * object is an instance, the at-sign character `{@code @}', and
269      * the unsigned hexadecimal representation of the hash code of the
270      * object. In other words, this method returns a string equal to the
271      * value of:
272      * {@snippet lang=java :
273      * getClass().getName() + '@' + Integer.toHexString(hashCode())
274      * }
275      * The {@link java.util.Objects#toIdentityString(Object)
276      * Objects.toIdentityString} method returns the string for an
277      * object equal to the string that would be returned if neither
278      * the {@code toString} nor {@code hashCode} methods were
279      * overridden by the object's class.
280      */
281     public String toString() {
282         return getClass().getName() + "@" + Integer.toHexString(hashCode());
283     }
284 
285     /**
286      * Wakes up a single thread that is waiting on this object's
287      * monitor. If any threads are waiting on this object, one of them
288      * is chosen to be awakened. The choice is arbitrary and occurs at
289      * the discretion of the implementation. A thread waits on an object's
290      * monitor by calling one of the {@code wait} methods.
291      * <p>
292      * The awakened thread will not be able to proceed until the current
293      * thread relinquishes the lock on this object. The awakened thread will
294      * compete in the usual manner with any other threads that might be
295      * actively competing to synchronize on this object; for example, the
296      * awakened thread enjoys no reliable privilege or disadvantage in being
297      * the next thread to lock this object.
298      * <p>
299      * This method should only be called by a thread that is the owner
300      * of this object's monitor. A thread becomes the owner of the
301      * object's monitor in one of three ways:
302      * <ul>
303      * <li>By executing a synchronized instance method of that object.
304      * <li>By executing the body of a {@code synchronized} statement
305      *     that synchronizes on the object.
306      * <li>For objects of type {@code Class,} by executing a
307      *     static synchronized method of that class.
308      * </ul>
309      * <p>
310      * Only one thread at a time can own an object's monitor.
311      * <div class="preview-block">
312      *      <div class="preview-comment">
313      *          If this object is a {@linkplain Class#isValue() value object},
314      *          it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
315      *      </div>
316      * </div>
317      *
318      * @throws  IllegalMonitorStateException  if the current thread is not
319      *               the owner of this object's monitor or
320      *               if this object is a {@linkplain Class#isValue() value object}.
321      * @see        java.lang.Object#notifyAll()
322      * @see        java.lang.Object#wait()
323      */
324     @IntrinsicCandidate
325     public final native void notify();
326 
327     /**
328      * Wakes up all threads that are waiting on this object's monitor. A
329      * thread waits on an object's monitor by calling one of the
330      * {@code wait} methods.
331      * <p>
332      * The awakened threads will not be able to proceed until the current
333      * thread relinquishes the lock on this object. The awakened threads
334      * will compete in the usual manner with any other threads that might
335      * be actively competing to synchronize on this object; for example,
336      * the awakened threads enjoy no reliable privilege or disadvantage in
337      * being the next thread to lock this object.
338      * <p>
339      * This method should only be called by a thread that is the owner
340      * of this object's monitor. See the {@code notify} method for a
341      * description of the ways in which a thread can become the owner of
342      * a monitor.
343      *
344      * <div class="preview-block">
345      *      <div class="preview-comment">
346      *          If this object is a {@linkplain Class#isValue() value object},
347      *          it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
348      *      </div>
349      * </div>
350      *
351      * @throws  IllegalMonitorStateException  if the current thread is not
352      *               the owner of this object's monitor or
353      *               if this object is a {@linkplain Class#isValue() value object}.
354      * @see        java.lang.Object#notify()
355      * @see        java.lang.Object#wait()
356      */
357     @IntrinsicCandidate
358     public final native void notifyAll();
359 
360     /**
361      * Causes the current thread to wait until it is awakened, typically
362      * by being <em>notified</em> or <em>interrupted</em>.
363      * <p>
364      * In all respects, this method behaves as if {@code wait(0L, 0)}
365      * had been called. See the specification of the {@link #wait(long, int)} method
366      * for details.
367      *
368      * <div class="preview-block">
369      *      <div class="preview-comment">
370      *          If this object is a {@linkplain Class#isValue() value object},
371      *          it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
372      *      </div>
373      * </div>
374      *
375      * @throws IllegalMonitorStateException if the current thread is not
376      *         the owner of the object's monitor or
377      *         if this object is a {@linkplain Class#isValue() value object}.
378      * @throws InterruptedException if any thread interrupted the current thread before or
379      *         while the current thread was waiting. The <em>interrupted status</em> of the
380      *         current thread is cleared when this exception is thrown.
381      * @see    #notify()
382      * @see    #notifyAll()
383      * @see    #wait(long)
384      * @see    #wait(long, int)
385      */
386     public final void wait() throws InterruptedException {
387         wait(0L);
388     }
389 
390     /**
391      * Causes the current thread to wait until it is awakened, typically
392      * by being <em>notified</em> or <em>interrupted</em>, or until a
393      * certain amount of real time has elapsed.
394      * <p>
395      * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
396      * had been called. See the specification of the {@link #wait(long, int)} method
397      * for details.
398      *
399      * <div class="preview-block">
400      *      <div class="preview-comment">
401      *          If this object is a {@linkplain Class#isValue() value object},
402      *          it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
403      *      </div>
404      * </div>
405      *
406      * @param  timeoutMillis the maximum time to wait, in milliseconds
407      * @throws IllegalArgumentException if {@code timeoutMillis} is negative
408      * @throws IllegalMonitorStateException if the current thread is not
409      *         the owner of the object's monitor or
410      *         if this object is a {@linkplain Class#isValue() value object}.
411      * @throws InterruptedException if any thread interrupted the current thread before or
412      *         while the current thread was waiting. The <em>interrupted status</em> of the
413      *         current thread is cleared when this exception is thrown.
414      * @see    #notify()
415      * @see    #notifyAll()
416      * @see    #wait()
417      * @see    #wait(long, int)
418      */
419     public final void wait(long timeoutMillis) throws InterruptedException {
420         if (timeoutMillis < 0) {
421             throw new IllegalArgumentException("timeout value is negative");
422         }
423 
424         if (Thread.currentThread() instanceof VirtualThread vthread) {
425             try {
426                 wait0(timeoutMillis);
427             } catch (InterruptedException e) {
428                 // virtual thread's interrupt status needs to be cleared
429                 vthread.getAndClearInterrupt();
430                 throw e;
431             }
432         } else {
433             wait0(timeoutMillis);
434         }
435     }
436 
437     // final modifier so method not in vtable
438     private final native void wait0(long timeoutMillis) throws InterruptedException;
439 
440     /**
441      * Causes the current thread to wait until it is awakened, typically
442      * by being <em>notified</em> or <em>interrupted</em>, or until a
443      * certain amount of real time has elapsed.
444      * <p>
445      * The current thread must own this object's monitor lock. See the
446      * {@link #notify notify} method for a description of the ways in which
447      * a thread can become the owner of a monitor lock.
448      * <p>
449      * This method causes the current thread (referred to here as <var>T</var>) to
450      * place itself in the wait set for this object and then to relinquish any
451      * and all synchronization claims on this object. Note that only the locks
452      * on this object are relinquished; any other objects on which the current
453      * thread may be synchronized remain locked while the thread waits.
454      * <p>
455      * Thread <var>T</var> then becomes disabled for thread scheduling purposes
456      * and lies dormant until one of the following occurs:
457      * <ul>
458      * <li>Some other thread invokes the {@code notify} method for this
459      * object and thread <var>T</var> happens to be arbitrarily chosen as
460      * the thread to be awakened.
461      * <li>Some other thread invokes the {@code notifyAll} method for this
462      * object.
463      * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
464      * thread <var>T</var>.
465      * <li>The specified amount of real time has elapsed, more or less.
466      * The amount of real time, in nanoseconds, is given by the expression
467      * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
468      * are both zero, then real time is not taken into consideration and the
469      * thread waits until awakened by one of the other causes.
470      * <li>Thread <var>T</var> is awakened spuriously. (See below.)
471      * </ul>
472      * <p>
473      * The thread <var>T</var> is then removed from the wait set for this
474      * object and re-enabled for thread scheduling. It competes in the
475      * usual manner with other threads for the right to synchronize on the
476      * object; once it has regained control of the object, all its
477      * synchronization claims on the object are restored to the status quo
478      * ante - that is, to the situation as of the time that the {@code wait}
479      * method was invoked. Thread <var>T</var> then returns from the
480      * invocation of the {@code wait} method. Thus, on return from the
481      * {@code wait} method, the synchronization state of the object and of
482      * thread {@code T} is exactly as it was when the {@code wait} method
483      * was invoked.
484      * <p>
485      * A thread can wake up without being notified, interrupted, or timing out, a
486      * so-called <em>spurious wakeup</em>.  While this will rarely occur in practice,
487      * applications must guard against it by testing for the condition that should
488      * have caused the thread to be awakened, and continuing to wait if the condition
489      * is not satisfied. See the example below.
490      * <p>
491      * For more information on this topic, see section 14.2,
492      * "Condition Queues," in Brian Goetz and others' <cite>Java Concurrency
493      * in Practice</cite> (Addison-Wesley, 2006) or Item 81 in Joshua
494      * Bloch's <cite>Effective Java, Third Edition</cite> (Addison-Wesley,
495      * 2018).
496      * <p>
497      * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
498      * by any thread before or while it is waiting, then an {@code InterruptedException}
499      * is thrown.  The <em>interrupted status</em> of the current thread is cleared when
500      * this exception is thrown. This exception is not thrown until the lock status of
501      * this object has been restored as described above.
502      *
503      * @apiNote
504      * The recommended approach to waiting is to check the condition being awaited in
505      * a {@code while} loop around the call to {@code wait}, as shown in the example
506      * below. Among other things, this approach avoids problems that can be caused
507      * by spurious wakeups.
508      *
509      * {@snippet lang=java :
510      *     synchronized (obj) {
511      *         while ( <condition does not hold and timeout not exceeded> ) {
512      *             long timeoutMillis = ... ; // recompute timeout values
513      *             int nanos = ... ;
514      *             obj.wait(timeoutMillis, nanos);
515      *         }
516      *         ... // Perform action appropriate to condition or timeout
517      *     }
518      * }
519      *
520      * <div class="preview-block">
521      *      <div class="preview-comment">
522      *          If this object is a {@linkplain Class#isValue() value object},
523      *          it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown.
524      *      </div>
525      * </div>
526      * @param  timeoutMillis the maximum time to wait, in milliseconds
527      * @param  nanos   additional time, in nanoseconds, in the range 0-999999 inclusive
528      * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
529      *         or if the value of {@code nanos} is out of range
530      * @throws IllegalMonitorStateException if the current thread is not
531      *         the owner of the object's monitor or
532      *         if this object is a {@linkplain Class#isValue() value object}.
533      * @throws InterruptedException if any thread interrupted the current thread before or
534      *         while the current thread was waiting. The <em>interrupted status</em> of the
535      *         current thread is cleared when this exception is thrown.
536      * @see    #notify()
537      * @see    #notifyAll()
538      * @see    #wait()
539      * @see    #wait(long)
540      */
541     public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
542         if (timeoutMillis < 0) {
543             throw new IllegalArgumentException("timeoutMillis value is negative");
544         }
545 
546         if (nanos < 0 || nanos > 999999) {
547             throw new IllegalArgumentException(
548                                 "nanosecond timeout value out of range");
549         }
550 
551         if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
552             timeoutMillis++;
553         }
554 
555         wait(timeoutMillis);
556     }
557 
558     /**
559      * Called by the garbage collector on an object when garbage collection
560      * determines that there are no more references to the object.
561      * A subclass overrides the {@code finalize} method to dispose of
562      * system resources or to perform other cleanup.
563      * <p>
564      * <b>When running in a Java virtual machine in which finalization has been
565      * disabled or removed, the garbage collector will never call
566      * {@code finalize()}. In a Java virtual machine in which finalization is
567      * enabled, the garbage collector might call {@code finalize} only after an
568      * indefinite delay.</b>
569      * <p>
570      * The general contract of {@code finalize} is that it is invoked
571      * if and when the Java virtual
572      * machine has determined that there is no longer any
573      * means by which this object can be accessed by any thread that has
574      * not yet died, except as a result of an action taken by the
575      * finalization of some other object or class which is ready to be
576      * finalized. The {@code finalize} method may take any action, including
577      * making this object available again to other threads; the usual purpose
578      * of {@code finalize}, however, is to perform cleanup actions before
579      * the object is irrevocably discarded. For example, the finalize method
580      * for an object that represents an input/output connection might perform
581      * explicit I/O transactions to break the connection before the object is
582      * permanently discarded.
583      * <p>
584      * The {@code finalize} method of class {@code Object} performs no
585      * special action; it simply returns normally. Subclasses of
586      * {@code Object} may override this definition.
587      * <p>
588      * The Java programming language does not guarantee which thread will
589      * invoke the {@code finalize} method for any given object. It is
590      * guaranteed, however, that the thread that invokes finalize will not
591      * be holding any user-visible synchronization locks when finalize is
592      * invoked. If an uncaught exception is thrown by the finalize method,
593      * the exception is ignored and finalization of that object terminates.
594      * <p>
595      * After the {@code finalize} method has been invoked for an object, no
596      * further action is taken until the Java virtual machine has again
597      * determined that there is no longer any means by which this object can
598      * be accessed by any thread that has not yet died, including possible
599      * actions by other objects or classes which are ready to be finalized,
600      * at which point the object may be discarded.
601      * <p>
602      * The {@code finalize} method is never invoked more than once by a Java
603      * virtual machine for any given object.
604      * <p>
605      * Any exception thrown by the {@code finalize} method causes
606      * the finalization of this object to be halted, but is otherwise
607      * ignored.
608      *
609      * @apiNote
610      * Classes that embed non-heap resources have many options
611      * for cleanup of those resources. The class must ensure that the
612      * lifetime of each instance is longer than that of any resource it embeds.
613      * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
614      * objects remain reachable while resources embedded in the object are in use.
615      * <p>
616      * A subclass should avoid overriding the {@code finalize} method
617      * unless the subclass embeds non-heap resources that must be cleaned up
618      * before the instance is collected.
619      * Finalizer invocations are not automatically chained, unlike constructors.
620      * If a subclass overrides {@code finalize} it must invoke the superclass
621      * finalizer explicitly.
622      * To guard against exceptions prematurely terminating the finalize chain,
623      * the subclass should use a {@code try-finally} block to ensure
624      * {@code super.finalize()} is always invoked. For example,
625      * {@snippet lang="java":
626      *     @Override
627      *     protected void finalize() throws Throwable {
628      *         try {
629      *             ... // cleanup subclass state
630      *         } finally {
631      *             super.finalize();
632      *         }
633      *     }
634      * }
635      *
636      * @deprecated Finalization is deprecated and subject to removal in a future
637      * release. The use of finalization can lead to problems with security,
638      * performance, and reliability.
639      * See <a href="https://openjdk.org/jeps/421">JEP 421</a> for
640      * discussion and alternatives.
641      * <p>
642      * Subclasses that override {@code finalize} to perform cleanup should use
643      * alternative cleanup mechanisms and remove the {@code finalize} method.
644      * Use {@link java.lang.ref.Cleaner} and
645      * {@link java.lang.ref.PhantomReference} as safer ways to release resources
646      * when an object becomes unreachable. Alternatively, add a {@code close}
647      * method to explicitly release resources, and implement
648      * {@code AutoCloseable} to enable use of the {@code try}-with-resources
649      * statement.
650      * <p>
651      * This method will remain in place until finalizers have been removed from
652      * most existing code.
653      *
654      * @throws Throwable the {@code Exception} raised by this method
655      * @see java.lang.ref.WeakReference
656      * @see java.lang.ref.PhantomReference
657      * @jls 12.6 Finalization of Class Instances
658      */
659     @Deprecated(since="9", forRemoval=true)
660     protected void finalize() throws Throwable { }
661 }