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