1 /*
  2  * Copyright (c) 1994, 2023, 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  *
 36  * @see     java.lang.Class
 37  * @since   1.0
 38  */
 39 public class Object {
 40 
 41     /**
 42      * Constructs a new object.
 43      */
 44     @IntrinsicCandidate
 45     public Object() {}
 46 
 47     /**
 48      * Returns the runtime class of this {@code Object}. The returned
 49      * {@code Class} object is the object that is locked by {@code
 50      * static synchronized} methods of the represented class.
 51      *
 52      * <p><b>The actual result type is {@code Class<? extends |X|>}
 53      * where {@code |X|} is the erasure of the static type of the
 54      * expression on which {@code getClass} is called.</b> For
 55      * example, no cast is required in this code fragment:</p>
 56      *
 57      * <p>
 58      * {@code Number n = 0;                             }<br>
 59      * {@code Class<? extends Number> c = n.getClass(); }
 60      * </p>
 61      *
 62      * @return The {@code Class} object that represents the runtime
 63      *         class of this object.
 64      * @jls 15.8.2 Class Literals
 65      */
 66     @IntrinsicCandidate
 67     public final native Class<?> getClass();
 68 
 69     /**
 70      * {@return a hash code value for this object} This method is
 71      * supported for the benefit of hash tables such as those provided by
 72      * {@link java.util.HashMap}.
 73      * <p>
 74      * The general contract of {@code hashCode} is:
 75      * <ul>
 76      * <li>Whenever it is invoked on the same object more than once during
 77      *     an execution of a Java application, the {@code hashCode} method
 78      *     must consistently return the same integer, provided no information
 79      *     used in {@code equals} comparisons on the object is modified.
 80      *     This integer need not remain consistent from one execution of an
 81      *     application to another execution of the same application.
 82      * <li>If two objects are equal according to the {@link
 83      *     #equals(Object) equals} method, then calling the {@code
 84      *     hashCode} method on each of the two objects must produce the
 85      *     same integer result.
 86      * <li>It is <em>not</em> required that if two objects are unequal
 87      *     according to the {@link #equals(Object) equals} method, then
 88      *     calling the {@code hashCode} method on each of the two objects
 89      *     must produce distinct integer results.  However, the programmer
 90      *     should be aware that producing distinct integer results for
 91      *     unequal objects may improve the performance of hash tables.
 92      * </ul>
 93      *
 94      * @implSpec
 95      * As far as is reasonably practical, the {@code hashCode} method defined
 96      * by class {@code Object} returns distinct integers for distinct objects.
 97      *
 98      * @apiNote
 99      * The {@link java.util.Objects#hash(Object...) hash} and {@link
100      * java.util.Objects#hashCode(Object) hashCode} methods of {@link
101      * java.util.Objects} can be used to help construct simple hash codes.
102      *
103      * @see     java.lang.Object#equals(java.lang.Object)
104      * @see     java.lang.System#identityHashCode
105      */
106     @IntrinsicCandidate
107     public native int hashCode();
108 
109     /**
110      * Indicates whether some other object is "equal to" this one.
111      * <p>
112      * The {@code equals} method implements an equivalence relation
113      * on non-null object references:
114      * <ul>
115      * <li>It is <i>reflexive</i>: for any non-null reference value
116      *     {@code x}, {@code x.equals(x)} should return
117      *     {@code true}.
118      * <li>It is <i>symmetric</i>: for any non-null reference values
119      *     {@code x} and {@code y}, {@code x.equals(y)}
120      *     should return {@code true} if and only if
121      *     {@code y.equals(x)} returns {@code true}.
122      * <li>It is <i>transitive</i>: for any non-null reference values
123      *     {@code x}, {@code y}, and {@code z}, if
124      *     {@code x.equals(y)} returns {@code true} and
125      *     {@code y.equals(z)} returns {@code true}, then
126      *     {@code x.equals(z)} should return {@code true}.
127      * <li>It is <i>consistent</i>: for any non-null reference values
128      *     {@code x} and {@code y}, multiple invocations of
129      *     {@code x.equals(y)} consistently return {@code true}
130      *     or consistently return {@code false}, provided no
131      *     information used in {@code equals} comparisons on the
132      *     objects is modified.
133      * <li>For any non-null reference value {@code x},
134      *     {@code x.equals(null)} should return {@code false}.
135      * </ul>
136      *
137      * <p>
138      * An equivalence relation partitions the elements it operates on
139      * into <i>equivalence classes</i>; all the members of an
140      * equivalence class are equal to each other. Members of an
141      * equivalence class are substitutable for each other, at least
142      * for some purposes.
143      *
144      * @implSpec
145      * The {@code equals} method for class {@code Object} implements
146      * the most discriminating possible equivalence relation on objects;
147      * that is, for any non-null reference values {@code x} and
148      * {@code y}, this method returns {@code true} if and only
149      * if {@code x} and {@code y} refer to the same object
150      * ({@code x == y} has the value {@code true}).
151      *
152      * In other words, under the reference equality equivalence
153      * relation, each equivalence class only has a single element.
154      *
155      * @apiNote
156      * It is generally necessary to override the {@link #hashCode() hashCode}
157      * method whenever this method is overridden, so as to maintain the
158      * general contract for the {@code hashCode} method, which states
159      * that equal objects must have equal hash codes.
160      * <p>The two-argument {@link java.util.Objects#equals(Object,
161      * Object) Objects.equals} method implements an equivalence relation
162      * on two possibly-null object references.
163      *
164      * @param   obj   the reference object with which to compare.
165      * @return  {@code true} if this object is the same as the obj
166      *          argument; {@code false} otherwise.
167      * @see     #hashCode()
168      * @see     java.util.HashMap
169      */
170     public boolean equals(Object obj) {
171         return (this == obj);
172     }
173 
174     /**
175      * Creates and returns a copy of this object.  The precise meaning
176      * of "copy" may depend on the class of the object. The general
177      * intent is that, for any object {@code x}, the expression:
178      * <blockquote>
179      * <pre>
180      * x.clone() != x</pre></blockquote>
181      * will be true, and that the expression:
182      * <blockquote>
183      * <pre>
184      * x.clone().getClass() == x.getClass()</pre></blockquote>
185      * will be {@code true}, but these are not absolute requirements.
186      * While it is typically the case that:
187      * <blockquote>
188      * <pre>
189      * x.clone().equals(x)</pre></blockquote>
190      * will be {@code true}, this is not an absolute requirement.
191      * <p>
192      * By convention, the returned object should be obtained by calling
193      * {@code super.clone}.  If a class and all of its superclasses (except
194      * {@code Object}) obey this convention, it will be the case that
195      * {@code x.clone().getClass() == x.getClass()}.
196      * <p>
197      * By convention, the object returned by this method should be independent
198      * of this object (which is being cloned).  To achieve this independence,
199      * it may be necessary to modify one or more fields of the object returned
200      * by {@code super.clone} before returning it.  Typically, this means
201      * copying any mutable objects that comprise the internal "deep structure"
202      * of the object being cloned and replacing the references to these
203      * objects with references to the copies.  If a class contains only
204      * primitive fields or references to immutable objects, then it is usually
205      * the case that no fields in the object returned by {@code super.clone}
206      * need to be modified.
207      *
208      * @implSpec
209      * The method {@code clone} for class {@code Object} performs a
210      * specific cloning operation. First, if the class of this object does
211      * not implement the interface {@code Cloneable}, then a
212      * {@code CloneNotSupportedException} is thrown. Note that all arrays
213      * are considered to implement the interface {@code Cloneable} and that
214      * the return type of the {@code clone} method of an array type {@code T[]}
215      * is {@code T[]} where T is any reference or primitive type.
216      * Otherwise, this method creates a new instance of the class of this
217      * object and initializes all its fields with exactly the contents of
218      * the corresponding fields of this object, as if by assignment; the
219      * contents of the fields are not themselves cloned. Thus, this method
220      * performs a "shallow copy" of this object, not a "deep copy" operation.
221      * <p>
222      * The class {@code Object} does not itself implement the interface
223      * {@code Cloneable}, so calling the {@code clone} method on an object
224      * whose class is {@code Object} will result in throwing an
225      * exception at run time.
226      *
227      * @return     a clone of this instance.
228      * @throws  CloneNotSupportedException  if the object's class does not
229      *               support the {@code Cloneable} interface. Subclasses
230      *               that override the {@code clone} method can also
231      *               throw this exception to indicate that an instance cannot
232      *               be cloned.
233      * @see java.lang.Cloneable
234      */
235     @IntrinsicCandidate
236     protected native Object clone() throws CloneNotSupportedException;
237 
238     /**
239      * {@return a string representation of the object}
240      * @apiNote
241      * In general, the
242      * {@code toString} method returns a string that
243      * "textually represents" this object. The result should
244      * be a concise but informative representation that is easy for a
245      * person to read.
246      * It is recommended that all subclasses override this method.
247      * The string output is not necessarily stable over time or across
248      * JVM invocations.
249      * @implSpec
250      * The {@code toString} method for class {@code Object}
251      * returns a string consisting of the name of the class of which the
252      * object is an instance, the at-sign character `{@code @}', and
253      * the unsigned hexadecimal representation of the hash code of the
254      * object. In other words, this method returns a string equal to the
255      * value of:
256      * {@snippet lang=java :
257      * getClass().getName() + '@' + Integer.toHexString(hashCode())
258      * }
259      * The {@link java.util.Objects#toIdentityString(Object)
260      * Objects.toIdentityString} method returns the string for an
261      * object equal to the string that would be returned if neither
262      * the {@code toString} nor {@code hashCode} methods were
263      * overridden by the object's class.
264      */
265     public String toString() {
266         return getClass().getName() + "@" + Integer.toHexString(hashCode());
267     }
268 
269     /**
270      * Wakes up a single thread that is waiting on this object's
271      * monitor. If any threads are waiting on this object, one of them
272      * is chosen to be awakened. The choice is arbitrary and occurs at
273      * the discretion of the implementation. A thread waits on an object's
274      * monitor by calling one of the {@code wait} methods.
275      * <p>
276      * The awakened thread will not be able to proceed until the current
277      * thread relinquishes the lock on this object. The awakened thread will
278      * compete in the usual manner with any other threads that might be
279      * actively competing to synchronize on this object; for example, the
280      * awakened thread enjoys no reliable privilege or disadvantage in being
281      * the next thread to lock this object.
282      * <p>
283      * This method should only be called by a thread that is the owner
284      * of this object's monitor. A thread becomes the owner of the
285      * object's monitor in one of three ways:
286      * <ul>
287      * <li>By executing a synchronized instance method of that object.
288      * <li>By executing the body of a {@code synchronized} statement
289      *     that synchronizes on the object.
290      * <li>For objects of type {@code Class,} by executing a
291      *     static synchronized method of that class.
292      * </ul>
293      * <p>
294      * Only one thread at a time can own an object's monitor.
295      *
296      * @throws  IllegalMonitorStateException  if the current thread is not
297      *               the owner of this object's monitor.
298      * @see        java.lang.Object#notifyAll()
299      * @see        java.lang.Object#wait()
300      */
301     @IntrinsicCandidate
302     public final native void notify();
303 
304     /**
305      * Wakes up all threads that are waiting on this object's monitor. A
306      * thread waits on an object's monitor by calling one of the
307      * {@code wait} methods.
308      * <p>
309      * The awakened threads will not be able to proceed until the current
310      * thread relinquishes the lock on this object. The awakened threads
311      * will compete in the usual manner with any other threads that might
312      * be actively competing to synchronize on this object; for example,
313      * the awakened threads enjoy no reliable privilege or disadvantage in
314      * being the next thread to lock this object.
315      * <p>
316      * This method should only be called by a thread that is the owner
317      * of this object's monitor. See the {@code notify} method for a
318      * description of the ways in which a thread can become the owner of
319      * a monitor.
320      *
321      * @throws  IllegalMonitorStateException  if the current thread is not
322      *               the owner of this object's monitor.
323      * @see        java.lang.Object#notify()
324      * @see        java.lang.Object#wait()
325      */
326     @IntrinsicCandidate
327     public final native void notifyAll();
328 
329     /**
330      * Causes the current thread to wait until it is awakened, typically
331      * by being <em>notified</em> or <em>interrupted</em>.
332      * <p>
333      * In all respects, this method behaves as if {@code wait(0L, 0)}
334      * had been called. See the specification of the {@link #wait(long, int)} method
335      * for details.
336      *
337      * @throws IllegalMonitorStateException if the current thread is not
338      *         the owner of the object's monitor
339      * @throws InterruptedException if any thread interrupted the current thread before or
340      *         while the current thread was waiting. The <em>interrupted status</em> of the
341      *         current thread is cleared when this exception is thrown.
342      * @see    #notify()
343      * @see    #notifyAll()
344      * @see    #wait(long)
345      * @see    #wait(long, int)
346      */
347     public final void wait() throws InterruptedException {
348         wait(0L);
349     }
350 
351     /**
352      * Causes the current thread to wait until it is awakened, typically
353      * by being <em>notified</em> or <em>interrupted</em>, or until a
354      * certain amount of real time has elapsed.
355      * <p>
356      * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
357      * had been called. See the specification of the {@link #wait(long, int)} method
358      * for details.
359      *
360      * @param  timeoutMillis the maximum time to wait, in milliseconds
361      * @throws IllegalArgumentException if {@code timeoutMillis} is negative
362      * @throws IllegalMonitorStateException if the current thread is not
363      *         the owner of the object's monitor
364      * @throws InterruptedException if any thread interrupted the current thread before or
365      *         while the current thread was waiting. The <em>interrupted status</em> of the
366      *         current thread is cleared when this exception is thrown.
367      * @see    #notify()
368      * @see    #notifyAll()
369      * @see    #wait()
370      * @see    #wait(long, int)
371      */
372     public final void wait(long timeoutMillis) throws InterruptedException {
373         if (!Thread.currentThread().isVirtual()) {
374             wait0(timeoutMillis);
375             return;
376         }
377 
378         // virtual thread waiting
379         boolean attempted = Blocker.begin();
380         try {
381             wait0(timeoutMillis);
382         } catch (InterruptedException e) {
383             // virtual thread's interrupt status needs to be cleared
384             Thread.currentThread().getAndClearInterrupt();
385             throw e;
386         } finally {
387             Blocker.end(attempted);
388         }
389     }
390 
391     // final modifier as method not in vtable
392     private final native void wait0(long timeoutMillis) throws InterruptedException;
393 
394     /**
395      * Causes the current thread to wait until it is awakened, typically
396      * by being <em>notified</em> or <em>interrupted</em>, or until a
397      * certain amount of real time has elapsed.
398      * <p>
399      * The current thread must own this object's monitor lock. See the
400      * {@link #notify notify} method for a description of the ways in which
401      * a thread can become the owner of a monitor lock.
402      * <p>
403      * This method causes the current thread (referred to here as <var>T</var>) to
404      * place itself in the wait set for this object and then to relinquish any
405      * and all synchronization claims on this object. Note that only the locks
406      * on this object are relinquished; any other objects on which the current
407      * thread may be synchronized remain locked while the thread waits.
408      * <p>
409      * Thread <var>T</var> then becomes disabled for thread scheduling purposes
410      * and lies dormant until one of the following occurs:
411      * <ul>
412      * <li>Some other thread invokes the {@code notify} method for this
413      * object and thread <var>T</var> happens to be arbitrarily chosen as
414      * the thread to be awakened.
415      * <li>Some other thread invokes the {@code notifyAll} method for this
416      * object.
417      * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
418      * thread <var>T</var>.
419      * <li>The specified amount of real time has elapsed, more or less.
420      * The amount of real time, in nanoseconds, is given by the expression
421      * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
422      * are both zero, then real time is not taken into consideration and the
423      * thread waits until awakened by one of the other causes.
424      * <li>Thread <var>T</var> is awakened spuriously. (See below.)
425      * </ul>
426      * <p>
427      * The thread <var>T</var> is then removed from the wait set for this
428      * object and re-enabled for thread scheduling. It competes in the
429      * usual manner with other threads for the right to synchronize on the
430      * object; once it has regained control of the object, all its
431      * synchronization claims on the object are restored to the status quo
432      * ante - that is, to the situation as of the time that the {@code wait}
433      * method was invoked. Thread <var>T</var> then returns from the
434      * invocation of the {@code wait} method. Thus, on return from the
435      * {@code wait} method, the synchronization state of the object and of
436      * thread {@code T} is exactly as it was when the {@code wait} method
437      * was invoked.
438      * <p>
439      * A thread can wake up without being notified, interrupted, or timing out, a
440      * so-called <em>spurious wakeup</em>.  While this will rarely occur in practice,
441      * applications must guard against it by testing for the condition that should
442      * have caused the thread to be awakened, and continuing to wait if the condition
443      * is not satisfied. See the example below.
444      * <p>
445      * For more information on this topic, see section 14.2,
446      * "Condition Queues," in Brian Goetz and others' <cite>Java Concurrency
447      * in Practice</cite> (Addison-Wesley, 2006) or Item 81 in Joshua
448      * Bloch's <cite>Effective Java, Third Edition</cite> (Addison-Wesley,
449      * 2018).
450      * <p>
451      * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
452      * by any thread before or while it is waiting, then an {@code InterruptedException}
453      * is thrown.  The <em>interrupted status</em> of the current thread is cleared when
454      * this exception is thrown. This exception is not thrown until the lock status of
455      * this object has been restored as described above.
456      *
457      * @apiNote
458      * The recommended approach to waiting is to check the condition being awaited in
459      * a {@code while} loop around the call to {@code wait}, as shown in the example
460      * below. Among other things, this approach avoids problems that can be caused
461      * by spurious wakeups.
462      *
463      * {@snippet lang=java :
464      *     synchronized (obj) {
465      *         while ( <condition does not hold and timeout not exceeded> ) {
466      *             long timeoutMillis = ... ; // recompute timeout values
467      *             int nanos = ... ;
468      *             obj.wait(timeoutMillis, nanos);
469      *         }
470      *         ... // Perform action appropriate to condition or timeout
471      *     }
472      * }
473      *
474      * @param  timeoutMillis the maximum time to wait, in milliseconds
475      * @param  nanos   additional time, in nanoseconds, in the range 0-999999 inclusive
476      * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
477      *         or if the value of {@code nanos} is out of range
478      * @throws IllegalMonitorStateException if the current thread is not
479      *         the owner of the object's monitor
480      * @throws InterruptedException if any thread interrupted the current thread before or
481      *         while the current thread was waiting. The <em>interrupted status</em> of the
482      *         current thread is cleared when this exception is thrown.
483      * @see    #notify()
484      * @see    #notifyAll()
485      * @see    #wait()
486      * @see    #wait(long)
487      */
488     public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
489         if (timeoutMillis < 0) {
490             throw new IllegalArgumentException("timeoutMillis value is negative");
491         }
492 
493         if (nanos < 0 || nanos > 999999) {
494             throw new IllegalArgumentException(
495                                 "nanosecond timeout value out of range");
496         }
497 
498         if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
499             timeoutMillis++;
500         }
501 
502         wait(timeoutMillis);
503     }
504 
505     /**
506      * Called by the garbage collector on an object when garbage collection
507      * determines that there are no more references to the object.
508      * A subclass overrides the {@code finalize} method to dispose of
509      * system resources or to perform other cleanup.
510      * <p>
511      * <b>When running in a Java virtual machine in which finalization has been
512      * disabled or removed, the garbage collector will never call
513      * {@code finalize()}. In a Java virtual machine in which finalization is
514      * enabled, the garbage collector might call {@code finalize} only after an
515      * indefinite delay.</b>
516      * <p>
517      * The general contract of {@code finalize} is that it is invoked
518      * if and when the Java virtual
519      * machine has determined that there is no longer any
520      * means by which this object can be accessed by any thread that has
521      * not yet died, except as a result of an action taken by the
522      * finalization of some other object or class which is ready to be
523      * finalized. The {@code finalize} method may take any action, including
524      * making this object available again to other threads; the usual purpose
525      * of {@code finalize}, however, is to perform cleanup actions before
526      * the object is irrevocably discarded. For example, the finalize method
527      * for an object that represents an input/output connection might perform
528      * explicit I/O transactions to break the connection before the object is
529      * permanently discarded.
530      * <p>
531      * The {@code finalize} method of class {@code Object} performs no
532      * special action; it simply returns normally. Subclasses of
533      * {@code Object} may override this definition.
534      * <p>
535      * The Java programming language does not guarantee which thread will
536      * invoke the {@code finalize} method for any given object. It is
537      * guaranteed, however, that the thread that invokes finalize will not
538      * be holding any user-visible synchronization locks when finalize is
539      * invoked. If an uncaught exception is thrown by the finalize method,
540      * the exception is ignored and finalization of that object terminates.
541      * <p>
542      * After the {@code finalize} method has been invoked for an object, no
543      * further action is taken until the Java virtual machine has again
544      * determined that there is no longer any means by which this object can
545      * be accessed by any thread that has not yet died, including possible
546      * actions by other objects or classes which are ready to be finalized,
547      * at which point the object may be discarded.
548      * <p>
549      * The {@code finalize} method is never invoked more than once by a Java
550      * virtual machine for any given object.
551      * <p>
552      * Any exception thrown by the {@code finalize} method causes
553      * the finalization of this object to be halted, but is otherwise
554      * ignored.
555      *
556      * @apiNote
557      * Classes that embed non-heap resources have many options
558      * for cleanup of those resources. The class must ensure that the
559      * lifetime of each instance is longer than that of any resource it embeds.
560      * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
561      * objects remain reachable while resources embedded in the object are in use.
562      * <p>
563      * A subclass should avoid overriding the {@code finalize} method
564      * unless the subclass embeds non-heap resources that must be cleaned up
565      * before the instance is collected.
566      * Finalizer invocations are not automatically chained, unlike constructors.
567      * If a subclass overrides {@code finalize} it must invoke the superclass
568      * finalizer explicitly.
569      * To guard against exceptions prematurely terminating the finalize chain,
570      * the subclass should use a {@code try-finally} block to ensure
571      * {@code super.finalize()} is always invoked. For example,
572      * {@snippet lang="java":
573      *     @Override
574      *     protected void finalize() throws Throwable {
575      *         try {
576      *             ... // cleanup subclass state
577      *         } finally {
578      *             super.finalize();
579      *         }
580      *     }
581      * }
582      *
583      * @deprecated Finalization is deprecated and subject to removal in a future
584      * release. The use of finalization can lead to problems with security,
585      * performance, and reliability.
586      * See <a href="https://openjdk.org/jeps/421">JEP 421</a> for
587      * discussion and alternatives.
588      * <p>
589      * Subclasses that override {@code finalize} to perform cleanup should use
590      * alternative cleanup mechanisms and remove the {@code finalize} method.
591      * Use {@link java.lang.ref.Cleaner} and
592      * {@link java.lang.ref.PhantomReference} as safer ways to release resources
593      * when an object becomes unreachable. Alternatively, add a {@code close}
594      * method to explicitly release resources, and implement
595      * {@code AutoCloseable} to enable use of the {@code try}-with-resources
596      * statement.
597      * <p>
598      * This method will remain in place until finalizers have been removed from
599      * most existing code.
600      *
601      * @throws Throwable the {@code Exception} raised by this method
602      * @see java.lang.ref.WeakReference
603      * @see java.lang.ref.PhantomReference
604      * @jls 12.6 Finalization of Class Instances
605      */
606     @Deprecated(since="9", forRemoval=true)
607     protected void finalize() throws Throwable { }
608 }