1 /*

     2  * Copyright (c) 1994, 2017, 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.HotSpotIntrinsicCandidate;

    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  *

    35  * @author  unascribed

    36  * @see     java.lang.Class

    37  * @since   1.0

    38  */

    39 public class Object {

    40 

    41     private static native void registerNatives();

    42     static {

    43         registerNatives();

    44     }

    45 

    46     /**

    47      * Constructs a new object.

    48      */

    49     @HotSpotIntrinsicCandidate

    50     public Object() {}

    51 

    52     /**

    53      * Returns the runtime class of this {@code Object}. The returned

    54      * {@code Class} object is the object that is locked by {@code

    55      * static synchronized} methods of the represented class.

    56      *

    57      * <p><b>The actual result type is {@code Class<? extends |X|>}

    58      * where {@code |X|} is the erasure of the static type of the

    59      * expression on which {@code getClass} is called.</b> For

    60      * example, no cast is required in this code fragment:</p>

    61      *

    62      * <p>

    63      * {@code Number n = 0;                             }<br>

    64      * {@code Class<? extends Number> c = n.getClass(); }

    65      * </p>

    66      *

    67      * @return The {@code Class} object that represents the runtime

    68      *         class of this object.

    69      * @jls 15.8.2 Class Literals

    70      */

    71     @HotSpotIntrinsicCandidate

    72     public final native Class<?> getClass();

    73 

    74     /**

    75      * Returns a hash code value for the object. This method is

    76      * supported for the benefit of hash tables such as those provided by

    77      * {@link java.util.HashMap}.

    78      * <p>

    79      * The general contract of {@code hashCode} is:

    80      * <ul>

    81      * <li>Whenever it is invoked on the same object more than once during

    82      *     an execution of a Java application, the {@code hashCode} method

    83      *     must consistently return the same integer, provided no information

    84      *     used in {@code equals} comparisons on the object is modified.

    85      *     This integer need not remain consistent from one execution of an

    86      *     application to another execution of the same application.

    87      * <li>If two objects are equal according to the {@code equals(Object)}

    88      *     method, then calling the {@code hashCode} method on each of

    89      *     the two objects must produce the same integer result.

    90      * <li>It is <em>not</em> required that if two objects are unequal

    91      *     according to the {@link java.lang.Object#equals(java.lang.Object)}

    92      *     method, then calling the {@code hashCode} method on each of the

    93      *     two objects must produce distinct integer results.  However, the

    94      *     programmer should be aware that producing distinct integer results

    95      *     for unequal objects may improve the performance of hash tables.

    96      * </ul>

    97      * <p>

    98      * As much as is reasonably practical, the hashCode method defined

    99      * by class {@code Object} does return distinct integers for

   100      * distinct objects. (The hashCode may or may not be implemented

   101      * as some function of an object's memory address at some point

   102      * in time.)

   103      *

   104      * @return  a hash code value for this object.

   105      * @see     java.lang.Object#equals(java.lang.Object)

   106      * @see     java.lang.System#identityHashCode

   107      */

   108     @HotSpotIntrinsicCandidate

   109     public native int hashCode();

   110 

   111     /**

   112      * Indicates whether some other object is "equal to" this one.

   113      * <p>

   114      * The {@code equals} method implements an equivalence relation

   115      * on non-null object references:

   116      * <ul>

   117      * <li>It is <i>reflexive</i>: for any non-null reference value

   118      *     {@code x}, {@code x.equals(x)} should return

   119      *     {@code true}.

   120      * <li>It is <i>symmetric</i>: for any non-null reference values

   121      *     {@code x} and {@code y}, {@code x.equals(y)}

   122      *     should return {@code true} if and only if

   123      *     {@code y.equals(x)} returns {@code true}.

   124      * <li>It is <i>transitive</i>: for any non-null reference values

   125      *     {@code x}, {@code y}, and {@code z}, if

   126      *     {@code x.equals(y)} returns {@code true} and

   127      *     {@code y.equals(z)} returns {@code true}, then

   128      *     {@code x.equals(z)} should return {@code true}.

   129      * <li>It is <i>consistent</i>: for any non-null reference values

   130      *     {@code x} and {@code y}, multiple invocations of

   131      *     {@code x.equals(y)} consistently return {@code true}

   132      *     or consistently return {@code false}, provided no

   133      *     information used in {@code equals} comparisons on the

   134      *     objects is modified.

   135      * <li>For any non-null reference value {@code x},

   136      *     {@code x.equals(null)} should return {@code false}.

   137      * </ul>

   138      * <p>

   139      * The {@code equals} method for class {@code Object} implements

   140      * the most discriminating possible equivalence relation on objects;

   141      * that is, for any non-null reference values {@code x} and

   142      * {@code y}, this method returns {@code true} if and only

   143      * if {@code x} and {@code y} refer to the same object

   144      * ({@code x == y} has the value {@code true}).

   145      * <p>

   146      * Note that it is generally necessary to override the {@code hashCode}

   147      * method whenever this method is overridden, so as to maintain the

   148      * general contract for the {@code hashCode} method, which states

   149      * that equal objects must have equal hash codes.

   150      *

   151      * @param   obj   the reference object with which to compare.

   152      * @return  {@code true} if this object is the same as the obj

   153      *          argument; {@code false} otherwise.

   154      * @see     #hashCode()

   155      * @see     java.util.HashMap

   156      */

   157     public boolean equals(Object obj) {

   158         return (this == obj);

   159     }

   160 

   161     /**

   162      * Creates and returns a copy of this object.  The precise meaning

   163      * of "copy" may depend on the class of the object. The general

   164      * intent is that, for any object {@code x}, the expression:

   165      * <blockquote>

   166      * <pre>

   167      * x.clone() != x</pre></blockquote>

   168      * will be true, and that the expression:

   169      * <blockquote>

   170      * <pre>

   171      * x.clone().getClass() == x.getClass()</pre></blockquote>

   172      * will be {@code true}, but these are not absolute requirements.

   173      * While it is typically the case that:

   174      * <blockquote>

   175      * <pre>

   176      * x.clone().equals(x)</pre></blockquote>

   177      * will be {@code true}, this is not an absolute requirement.

   178      * <p>

   179      * By convention, the returned object should be obtained by calling

   180      * {@code super.clone}.  If a class and all of its superclasses (except

   181      * {@code Object}) obey this convention, it will be the case that

   182      * {@code x.clone().getClass() == x.getClass()}.

   183      * <p>

   184      * By convention, the object returned by this method should be independent

   185      * of this object (which is being cloned).  To achieve this independence,

   186      * it may be necessary to modify one or more fields of the object returned

   187      * by {@code super.clone} before returning it.  Typically, this means

   188      * copying any mutable objects that comprise the internal "deep structure"

   189      * of the object being cloned and replacing the references to these

   190      * objects with references to the copies.  If a class contains only

   191      * primitive fields or references to immutable objects, then it is usually

   192      * the case that no fields in the object returned by {@code super.clone}

   193      * need to be modified.

   194      * <p>

   195      * The method {@code clone} for class {@code Object} performs a

   196      * specific cloning operation. First, if the class of this object does

   197      * not implement the interface {@code Cloneable}, then a

   198      * {@code CloneNotSupportedException} is thrown. Note that all arrays

   199      * are considered to implement the interface {@code Cloneable} and that

   200      * the return type of the {@code clone} method of an array type {@code T[]}

   201      * is {@code T[]} where T is any reference or primitive type.

   202      * Otherwise, this method creates a new instance of the class of this

   203      * object and initializes all its fields with exactly the contents of

   204      * the corresponding fields of this object, as if by assignment; the

   205      * contents of the fields are not themselves cloned. Thus, this method

   206      * performs a "shallow copy" of this object, not a "deep copy" operation.

   207      * <p>

   208      * The class {@code Object} does not itself implement the interface

   209      * {@code Cloneable}, so calling the {@code clone} method on an object

   210      * whose class is {@code Object} will result in throwing an

   211      * exception at run time.

   212      *

   213      * @return     a clone of this instance.

   214      * @throws  CloneNotSupportedException  if the object's class does not

   215      *               support the {@code Cloneable} interface. Subclasses

   216      *               that override the {@code clone} method can also

   217      *               throw this exception to indicate that an instance cannot

   218      *               be cloned.

   219      * @see java.lang.Cloneable

   220      */

   221     @HotSpotIntrinsicCandidate

   222     protected native Object clone() throws CloneNotSupportedException;

   223 

   224     /**

   225      * Returns a string representation of the object. In general, the

   226      * {@code toString} method returns a string that

   227      * "textually represents" this object. The result should

   228      * be a concise but informative representation that is easy for a

   229      * person to read.

   230      * It is recommended that all subclasses override this method.

   231      * <p>

   232      * The {@code toString} method for class {@code Object}

   233      * returns a string consisting of the name of the class of which the

   234      * object is an instance, the at-sign character `{@code @}', and

   235      * the unsigned hexadecimal representation of the hash code of the

   236      * object. In other words, this method returns a string equal to the

   237      * value of:

   238      * <blockquote>

   239      * <pre>

   240      * getClass().getName() + '@' + Integer.toHexString(hashCode())

   241      * </pre></blockquote>

   242      *

   243      * @return  a string representation of the object.

   244      */

   245     public String toString() {

   246         return getClass().getName() + "@" + Integer.toHexString(hashCode());

   247     }

   248 

   249     /**

   250      * Wakes up a single thread that is waiting on this object's

   251      * monitor. If any threads are waiting on this object, one of them

   252      * is chosen to be awakened. The choice is arbitrary and occurs at

   253      * the discretion of the implementation. A thread waits on an object's

   254      * monitor by calling one of the {@code wait} methods.

   255      * <p>

   256      * The awakened thread will not be able to proceed until the current

   257      * thread relinquishes the lock on this object. The awakened thread will

   258      * compete in the usual manner with any other threads that might be

   259      * actively competing to synchronize on this object; for example, the

   260      * awakened thread enjoys no reliable privilege or disadvantage in being

   261      * the next thread to lock this object.

   262      * <p>

   263      * This method should only be called by a thread that is the owner

   264      * of this object's monitor. A thread becomes the owner of the

   265      * object's monitor in one of three ways:

   266      * <ul>

   267      * <li>By executing a synchronized instance method of that object.

   268      * <li>By executing the body of a {@code synchronized} statement

   269      *     that synchronizes on the object.

   270      * <li>For objects of type {@code Class,} by executing a

   271      *     synchronized static method of that class.

   272      * </ul>

   273      * <p>

   274      * Only one thread at a time can own an object's monitor.

   275      *

   276      * @throws  IllegalMonitorStateException  if the current thread is not

   277      *               the owner of this object's monitor.

   278      * @see        java.lang.Object#notifyAll()

   279      * @see        java.lang.Object#wait()

   280      */

   281     @HotSpotIntrinsicCandidate

   282     public final native void notify();

   283 

   284     /**

   285      * Wakes up all threads that are waiting on this object's monitor. A

   286      * thread waits on an object's monitor by calling one of the

   287      * {@code wait} methods.

   288      * <p>

   289      * The awakened threads will not be able to proceed until the current

   290      * thread relinquishes the lock on this object. The awakened threads

   291      * will compete in the usual manner with any other threads that might

   292      * be actively competing to synchronize on this object; for example,

   293      * the awakened threads enjoy no reliable privilege or disadvantage in

   294      * being the next thread to lock this object.

   295      * <p>

   296      * This method should only be called by a thread that is the owner

   297      * of this object's monitor. See the {@code notify} method for a

   298      * description of the ways in which a thread can become the owner of

   299      * a monitor.

   300      *

   301      * @throws  IllegalMonitorStateException  if the current thread is not

   302      *               the owner of this object's monitor.

   303      * @see        java.lang.Object#notify()

   304      * @see        java.lang.Object#wait()

   305      */

   306     @HotSpotIntrinsicCandidate

   307     public final native void notifyAll();

   308 

   309     /**

   310      * Causes the current thread to wait until either another thread invokes the

   311      * {@link java.lang.Object#notify()} method or the

   312      * {@link java.lang.Object#notifyAll()} method for this object, or a

   313      * specified amount of time has elapsed.

   314      * <p>

   315      * The current thread must own this object's monitor.

   316      * <p>

   317      * This method causes the current thread (call it <var>T</var>) to

   318      * place itself in the wait set for this object and then to relinquish

   319      * any and all synchronization claims on this object. Thread <var>T</var>

   320      * becomes disabled for thread scheduling purposes and lies dormant

   321      * until one of four things happens:

   322      * <ul>

   323      * <li>Some other thread invokes the {@code notify} method for this

   324      * object and thread <var>T</var> happens to be arbitrarily chosen as

   325      * the thread to be awakened.

   326      * <li>Some other thread invokes the {@code notifyAll} method for this

   327      * object.

   328      * <li>Some other thread {@linkplain Thread#interrupt() interrupts}

   329      * thread <var>T</var>.

   330      * <li>The specified amount of real time has elapsed, more or less.  If

   331      * {@code timeout} is zero, however, then real time is not taken into

   332      * consideration and the thread simply waits until notified.

   333      * </ul>

   334      * The thread <var>T</var> is then removed from the wait set for this

   335      * object and re-enabled for thread scheduling. It then competes in the

   336      * usual manner with other threads for the right to synchronize on the

   337      * object; once it has gained control of the object, all its

   338      * synchronization claims on the object are restored to the status quo

   339      * ante - that is, to the situation as of the time that the {@code wait}

   340      * method was invoked. Thread <var>T</var> then returns from the

   341      * invocation of the {@code wait} method. Thus, on return from the

   342      * {@code wait} method, the synchronization state of the object and of

   343      * thread {@code T} is exactly as it was when the {@code wait} method

   344      * was invoked.

   345      * <p>

   346      * A thread can also wake up without being notified, interrupted, or

   347      * timing out, a so-called <i>spurious wakeup</i>.  While this will rarely

   348      * occur in practice, applications must guard against it by testing for

   349      * the condition that should have caused the thread to be awakened, and

   350      * continuing to wait if the condition is not satisfied.  In other words,

   351      * waits should always occur in loops, like this one:

   352      * <pre>

   353      *     synchronized (obj) {

   354      *         while (<condition does not hold>)

   355      *             obj.wait(timeout);

   356      *         ... // Perform action appropriate to condition

   357      *     }

   358      * </pre>

   359      *

   360      * (For more information on this topic, see section 14.2,

   361      * Condition Queues, in Brian Goetz and others' "Java Concurrency

   362      * in Practice" (Addison-Wesley, 2006) or Item 69 in Joshua

   363      * Bloch's "Effective Java (Second Edition)" (Addison-Wesley,

   364      * 2008).

   365      *

   366      * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()

   367      * interrupted} by any thread before or while it is waiting, then an

   368      * {@code InterruptedException} is thrown.  This exception is not

   369      * thrown until the lock status of this object has been restored as

   370      * described above.

   371      *

   372      * <p>

   373      * Note that the {@code wait} method, as it places the current thread

   374      * into the wait set for this object, unlocks only this object; any

   375      * other objects on which the current thread may be synchronized remain

   376      * locked while the thread waits.

   377      * <p>

   378      * This method should only be called by a thread that is the owner

   379      * of this object's monitor. See the {@code notify} method for a

   380      * description of the ways in which a thread can become the owner of

   381      * a monitor.

   382      *

   383      * @param      timeout   the maximum time to wait in milliseconds.

   384      * @throws  IllegalArgumentException      if the value of timeout is

   385      *               negative.

   386      * @throws  IllegalMonitorStateException  if the current thread is not

   387      *               the owner of the object's monitor.

   388      * @throws  InterruptedException if any thread interrupted the

   389      *             current thread before or while the current thread

   390      *             was waiting for a notification.  The <i>interrupted

   391      *             status</i> of the current thread is cleared when

   392      *             this exception is thrown.

   393      * @see        java.lang.Object#notify()

   394      * @see        java.lang.Object#notifyAll()

   395      */

   396     public final native void wait(long timeout) throws InterruptedException;

   397 

   398     /**

   399      * Causes the current thread to wait until another thread invokes the

   400      * {@link java.lang.Object#notify()} method or the

   401      * {@link java.lang.Object#notifyAll()} method for this object, or

   402      * some other thread interrupts the current thread, or a certain

   403      * amount of real time has elapsed.

   404      * <p>

   405      * This method is similar to the {@code wait} method of one

   406      * argument, but it allows finer control over the amount of time to

   407      * wait for a notification before giving up. The amount of real time,

   408      * measured in nanoseconds, is given by:

   409      * <blockquote>

   410      * <pre>

   411      * 1000000*timeout+nanos</pre></blockquote>

   412      * <p>

   413      * In all other respects, this method does the same thing as the

   414      * method {@link #wait(long)} of one argument. In particular,

   415      * {@code wait(0, 0)} means the same thing as {@code wait(0)}.

   416      * <p>

   417      * The current thread must own this object's monitor. The thread

   418      * releases ownership of this monitor and waits until either of the

   419      * following two conditions has occurred:

   420      * <ul>

   421      * <li>Another thread notifies threads waiting on this object's monitor

   422      *     to wake up either through a call to the {@code notify} method

   423      *     or the {@code notifyAll} method.

   424      * <li>The timeout period, specified by {@code timeout}

   425      *     milliseconds plus {@code nanos} nanoseconds arguments, has

   426      *     elapsed.

   427      * </ul>

   428      * <p>

   429      * The thread then waits until it can re-obtain ownership of the

   430      * monitor and resumes execution.

   431      * <p>

   432      * As in the one argument version, interrupts and spurious wakeups are

   433      * possible, and this method should always be used in a loop:

   434      * <pre>

   435      *     synchronized (obj) {

   436      *         while (<condition does not hold>)

   437      *             obj.wait(timeout, nanos);

   438      *         ... // Perform action appropriate to condition

   439      *     }

   440      * </pre>

   441      * This method should only be called by a thread that is the owner

   442      * of this object's monitor. See the {@code notify} method for a

   443      * description of the ways in which a thread can become the owner of

   444      * a monitor.

   445      *

   446      * @param      timeout   the maximum time to wait in milliseconds.

   447      * @param      nanos      additional time, in nanoseconds range

   448      *                       0-999999.

   449      * @throws  IllegalArgumentException      if the value of timeout is

   450      *                      negative or the value of nanos is

   451      *                      not in the range 0-999999.

   452      * @throws  IllegalMonitorStateException  if the current thread is not

   453      *               the owner of this object's monitor.

   454      * @throws  InterruptedException if any thread interrupted the

   455      *             current thread before or while the current thread

   456      *             was waiting for a notification.  The <i>interrupted

   457      *             status</i> of the current thread is cleared when

   458      *             this exception is thrown.

   459      */

   460     public final void wait(long timeout, int nanos) throws InterruptedException {

   461         if (timeout < 0) {

   462             throw new IllegalArgumentException("timeout value is negative");

   463         }

   464 

   465         if (nanos < 0 || nanos > 999999) {

   466             throw new IllegalArgumentException(

   467                                 "nanosecond timeout value out of range");

   468         }

   469 

   470         if (nanos > 0) {

   471             timeout++;

   472         }

   473 

   474         wait(timeout);

   475     }

   476 

   477     /**

   478      * Causes the current thread to wait until another thread invokes the

   479      * {@link java.lang.Object#notify()} method or the

   480      * {@link java.lang.Object#notifyAll()} method for this object.

   481      * In other words, this method behaves exactly as if it simply

   482      * performs the call {@code wait(0)}.

   483      * <p>

   484      * The current thread must own this object's monitor. The thread

   485      * releases ownership of this monitor and waits until another thread

   486      * notifies threads waiting on this object's monitor to wake up

   487      * either through a call to the {@code notify} method or the

   488      * {@code notifyAll} method. The thread then waits until it can

   489      * re-obtain ownership of the monitor and resumes execution.

   490      * <p>

   491      * As in the one argument version, interrupts and spurious wakeups are

   492      * possible, and this method should always be used in a loop:

   493      * <pre>

   494      *     synchronized (obj) {

   495      *         while (<condition does not hold>)

   496      *             obj.wait();

   497      *         ... // Perform action appropriate to condition

   498      *     }

   499      * </pre>

   500      * This method should only be called by a thread that is the owner

   501      * of this object's monitor. See the {@code notify} method for a

   502      * description of the ways in which a thread can become the owner of

   503      * a monitor.

   504      *

   505      * @throws  IllegalMonitorStateException  if the current thread is not

   506      *               the owner of the object's monitor.

   507      * @throws  InterruptedException if any thread interrupted the

   508      *             current thread before or while the current thread

   509      *             was waiting for a notification.  The <i>interrupted

   510      *             status</i> of the current thread is cleared when

   511      *             this exception is thrown.

   512      * @see        java.lang.Object#notify()

   513      * @see        java.lang.Object#notifyAll()

   514      */

   515     public final void wait() throws InterruptedException {

   516         wait(0);

   517     }

   518 

   519     /**

   520      * Called by the garbage collector on an object when garbage collection

   521      * determines that there are no more references to the object.

   522      * A subclass overrides the {@code finalize} method to dispose of

   523      * system resources or to perform other cleanup.

   524      * <p>

   525      * The general contract of {@code finalize} is that it is invoked

   526      * if and when the Java™ virtual

   527      * machine has determined that there is no longer any

   528      * means by which this object can be accessed by any thread that has

   529      * not yet died, except as a result of an action taken by the

   530      * finalization of some other object or class which is ready to be

   531      * finalized. The {@code finalize} method may take any action, including

   532      * making this object available again to other threads; the usual purpose

   533      * of {@code finalize}, however, is to perform cleanup actions before

   534      * the object is irrevocably discarded. For example, the finalize method

   535      * for an object that represents an input/output connection might perform

   536      * explicit I/O transactions to break the connection before the object is

   537      * permanently discarded.

   538      * <p>

   539      * The {@code finalize} method of class {@code Object} performs no

   540      * special action; it simply returns normally. Subclasses of

   541      * {@code Object} may override this definition.

   542      * <p>

   543      * The Java programming language does not guarantee which thread will

   544      * invoke the {@code finalize} method for any given object. It is

   545      * guaranteed, however, that the thread that invokes finalize will not

   546      * be holding any user-visible synchronization locks when finalize is

   547      * invoked. If an uncaught exception is thrown by the finalize method,

   548      * the exception is ignored and finalization of that object terminates.

   549      * <p>

   550      * After the {@code finalize} method has been invoked for an object, no

   551      * further action is taken until the Java virtual machine has again

   552      * determined that there is no longer any means by which this object can

   553      * be accessed by any thread that has not yet died, including possible

   554      * actions by other objects or classes which are ready to be finalized,

   555      * at which point the object may be discarded.

   556      * <p>

   557      * The {@code finalize} method is never invoked more than once by a Java

   558      * virtual machine for any given object.

   559      * <p>

   560      * Any exception thrown by the {@code finalize} method causes

   561      * the finalization of this object to be halted, but is otherwise

   562      * ignored.

   563      *

   564      * @apiNote

   565      * Classes that embed non-heap resources have many options

   566      * for cleanup of those resources. The class must ensure that the

   567      * lifetime of each instance is longer than that of any resource it embeds.

   568      * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that

   569      * objects remain reachable while resources embedded in the object are in use.

   570      * <p>

   571      * A subclass should avoid overriding the {@code finalize} method

   572      * unless the subclass embeds non-heap resources that must be cleaned up

   573      * before the instance is collected.

   574      * Finalizer invocations are not automatically chained, unlike constructors.

   575      * If a subclass overrides {@code finalize} it must invoke the superclass

   576      * finalizer explicitly.

   577      * To guard against exceptions prematurely terminating the finalize chain,

   578      * the subclass should use a {@code try-finally} block to ensure

   579      * {@code super.finalize()} is always invoked. For example,

   580      * <pre>{@code      @Override

   581      *     protected void finalize() throws Throwable {

   582      *         try {

   583      *             ... // cleanup subclass state

   584      *         } finally {

   585      *             super.finalize();

   586      *         }

   587      *     }

   588      * }</pre>

   589      *

   590      * @deprecated The finalization mechanism is inherently problematic.

   591      * Finalization can lead to performance issues, deadlocks, and hangs.

   592      * Errors in finalizers can lead to resource leaks; there is no way to cancel

   593      * finalization if it is no longer necessary; and no ordering is specified

   594      * among calls to {@code finalize} methods of different objects.

   595      * Furthermore, there are no guarantees regarding the timing of finalization.

   596      * The {@code finalize} method might be called on a finalizable object

   597      * only after an indefinite delay, if at all.

   598      *

   599      * Classes whose instances hold non-heap resources should provide a method

   600      * to enable explicit release of those resources, and they should also

   601      * implement {@link AutoCloseable} if appropriate.

   602      * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference}

   603      * provide more flexible and efficient ways to release resources when an object

   604      * becomes unreachable.

   605      *

   606      * @throws Throwable the {@code Exception} raised by this method

   607      * @see java.lang.ref.WeakReference

   608      * @see java.lang.ref.PhantomReference

   609      * @jls 12.6 Finalization of Class Instances

   610      */

   611     @Deprecated(since="9")

   612     protected void finalize() throws Throwable { }

   613 }