1 /*
  2  * Copyright (c) 1994, 2024, Oracle and/or its affiliates. All rights reserved.
  3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  4  *
  5  * This code is free software; you can redistribute it and/or modify it
  6  * under the terms of the GNU General Public License version 2 only, as
  7  * published by the Free Software Foundation.  Oracle designates this
  8  * particular file as subject to the "Classpath" exception as provided
  9  * by Oracle in the LICENSE file that accompanied this code.
 10  *
 11  * This code is distributed in the hope that it will be useful, but WITHOUT
 12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 14  * version 2 for more details (a copy is included in the LICENSE file that
 15  * accompanied this code).
 16  *
 17  * You should have received a copy of the GNU General Public License version
 18  * 2 along with this work; if not, write to the Free Software Foundation,
 19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 20  *
 21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 22  * or visit www.oracle.com if you need additional information or have any
 23  * questions.
 24  */
 25 
 26 package java.lang;
 27 
 28 import jdk.internal.misc.Blocker;
 29 import jdk.internal.vm.annotation.IntrinsicCandidate;
 30 
 31 /**
 32  * Class {@code Object} is the root of the class hierarchy.
 33  * Every class has {@code Object} as a superclass. All objects,
 34  * including arrays, implement the methods of this class.
 35  * <p>
 36  * Subclasses of {@code java.lang.Object} can be either an {@linkplain Class#isIdentity identity class}
 37  * or a {@linkplain Class#isValue value class}.
 38  * See {@jls The Java Language Specification 8.1.1.5 value Classes}.
 39  *
 40  * @see     java.lang.Class
 41  * @since   1.0
 42  */
 43 public class Object {
 44 
 45     /**
 46      * Constructs a new object.
 47      */
 48     @IntrinsicCandidate
 49     public Object() {}
 50 
 51     /**
 52      * Returns the runtime class of this {@code Object}. The returned
 53      * {@code Class} object is the object that is locked by {@code
 54      * static synchronized} methods of the represented class.
 55      *
 56      * <p><b>The actual result type is {@code Class<? extends |X|>}
 57      * where {@code |X|} is the erasure of the static type of the
 58      * expression on which {@code getClass} is called.</b> For
 59      * example, no cast is required in this code fragment:</p>
 60      *
 61      * <p>
 62      * {@code Number n = 0;                             }<br>
 63      * {@code Class<? extends Number> c = n.getClass(); }
 64      * </p>
 65      *
 66      * @return The {@code Class} object that represents the runtime
 67      *         class of this object.
 68      * @jls 15.8.2 Class Literals
 69      */
 70     @IntrinsicCandidate
 71     public final native Class<?> getClass();
 72 
 73     /**
 74      * {@return a hash code value for this object} This method is
 75      * supported for the benefit of hash tables such as those provided by
 76      * {@link java.util.HashMap}.
 77      * <p>
 78      * The general contract of {@code hashCode} is:
 79      * <ul>
 80      * <li>Whenever it is invoked on the same object more than once during
 81      *     an execution of a Java application, the {@code hashCode} method
 82      *     must consistently return the same integer, provided no information
 83      *     used in {@code equals} comparisons on the object is modified.
 84      *     This integer need not remain consistent from one execution of an
 85      *     application to another execution of the same application.
 86      * <li>If two objects are equal according to the {@link
 87      *     #equals(Object) equals} method, then calling the {@code
 88      *     hashCode} method on each of the two objects must produce the
 89      *     same integer result.
 90      * <li>It is <em>not</em> required that if two objects are unequal
 91      *     according to the {@link #equals(Object) equals} method, then
 92      *     calling the {@code hashCode} method on each of the two objects
 93      *     must produce distinct integer results.  However, the programmer
 94      *     should be aware that producing distinct integer results for
 95      *     unequal objects may improve the performance of hash tables.
 96      * </ul>
 97      *
 98      * @implSpec
 99      * As far as is reasonably practical, the {@code hashCode} method defined
100      * by class {@code Object} returns distinct integers for distinct objects.
101      *
102      * @apiNote
103      * The {@link java.util.Objects#hash(Object...) hash} and {@link
104      * java.util.Objects#hashCode(Object) hashCode} methods of {@link
105      * java.util.Objects} can be used to help construct simple hash codes.
106      *
107      * @see     java.lang.Object#equals(java.lang.Object)
108      * @see     java.lang.System#identityHashCode
109      */
110     @IntrinsicCandidate
111     public native int hashCode();
112 
113     /**
114      * Indicates whether some other object is "equal to" this one.
115      * <p>
116      * The {@code equals} method implements an equivalence relation
117      * on non-null object references:
118      * <ul>
119      * <li>It is <i>reflexive</i>: for any non-null reference value
120      *     {@code x}, {@code x.equals(x)} should return
121      *     {@code true}.
122      * <li>It is <i>symmetric</i>: for any non-null reference values
123      *     {@code x} and {@code y}, {@code x.equals(y)}
124      *     should return {@code true} if and only if
125      *     {@code y.equals(x)} returns {@code true}.
126      * <li>It is <i>transitive</i>: for any non-null reference values
127      *     {@code x}, {@code y}, and {@code z}, if
128      *     {@code x.equals(y)} returns {@code true} and
129      *     {@code y.equals(z)} returns {@code true}, then
130      *     {@code x.equals(z)} should return {@code true}.
131      * <li>It is <i>consistent</i>: for any non-null reference values
132      *     {@code x} and {@code y}, multiple invocations of
133      *     {@code x.equals(y)} consistently return {@code true}
134      *     or consistently return {@code false}, provided no
135      *     information used in {@code equals} comparisons on the
136      *     objects is modified.
137      * <li>For any non-null reference value {@code x},
138      *     {@code x.equals(null)} should return {@code false}.
139      * </ul>
140      *
141      * <p>
142      * An equivalence relation partitions the elements it operates on
143      * into <i>equivalence classes</i>; all the members of an
144      * equivalence class are equal to each other. Members of an
145      * equivalence class are substitutable for each other, at least
146      * for some purposes.
147      *
148      * @implSpec
149      * The {@code equals} method for class {@code Object} implements
150      * the most discriminating possible equivalence relation on objects;
151      * that is, for any non-null reference values {@code x} and
152      * {@code y}, this method returns {@code true} if and only
153      * if {@code x} and {@code y} refer to the same object
154      * ({@code x == y} has the value {@code true}).
155      *
156      * In other words, under the reference equality equivalence
157      * relation, each equivalence class only has a single element.
158      *
159      * @apiNote
160      * It is generally necessary to override the {@link #hashCode() hashCode}
161      * method whenever this method is overridden, so as to maintain the
162      * general contract for the {@code hashCode} method, which states
163      * that equal objects must have equal hash codes.
164      * <p>The two-argument {@link java.util.Objects#equals(Object,
165      * Object) Objects.equals} method implements an equivalence relation
166      * on two possibly-null object references.
167      *
168      * @param   obj   the reference object with which to compare.
169      * @return  {@code true} if this object is the same as the obj
170      *          argument; {@code false} otherwise.
171      * @see     #hashCode()
172      * @see     java.util.HashMap
173      */
174     public boolean equals(Object obj) {
175         return (this == obj);
176     }
177 
178     /**
179      * Creates and returns a copy of this object.  The precise meaning
180      * of "copy" may depend on the class of the object. The general
181      * intent is that, for any object {@code x}, the expression:
182      * <blockquote>
183      * <pre>
184      * x.clone() != x</pre></blockquote>
185      * will be true, and that the expression:
186      * <blockquote>
187      * <pre>
188      * x.clone().getClass() == x.getClass()</pre></blockquote>
189      * will be {@code true}, but these are not absolute requirements.
190      * While it is typically the case that:
191      * <blockquote>
192      * <pre>
193      * x.clone().equals(x)</pre></blockquote>
194      * will be {@code true}, this is not an absolute requirement.
195      * <p>
196      * By convention, the returned object should be obtained by calling
197      * {@code super.clone}.  If a class and all of its superclasses (except
198      * {@code Object}) obey this convention, it will be the case that
199      * {@code x.clone().getClass() == x.getClass()}.
200      * <p>
201      * By convention, the object returned by this method should be independent
202      * of this object (which is being cloned).  To achieve this independence,
203      * it may be necessary to modify one or more fields of the object returned
204      * by {@code super.clone} before returning it.  Typically, this means
205      * copying any mutable objects that comprise the internal "deep structure"
206      * of the object being cloned and replacing the references to these
207      * objects with references to the copies.  If a class contains only
208      * primitive fields or references to immutable objects, then it is usually
209      * the case that no fields in the object returned by {@code super.clone}
210      * need to be modified.
211      *
212      * @implSpec
213      * The method {@code clone} for class {@code Object} performs a
214      * specific cloning operation. First, if the class of this object does
215      * not implement the interface {@code Cloneable}, then a
216      * {@code CloneNotSupportedException} is thrown. Note that all arrays
217      * are considered to implement the interface {@code Cloneable} and that
218      * the return type of the {@code clone} method of an array type {@code T[]}
219      * is {@code T[]} where T is any reference or primitive type.
220      * Otherwise, this method creates a new instance of the class of this
221      * object and initializes all its fields with exactly the contents of
222      * the corresponding fields of this object, as if by assignment; the
223      * contents of the fields are not themselves cloned. Thus, this method
224      * performs a "shallow copy" of this object, not a "deep copy" operation.
225      * <p>
226      * The class {@code Object} does not itself implement the interface
227      * {@code Cloneable}, so calling the {@code clone} method on an object
228      * whose class is {@code Object} will result in throwing an
229      * exception at run time.
230      *
231      * @return     a clone of this instance.
232      * @throws  CloneNotSupportedException  if the object's class does not
233      *               support the {@code Cloneable} interface. Subclasses
234      *               that override the {@code clone} method can also
235      *               throw this exception to indicate that an instance cannot
236      *               be cloned.
237      * @see java.lang.Cloneable
238      */
239     @IntrinsicCandidate
240     protected native Object clone() throws CloneNotSupportedException;
241 
242     /**
243      * {@return a string representation of the object}
244      * @apiNote
245      * In general, the
246      * {@code toString} method returns a string that
247      * "textually represents" this object. The result should
248      * be a concise but informative representation that is easy for a
249      * person to read.
250      * It is recommended that all subclasses override this method.
251      * The string output is not necessarily stable over time or across
252      * JVM invocations.
253      * @implSpec
254      * The {@code toString} method for class {@code Object}
255      * returns a string consisting of the name of the class of which the
256      * object is an instance, the at-sign character `{@code @}', and
257      * the unsigned hexadecimal representation of the hash code of the
258      * object. In other words, this method returns a string equal to the
259      * value of:
260      * {@snippet lang=java :
261      * getClass().getName() + '@' + Integer.toHexString(hashCode())
262      * }
263      * The {@link java.util.Objects#toIdentityString(Object)
264      * Objects.toIdentityString} method returns the string for an
265      * object equal to the string that would be returned if neither
266      * the {@code toString} nor {@code hashCode} methods were
267      * overridden by the object's class.
268      */
269     public String toString() {
270         return getClass().getName() + "@" + Integer.toHexString(hashCode());
271     }
272 
273     /**
274      * Wakes up a single thread that is waiting on this object's
275      * monitor. If any threads are waiting on this object, one of them
276      * is chosen to be awakened. The choice is arbitrary and occurs at
277      * the discretion of the implementation. A thread waits on an object's
278      * monitor by calling one of the {@code wait} methods.
279      * <p>
280      * The awakened thread will not be able to proceed until the current
281      * thread relinquishes the lock on this object. The awakened thread will
282      * compete in the usual manner with any other threads that might be
283      * actively competing to synchronize on this object; for example, the
284      * awakened thread enjoys no reliable privilege or disadvantage in being
285      * the next thread to lock this object.
286      * <p>
287      * This method should only be called by a thread that is the owner
288      * of this object's monitor. A thread becomes the owner of the
289      * object's monitor in one of three ways:
290      * <ul>
291      * <li>By executing a synchronized instance method of that object.
292      * <li>By executing the body of a {@code synchronized} statement
293      *     that synchronizes on the object.
294      * <li>For objects of type {@code Class,} by executing a
295      *     static synchronized method of that class.
296      * </ul>
297      * <p>
298      * Only one thread at a time can own an object's monitor.
299      *
300      * @throws  IllegalMonitorStateException  if the current thread is not
301      *               the owner of this object's monitor.
302      * @see        java.lang.Object#notifyAll()
303      * @see        java.lang.Object#wait()
304      */
305     @IntrinsicCandidate
306     public final native void notify();
307 
308     /**
309      * Wakes up all threads that are waiting on this object's monitor. A
310      * thread waits on an object's monitor by calling one of the
311      * {@code wait} methods.
312      * <p>
313      * The awakened threads will not be able to proceed until the current
314      * thread relinquishes the lock on this object. The awakened threads
315      * will compete in the usual manner with any other threads that might
316      * be actively competing to synchronize on this object; for example,
317      * the awakened threads enjoy no reliable privilege or disadvantage in
318      * being the next thread to lock this object.
319      * <p>
320      * This method should only be called by a thread that is the owner
321      * of this object's monitor. See the {@code notify} method for a
322      * description of the ways in which a thread can become the owner of
323      * a monitor.
324      *
325      * @throws  IllegalMonitorStateException  if the current thread is not
326      *               the owner of this object's monitor.
327      * @see        java.lang.Object#notify()
328      * @see        java.lang.Object#wait()
329      */
330     @IntrinsicCandidate
331     public final native void notifyAll();
332 
333     /**
334      * Causes the current thread to wait until it is awakened, typically
335      * by being <em>notified</em> or <em>interrupted</em>.
336      * <p>
337      * In all respects, this method behaves as if {@code wait(0L, 0)}
338      * had been called. See the specification of the {@link #wait(long, int)} method
339      * for details.
340      *
341      * @throws IllegalMonitorStateException if the current thread is not
342      *         the owner of the object's monitor
343      * @throws InterruptedException if any thread interrupted the current thread before or
344      *         while the current thread was waiting. The <em>interrupted status</em> of the
345      *         current thread is cleared when this exception is thrown.
346      * @see    #notify()
347      * @see    #notifyAll()
348      * @see    #wait(long)
349      * @see    #wait(long, int)
350      */
351     public final void wait() throws InterruptedException {
352         wait(0L);
353     }
354 
355     /**
356      * Causes the current thread to wait until it is awakened, typically
357      * by being <em>notified</em> or <em>interrupted</em>, or until a
358      * certain amount of real time has elapsed.
359      * <p>
360      * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
361      * had been called. See the specification of the {@link #wait(long, int)} method
362      * for details.
363      *
364      * @param  timeoutMillis the maximum time to wait, in milliseconds
365      * @throws IllegalArgumentException if {@code timeoutMillis} is negative
366      * @throws IllegalMonitorStateException if the current thread is not
367      *         the owner of the object's monitor
368      * @throws InterruptedException if any thread interrupted the current thread before or
369      *         while the current thread was waiting. The <em>interrupted status</em> of the
370      *         current thread is cleared when this exception is thrown.
371      * @see    #notify()
372      * @see    #notifyAll()
373      * @see    #wait()
374      * @see    #wait(long, int)
375      */
376     public final void wait(long timeoutMillis) throws InterruptedException {
377         if (!Thread.currentThread().isVirtual()) {
378             wait0(timeoutMillis);
379             return;
380         }
381 
382         // virtual thread waiting
383         boolean attempted = Blocker.begin();
384         try {
385             wait0(timeoutMillis);
386         } catch (InterruptedException e) {
387             // virtual thread's interrupt status needs to be cleared
388             Thread.currentThread().getAndClearInterrupt();
389             throw e;
390         } finally {
391             Blocker.end(attempted);
392         }
393     }
394 
395     // final modifier so method not in vtable
396     private final native void wait0(long timeoutMillis) throws InterruptedException;
397 
398     /**
399      * Causes the current thread to wait until it is awakened, typically
400      * by being <em>notified</em> or <em>interrupted</em>, or until a
401      * certain amount of real time has elapsed.
402      * <p>
403      * The current thread must own this object's monitor lock. See the
404      * {@link #notify notify} method for a description of the ways in which
405      * a thread can become the owner of a monitor lock.
406      * <p>
407      * This method causes the current thread (referred to here as <var>T</var>) to
408      * place itself in the wait set for this object and then to relinquish any
409      * and all synchronization claims on this object. Note that only the locks
410      * on this object are relinquished; any other objects on which the current
411      * thread may be synchronized remain locked while the thread waits.
412      * <p>
413      * Thread <var>T</var> then becomes disabled for thread scheduling purposes
414      * and lies dormant until one of the following occurs:
415      * <ul>
416      * <li>Some other thread invokes the {@code notify} method for this
417      * object and thread <var>T</var> happens to be arbitrarily chosen as
418      * the thread to be awakened.
419      * <li>Some other thread invokes the {@code notifyAll} method for this
420      * object.
421      * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
422      * thread <var>T</var>.
423      * <li>The specified amount of real time has elapsed, more or less.
424      * The amount of real time, in nanoseconds, is given by the expression
425      * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
426      * are both zero, then real time is not taken into consideration and the
427      * thread waits until awakened by one of the other causes.
428      * <li>Thread <var>T</var> is awakened spuriously. (See below.)
429      * </ul>
430      * <p>
431      * The thread <var>T</var> is then removed from the wait set for this
432      * object and re-enabled for thread scheduling. It competes in the
433      * usual manner with other threads for the right to synchronize on the
434      * object; once it has regained control of the object, all its
435      * synchronization claims on the object are restored to the status quo
436      * ante - that is, to the situation as of the time that the {@code wait}
437      * method was invoked. Thread <var>T</var> then returns from the
438      * invocation of the {@code wait} method. Thus, on return from the
439      * {@code wait} method, the synchronization state of the object and of
440      * thread {@code T} is exactly as it was when the {@code wait} method
441      * was invoked.
442      * <p>
443      * A thread can wake up without being notified, interrupted, or timing out, a
444      * so-called <em>spurious wakeup</em>.  While this will rarely occur in practice,
445      * applications must guard against it by testing for the condition that should
446      * have caused the thread to be awakened, and continuing to wait if the condition
447      * is not satisfied. See the example below.
448      * <p>
449      * For more information on this topic, see section 14.2,
450      * "Condition Queues," in Brian Goetz and others' <cite>Java Concurrency
451      * in Practice</cite> (Addison-Wesley, 2006) or Item 81 in Joshua
452      * Bloch's <cite>Effective Java, Third Edition</cite> (Addison-Wesley,
453      * 2018).
454      * <p>
455      * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
456      * by any thread before or while it is waiting, then an {@code InterruptedException}
457      * is thrown.  The <em>interrupted status</em> of the current thread is cleared when
458      * this exception is thrown. This exception is not thrown until the lock status of
459      * this object has been restored as described above.
460      *
461      * @apiNote
462      * The recommended approach to waiting is to check the condition being awaited in
463      * a {@code while} loop around the call to {@code wait}, as shown in the example
464      * below. Among other things, this approach avoids problems that can be caused
465      * by spurious wakeups.
466      *
467      * {@snippet lang=java :
468      *     synchronized (obj) {
469      *         while ( <condition does not hold and timeout not exceeded> ) {
470      *             long timeoutMillis = ... ; // recompute timeout values
471      *             int nanos = ... ;
472      *             obj.wait(timeoutMillis, nanos);
473      *         }
474      *         ... // Perform action appropriate to condition or timeout
475      *     }
476      * }
477      *
478      * @param  timeoutMillis the maximum time to wait, in milliseconds
479      * @param  nanos   additional time, in nanoseconds, in the range 0-999999 inclusive
480      * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
481      *         or if the value of {@code nanos} is out of range
482      * @throws IllegalMonitorStateException if the current thread is not
483      *         the owner of the object's monitor
484      * @throws InterruptedException if any thread interrupted the current thread before or
485      *         while the current thread was waiting. The <em>interrupted status</em> of the
486      *         current thread is cleared when this exception is thrown.
487      * @see    #notify()
488      * @see    #notifyAll()
489      * @see    #wait()
490      * @see    #wait(long)
491      */
492     public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
493         if (timeoutMillis < 0) {
494             throw new IllegalArgumentException("timeoutMillis value is negative");
495         }
496 
497         if (nanos < 0 || nanos > 999999) {
498             throw new IllegalArgumentException(
499                                 "nanosecond timeout value out of range");
500         }
501 
502         if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
503             timeoutMillis++;
504         }
505 
506         wait(timeoutMillis);
507     }
508 
509     /**
510      * Called by the garbage collector on an object when garbage collection
511      * determines that there are no more references to the object.
512      * A subclass overrides the {@code finalize} method to dispose of
513      * system resources or to perform other cleanup.
514      * <p>
515      * <b>When running in a Java virtual machine in which finalization has been
516      * disabled or removed, the garbage collector will never call
517      * {@code finalize()}. In a Java virtual machine in which finalization is
518      * enabled, the garbage collector might call {@code finalize} only after an
519      * indefinite delay.</b>
520      * <p>
521      * The general contract of {@code finalize} is that it is invoked
522      * if and when the Java virtual
523      * machine has determined that there is no longer any
524      * means by which this object can be accessed by any thread that has
525      * not yet died, except as a result of an action taken by the
526      * finalization of some other object or class which is ready to be
527      * finalized. The {@code finalize} method may take any action, including
528      * making this object available again to other threads; the usual purpose
529      * of {@code finalize}, however, is to perform cleanup actions before
530      * the object is irrevocably discarded. For example, the finalize method
531      * for an object that represents an input/output connection might perform
532      * explicit I/O transactions to break the connection before the object is
533      * permanently discarded.
534      * <p>
535      * The {@code finalize} method of class {@code Object} performs no
536      * special action; it simply returns normally. Subclasses of
537      * {@code Object} may override this definition.
538      * <p>
539      * The Java programming language does not guarantee which thread will
540      * invoke the {@code finalize} method for any given object. It is
541      * guaranteed, however, that the thread that invokes finalize will not
542      * be holding any user-visible synchronization locks when finalize is
543      * invoked. If an uncaught exception is thrown by the finalize method,
544      * the exception is ignored and finalization of that object terminates.
545      * <p>
546      * After the {@code finalize} method has been invoked for an object, no
547      * further action is taken until the Java virtual machine has again
548      * determined that there is no longer any means by which this object can
549      * be accessed by any thread that has not yet died, including possible
550      * actions by other objects or classes which are ready to be finalized,
551      * at which point the object may be discarded.
552      * <p>
553      * The {@code finalize} method is never invoked more than once by a Java
554      * virtual machine for any given object.
555      * <p>
556      * Any exception thrown by the {@code finalize} method causes
557      * the finalization of this object to be halted, but is otherwise
558      * ignored.
559      *
560      * @apiNote
561      * Classes that embed non-heap resources have many options
562      * for cleanup of those resources. The class must ensure that the
563      * lifetime of each instance is longer than that of any resource it embeds.
564      * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
565      * objects remain reachable while resources embedded in the object are in use.
566      * <p>
567      * A subclass should avoid overriding the {@code finalize} method
568      * unless the subclass embeds non-heap resources that must be cleaned up
569      * before the instance is collected.
570      * Finalizer invocations are not automatically chained, unlike constructors.
571      * If a subclass overrides {@code finalize} it must invoke the superclass
572      * finalizer explicitly.
573      * To guard against exceptions prematurely terminating the finalize chain,
574      * the subclass should use a {@code try-finally} block to ensure
575      * {@code super.finalize()} is always invoked. For example,
576      * {@snippet lang="java":
577      *     @Override
578      *     protected void finalize() throws Throwable {
579      *         try {
580      *             ... // cleanup subclass state
581      *         } finally {
582      *             super.finalize();
583      *         }
584      *     }
585      * }
586      *
587      * @deprecated Finalization is deprecated and subject to removal in a future
588      * release. The use of finalization can lead to problems with security,
589      * performance, and reliability.
590      * See <a href="https://openjdk.org/jeps/421">JEP 421</a> for
591      * discussion and alternatives.
592      * <p>
593      * Subclasses that override {@code finalize} to perform cleanup should use
594      * alternative cleanup mechanisms and remove the {@code finalize} method.
595      * Use {@link java.lang.ref.Cleaner} and
596      * {@link java.lang.ref.PhantomReference} as safer ways to release resources
597      * when an object becomes unreachable. Alternatively, add a {@code close}
598      * method to explicitly release resources, and implement
599      * {@code AutoCloseable} to enable use of the {@code try}-with-resources
600      * statement.
601      * <p>
602      * This method will remain in place until finalizers have been removed from
603      * most existing code.
604      *
605      * @throws Throwable the {@code Exception} raised by this method
606      * @see java.lang.ref.WeakReference
607      * @see java.lang.ref.PhantomReference
608      * @jls 12.6 Finalization of Class Instances
609      */
610     @Deprecated(since="9", forRemoval=true)
611     protected void finalize() throws Throwable { }
612 }