1 /*
   2  * Copyright (c) 1994, 2019, 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      *
  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      * @return  a hash code value for this object.
 103      * @see     java.lang.Object#equals(java.lang.Object)
 104      * @see     java.lang.System#identityHashCode
 105      */
 106     @HotSpotIntrinsicCandidate
 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      * <p>
 137      * The {@code equals} method for class {@code Object} implements
 138      * the most discriminating possible equivalence relation on objects;
 139      * that is, for any non-null reference values {@code x} and
 140      * {@code y}, this method returns {@code true} if and only
 141      * if {@code x} and {@code y} refer to the same object
 142      * ({@code x == y} has the value {@code true}).
 143      * <p>
 144      * Note that it is generally necessary to override the {@code hashCode}
 145      * method whenever this method is overridden, so as to maintain the
 146      * general contract for the {@code hashCode} method, which states
 147      * that equal objects must have equal hash codes.
 148      *
 149      * @param   obj   the reference object with which to compare.
 150      * @return  {@code true} if this object is the same as the obj
 151      *          argument; {@code false} otherwise.
 152      * @see     #hashCode()
 153      * @see     java.util.HashMap
 154      */
 155     public boolean equals(Object obj) {
 156         return (this == obj);
 157     }
 158 
 159     /**
 160      * Creates and returns a copy of this object.  The precise meaning
 161      * of "copy" may depend on the class of the object. The general
 162      * intent is that, for any object {@code x}, the expression:
 163      * <blockquote>
 164      * <pre>
 165      * x.clone() != x</pre></blockquote>
 166      * will be true, and that the expression:
 167      * <blockquote>
 168      * <pre>
 169      * x.clone().getClass() == x.getClass()</pre></blockquote>
 170      * will be {@code true}, but these are not absolute requirements.
 171      * While it is typically the case that:
 172      * <blockquote>
 173      * <pre>
 174      * x.clone().equals(x)</pre></blockquote>
 175      * will be {@code true}, this is not an absolute requirement.
 176      * <p>
 177      * By convention, the returned object should be obtained by calling
 178      * {@code super.clone}.  If a class and all of its superclasses (except
 179      * {@code Object}) obey this convention, it will be the case that
 180      * {@code x.clone().getClass() == x.getClass()}.
 181      * <p>
 182      * By convention, the object returned by this method should be independent
 183      * of this object (which is being cloned).  To achieve this independence,
 184      * it may be necessary to modify one or more fields of the object returned
 185      * by {@code super.clone} before returning it.  Typically, this means
 186      * copying any mutable objects that comprise the internal "deep structure"
 187      * of the object being cloned and replacing the references to these
 188      * objects with references to the copies.  If a class contains only
 189      * primitive fields or references to immutable objects, then it is usually
 190      * the case that no fields in the object returned by {@code super.clone}
 191      * need to be modified.
 192      * <p>
 193      * The method {@code clone} for class {@code Object} performs a
 194      * specific cloning operation. First, if the class of this object does
 195      * not implement the interface {@code Cloneable}, then a
 196      * {@code CloneNotSupportedException} is thrown. Note that all arrays
 197      * are considered to implement the interface {@code Cloneable} and that
 198      * the return type of the {@code clone} method of an array type {@code T[]}
 199      * is {@code T[]} where T is any reference or primitive type.
 200      * Otherwise, this method creates a new instance of the class of this
 201      * object and initializes all its fields with exactly the contents of
 202      * the corresponding fields of this object, as if by assignment; the
 203      * contents of the fields are not themselves cloned. Thus, this method
 204      * performs a "shallow copy" of this object, not a "deep copy" operation.
 205      * <p>
 206      * The class {@code Object} does not itself implement the interface
 207      * {@code Cloneable}, so calling the {@code clone} method on an object
 208      * whose class is {@code Object} will result in throwing an
 209      * exception at run time.
 210      *
 211      * @return     a clone of this instance.
 212      * @throws  CloneNotSupportedException  if the object's class does not
 213      *               support the {@code Cloneable} interface. Subclasses
 214      *               that override the {@code clone} method can also
 215      *               throw this exception to indicate that an instance cannot
 216      *               be cloned.
 217      * @see java.lang.Cloneable
 218      */
 219     @HotSpotIntrinsicCandidate
 220     protected native Object clone() throws CloneNotSupportedException;
 221 
 222     /**
 223      * Returns a string representation of the object. In general, the
 224      * {@code toString} method returns a string that
 225      * "textually represents" this object. The result should
 226      * be a concise but informative representation that is easy for a
 227      * person to read.
 228      * It is recommended that all subclasses override this method.
 229      * <p>
 230      * The {@code toString} method for class {@code Object}
 231      * returns a string consisting of the name of the class of which the
 232      * object is an instance, the at-sign character `{@code @}', and
 233      * the unsigned hexadecimal representation of the hash code of the
 234      * object. In other words, this method returns a string equal to the
 235      * value of:
 236      * <blockquote>
 237      * <pre>
 238      * getClass().getName() + '@' + Integer.toHexString(hashCode())
 239      * </pre></blockquote>
 240      *
 241      * @return  a string representation of the object.
 242      */
 243     public String toString() {
 244         return getClass().getName() + "@" + Integer.toHexString(hashCode());
 245     }
 246 
 247     /**
 248      * Wakes up a single thread that is waiting on this object's
 249      * monitor. If any threads are waiting on this object, one of them
 250      * is chosen to be awakened. The choice is arbitrary and occurs at
 251      * the discretion of the implementation. A thread waits on an object's
 252      * monitor by calling one of the {@code wait} methods.
 253      * <p>
 254      * The awakened thread will not be able to proceed until the current
 255      * thread relinquishes the lock on this object. The awakened thread will
 256      * compete in the usual manner with any other threads that might be
 257      * actively competing to synchronize on this object; for example, the
 258      * awakened thread enjoys no reliable privilege or disadvantage in being
 259      * the next thread to lock this object.
 260      * <p>
 261      * This method should only be called by a thread that is the owner
 262      * of this object's monitor. A thread becomes the owner of the
 263      * object's monitor in one of three ways:
 264      * <ul>
 265      * <li>By executing a synchronized instance method of that object.
 266      * <li>By executing the body of a {@code synchronized} statement
 267      *     that synchronizes on the object.
 268      * <li>For objects of type {@code Class,} by executing a
 269      *     synchronized static method of that class.
 270      * </ul>
 271      * <p>
 272      * Only one thread at a time can own an object's monitor.
 273      *
 274      * @throws  IllegalMonitorStateException  if the current thread is not
 275      *               the owner of this object's monitor.
 276      * @see        java.lang.Object#notifyAll()
 277      * @see        java.lang.Object#wait()
 278      */
 279     @HotSpotIntrinsicCandidate
 280     public final native void notify();
 281 
 282     /**
 283      * Wakes up all threads that are waiting on this object's monitor. A
 284      * thread waits on an object's monitor by calling one of the
 285      * {@code wait} methods.
 286      * <p>
 287      * The awakened threads will not be able to proceed until the current
 288      * thread relinquishes the lock on this object. The awakened threads
 289      * will compete in the usual manner with any other threads that might
 290      * be actively competing to synchronize on this object; for example,
 291      * the awakened threads enjoy no reliable privilege or disadvantage in
 292      * being the next thread to lock this object.
 293      * <p>
 294      * This method should only be called by a thread that is the owner
 295      * of this object's monitor. See the {@code notify} method for a
 296      * description of the ways in which a thread can become the owner of
 297      * a monitor.
 298      *
 299      * @throws  IllegalMonitorStateException  if the current thread is not
 300      *               the owner of this object's monitor.
 301      * @see        java.lang.Object#notify()
 302      * @see        java.lang.Object#wait()
 303      */
 304     @HotSpotIntrinsicCandidate
 305     public final native void notifyAll();
 306 
 307     /**
 308      * Causes the current thread to wait until it is awakened, typically
 309      * by being <em>notified</em> or <em>interrupted</em>.
 310      * <p>
 311      * In all respects, this method behaves as if {@code wait(0L, 0)}
 312      * had been called. See the specification of the {@link #wait(long, int)} method
 313      * for details.
 314      *
 315      * @throws IllegalMonitorStateException if the current thread is not
 316      *         the owner of the object's monitor
 317      * @throws InterruptedException if any thread interrupted the current thread before or
 318      *         while the current thread was waiting. The <em>interrupted status</em> of the
 319      *         current thread is cleared when this exception is thrown.
 320      * @see    #notify()
 321      * @see    #notifyAll()
 322      * @see    #wait(long)
 323      * @see    #wait(long, int)
 324      */
 325     public final void wait() throws InterruptedException {
 326         wait(0L);
 327     }
 328 
 329     /**
 330      * Causes the current thread to wait until it is awakened, typically
 331      * by being <em>notified</em> or <em>interrupted</em>, or until a
 332      * certain amount of real time has elapsed.
 333      * <p>
 334      * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
 335      * had been called. See the specification of the {@link #wait(long, int)} method
 336      * for details.
 337      *
 338      * @param  timeoutMillis the maximum time to wait, in milliseconds
 339      * @throws IllegalArgumentException if {@code timeoutMillis} is negative
 340      * @throws IllegalMonitorStateException if the current thread is not
 341      *         the owner of the object's monitor
 342      * @throws InterruptedException if any thread interrupted the current thread before or
 343      *         while the current thread was waiting. The <em>interrupted status</em> of the
 344      *         current thread is cleared when this exception is thrown.
 345      * @see    #notify()
 346      * @see    #notifyAll()
 347      * @see    #wait()
 348      * @see    #wait(long, int)
 349      */
 350     public final void wait(long timeoutMillis) throws InterruptedException {
 351         try {
 352             wait0(timeoutMillis);
 353         } catch (InterruptedException e) {
 354             Fiber<?> fiber = Fiber.currentFiber();
 355             if (fiber != null) {
 356                 fiber.isInterrupted(true);
 357             }
 358             throw e;
 359         }
 360     }
 361 
 362     // final modifier so method not in vtable
 363     private final native void wait0(long timeoutMillis) throws InterruptedException;
 364 
 365     /**
 366      * Causes the current thread to wait until it is awakened, typically
 367      * by being <em>notified</em> or <em>interrupted</em>, or until a
 368      * certain amount of real time has elapsed.
 369      * <p>
 370      * The current thread must own this object's monitor lock. See the
 371      * {@link #notify notify} method for a description of the ways in which
 372      * a thread can become the owner of a monitor lock.
 373      * <p>
 374      * This method causes the current thread (referred to here as <var>T</var>) to
 375      * place itself in the wait set for this object and then to relinquish any
 376      * and all synchronization claims on this object. Note that only the locks
 377      * on this object are relinquished; any other objects on which the current
 378      * thread may be synchronized remain locked while the thread waits.
 379      * <p>
 380      * Thread <var>T</var> then becomes disabled for thread scheduling purposes
 381      * and lies dormant until one of the following occurs:
 382      * <ul>
 383      * <li>Some other thread invokes the {@code notify} method for this
 384      * object and thread <var>T</var> happens to be arbitrarily chosen as
 385      * the thread to be awakened.
 386      * <li>Some other thread invokes the {@code notifyAll} method for this
 387      * object.
 388      * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
 389      * thread <var>T</var>.
 390      * <li>The specified amount of real time has elapsed, more or less.
 391      * The amount of real time, in nanoseconds, is given by the expression
 392      * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
 393      * are both zero, then real time is not taken into consideration and the
 394      * thread waits until awakened by one of the other causes.
 395      * <li>Thread <var>T</var> is awakened spuriously. (See below.)
 396      * </ul>
 397      * <p>
 398      * The thread <var>T</var> is then removed from the wait set for this
 399      * object and re-enabled for thread scheduling. It competes in the
 400      * usual manner with other threads for the right to synchronize on the
 401      * object; once it has regained control of the object, all its
 402      * synchronization claims on the object are restored to the status quo
 403      * ante - that is, to the situation as of the time that the {@code wait}
 404      * method was invoked. Thread <var>T</var> then returns from the
 405      * invocation of the {@code wait} method. Thus, on return from the
 406      * {@code wait} method, the synchronization state of the object and of
 407      * thread {@code T} is exactly as it was when the {@code wait} method
 408      * was invoked.
 409      * <p>
 410      * A thread can wake up without being notified, interrupted, or timing out, a
 411      * so-called <em>spurious wakeup</em>.  While this will rarely occur in practice,
 412      * applications must guard against it by testing for the condition that should
 413      * have caused the thread to be awakened, and continuing to wait if the condition
 414      * is not satisfied. See the example below.
 415      * <p>
 416      * For more information on this topic, see section 14.2,
 417      * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency
 418      * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua
 419      * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley,
 420      * 2008).
 421      * <p>
 422      * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
 423      * by any thread before or while it is waiting, then an {@code InterruptedException}
 424      * is thrown.  The <em>interrupted status</em> of the current thread is cleared when
 425      * this exception is thrown. This exception is not thrown until the lock status of
 426      * this object has been restored as described above.
 427      *
 428      * @apiNote
 429      * The recommended approach to waiting is to check the condition being awaited in
 430      * a {@code while} loop around the call to {@code wait}, as shown in the example
 431      * below. Among other things, this approach avoids problems that can be caused
 432      * by spurious wakeups.
 433      *
 434      * <pre>{@code
 435      *     synchronized (obj) {
 436      *         while (<condition does not hold> and <timeout not exceeded>) {
 437      *             long timeoutMillis = ... ; // recompute timeout values
 438      *             int nanos = ... ;
 439      *             obj.wait(timeoutMillis, nanos);
 440      *         }
 441      *         ... // Perform action appropriate to condition or timeout
 442      *     }
 443      * }</pre>
 444      *
 445      * @param  timeoutMillis the maximum time to wait, in milliseconds
 446      * @param  nanos   additional time, in nanoseconds, in the range 0-999999 inclusive
 447      * @throws IllegalArgumentException if {@code timeoutMillis} is negative,
 448      *         or if the value of {@code nanos} is out of range
 449      * @throws IllegalMonitorStateException if the current thread is not
 450      *         the owner of the object's monitor
 451      * @throws InterruptedException if any thread interrupted the current thread before or
 452      *         while the current thread was waiting. The <em>interrupted status</em> of the
 453      *         current thread is cleared when this exception is thrown.
 454      * @see    #notify()
 455      * @see    #notifyAll()
 456      * @see    #wait()
 457      * @see    #wait(long)
 458      */
 459     public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
 460         if (timeoutMillis < 0) {
 461             throw new IllegalArgumentException("timeoutMillis value is negative");
 462         }
 463 
 464         if (nanos < 0 || nanos > 999999) {
 465             throw new IllegalArgumentException(
 466                                 "nanosecond timeout value out of range");
 467         }
 468 
 469         if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) {
 470             timeoutMillis++;
 471         }
 472 
 473         wait(timeoutMillis);
 474     }
 475 
 476     /**
 477      * Called by the garbage collector on an object when garbage collection
 478      * determines that there are no more references to the object.
 479      * A subclass overrides the {@code finalize} method to dispose of
 480      * system resources or to perform other cleanup.
 481      * <p>
 482      * The general contract of {@code finalize} is that it is invoked
 483      * if and when the Java&trade; virtual
 484      * machine has determined that there is no longer any
 485      * means by which this object can be accessed by any thread that has
 486      * not yet died, except as a result of an action taken by the
 487      * finalization of some other object or class which is ready to be
 488      * finalized. The {@code finalize} method may take any action, including
 489      * making this object available again to other threads; the usual purpose
 490      * of {@code finalize}, however, is to perform cleanup actions before
 491      * the object is irrevocably discarded. For example, the finalize method
 492      * for an object that represents an input/output connection might perform
 493      * explicit I/O transactions to break the connection before the object is
 494      * permanently discarded.
 495      * <p>
 496      * The {@code finalize} method of class {@code Object} performs no
 497      * special action; it simply returns normally. Subclasses of
 498      * {@code Object} may override this definition.
 499      * <p>
 500      * The Java programming language does not guarantee which thread will
 501      * invoke the {@code finalize} method for any given object. It is
 502      * guaranteed, however, that the thread that invokes finalize will not
 503      * be holding any user-visible synchronization locks when finalize is
 504      * invoked. If an uncaught exception is thrown by the finalize method,
 505      * the exception is ignored and finalization of that object terminates.
 506      * <p>
 507      * After the {@code finalize} method has been invoked for an object, no
 508      * further action is taken until the Java virtual machine has again
 509      * determined that there is no longer any means by which this object can
 510      * be accessed by any thread that has not yet died, including possible
 511      * actions by other objects or classes which are ready to be finalized,
 512      * at which point the object may be discarded.
 513      * <p>
 514      * The {@code finalize} method is never invoked more than once by a Java
 515      * virtual machine for any given object.
 516      * <p>
 517      * Any exception thrown by the {@code finalize} method causes
 518      * the finalization of this object to be halted, but is otherwise
 519      * ignored.
 520      *
 521      * @apiNote
 522      * Classes that embed non-heap resources have many options
 523      * for cleanup of those resources. The class must ensure that the
 524      * lifetime of each instance is longer than that of any resource it embeds.
 525      * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
 526      * objects remain reachable while resources embedded in the object are in use.
 527      * <p>
 528      * A subclass should avoid overriding the {@code finalize} method
 529      * unless the subclass embeds non-heap resources that must be cleaned up
 530      * before the instance is collected.
 531      * Finalizer invocations are not automatically chained, unlike constructors.
 532      * If a subclass overrides {@code finalize} it must invoke the superclass
 533      * finalizer explicitly.
 534      * To guard against exceptions prematurely terminating the finalize chain,
 535      * the subclass should use a {@code try-finally} block to ensure
 536      * {@code super.finalize()} is always invoked. For example,
 537      * <pre>{@code      @Override
 538      *     protected void finalize() throws Throwable {
 539      *         try {
 540      *             ... // cleanup subclass state
 541      *         } finally {
 542      *             super.finalize();
 543      *         }
 544      *     }
 545      * }</pre>
 546      *
 547      * @deprecated The finalization mechanism is inherently problematic.
 548      * Finalization can lead to performance issues, deadlocks, and hangs.
 549      * Errors in finalizers can lead to resource leaks; there is no way to cancel
 550      * finalization if it is no longer necessary; and no ordering is specified
 551      * among calls to {@code finalize} methods of different objects.
 552      * Furthermore, there are no guarantees regarding the timing of finalization.
 553      * The {@code finalize} method might be called on a finalizable object
 554      * only after an indefinite delay, if at all.
 555      *
 556      * Classes whose instances hold non-heap resources should provide a method
 557      * to enable explicit release of those resources, and they should also
 558      * implement {@link AutoCloseable} if appropriate.
 559      * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference}
 560      * provide more flexible and efficient ways to release resources when an object
 561      * becomes unreachable.
 562      *
 563      * @throws Throwable the {@code Exception} raised by this method
 564      * @see java.lang.ref.WeakReference
 565      * @see java.lang.ref.PhantomReference
 566      * @jls 12.6 Finalization of Class Instances
 567      */
 568     @Deprecated(since="9")
 569     protected void finalize() throws Throwable { }
 570 }