1 /* 2 * Copyright (c) 1994, 2024, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26 package java.lang; 27 28 import jdk.internal.vm.annotation.AOTSafeClassInitializer; 29 import jdk.internal.vm.annotation.IntrinsicCandidate; 30 31 /** 32 * Class {@code Object} is the root of the class hierarchy. 33 * Every class has {@code Object} as a superclass. All objects, 34 * including arrays, implement the methods of this class. 35 * 36 * <div class="preview-block"> 37 * <div class="preview-comment"> 38 * When preview features are enabled, subclasses of {@code java.lang.Object} can be either 39 * an {@linkplain Class#isIdentity identity class} or a {@linkplain Class#isValue value class}. 40 * See {@jls The Java Language Specification 8.1.1.5 Value Classes}. 41 * Use of value class instances for synchronization, mutexes, or with 42 * {@linkplain java.lang.ref.Reference object references} result in 43 * {@link IdentityException}. 44 * </div> 45 * </div> 46 * 47 * @see java.lang.Class 48 * @since 1.0 49 */ 50 @AOTSafeClassInitializer // for hierarchy checks 51 public class Object { 52 53 /** 54 * Constructs a new object. 55 */ 56 @IntrinsicCandidate 57 public Object() {} 58 59 /** 60 * Returns the runtime class of this {@code Object}. The returned 61 * {@code Class} object is the object that is locked by {@code 62 * static synchronized} methods of the represented class. 63 * 64 * <p><b>The actual result type is {@code Class<? extends |X|>} 65 * where {@code |X|} is the erasure of the static type of the 66 * expression on which {@code getClass} is called.</b> For 67 * example, no cast is required in this code fragment:</p> 68 * 69 * <p> 70 * {@code Number n = 0; }<br> 71 * {@code Class<? extends Number> c = n.getClass(); } 72 * </p> 73 * 74 * @return The {@code Class} object that represents the runtime 75 * class of this object. 76 * @jls 15.8.2 Class Literals 77 */ 78 @IntrinsicCandidate 79 public final native Class<?> getClass(); 80 81 /** 82 * {@return a hash code value for this object} This method is 83 * supported for the benefit of hash tables such as those provided by 84 * {@link java.util.HashMap}. 85 * <p> 86 * The general contract of {@code hashCode} is: 87 * <ul> 88 * <li>Whenever it is invoked on the same object more than once during 89 * an execution of a Java application, the {@code hashCode} method 90 * must consistently return the same integer, provided no information 91 * used in {@code equals} comparisons on the object is modified. 92 * This integer need not remain consistent from one execution of an 93 * application to another execution of the same application. 94 * <li>If two objects are equal according to the {@link 95 * #equals(Object) equals} method, then calling the {@code 96 * hashCode} method on each of the two objects must produce the 97 * same integer result. 98 * <li>It is <em>not</em> required that if two objects are unequal 99 * according to the {@link #equals(Object) equals} method, then 100 * calling the {@code hashCode} method on each of the two objects 101 * must produce distinct integer results. However, the programmer 102 * should be aware that producing distinct integer results for 103 * unequal objects may improve the performance of hash tables. 104 * </ul> 105 * 106 * @implSpec 107 * As far as is reasonably practical, the {@code hashCode} method defined 108 * by class {@code Object} returns distinct integers for distinct objects. 109 * 110 * @apiNote 111 * The {@link java.util.Objects#hash(Object...) hash} and {@link 112 * java.util.Objects#hashCode(Object) hashCode} methods of {@link 113 * java.util.Objects} can be used to help construct simple hash codes. 114 * 115 * @see java.lang.Object#equals(java.lang.Object) 116 * @see java.lang.System#identityHashCode 117 */ 118 @IntrinsicCandidate 119 public native int hashCode(); 120 121 /** 122 * Indicates whether some other object is "equal to" this one. 123 * <p> 124 * The {@code equals} method implements an <dfn>{@index "equivalence relation"}</dfn> 125 * on non-null object references: 126 * <ul> 127 * <li>It is <i>reflexive</i>: for any non-null reference value 128 * {@code x}, {@code x.equals(x)} should return 129 * {@code true}. 130 * <li>It is <i>symmetric</i>: for any non-null reference values 131 * {@code x} and {@code y}, {@code x.equals(y)} 132 * should return {@code true} if and only if 133 * {@code y.equals(x)} returns {@code true}. 134 * <li>It is <i>transitive</i>: for any non-null reference values 135 * {@code x}, {@code y}, and {@code z}, if 136 * {@code x.equals(y)} returns {@code true} and 137 * {@code y.equals(z)} returns {@code true}, then 138 * {@code x.equals(z)} should return {@code true}. 139 * <li>It is <i>consistent</i>: for any non-null reference values 140 * {@code x} and {@code y}, multiple invocations of 141 * {@code x.equals(y)} consistently return {@code true} 142 * or consistently return {@code false}, provided no 143 * information used in {@code equals} comparisons on the 144 * objects is modified. 145 * <li>For any non-null reference value {@code x}, 146 * {@code x.equals(null)} should return {@code false}. 147 * </ul> 148 * 149 * <p> 150 * An equivalence relation partitions the elements it operates on 151 * into <i>equivalence classes</i>; all the members of an 152 * equivalence class are equal to each other. Members of an 153 * equivalence class are substitutable for each other, at least 154 * for some purposes. 155 * 156 * @implSpec 157 * The {@code equals} method for class {@code Object} implements 158 * the most discriminating possible equivalence relation on objects; 159 * that is, for any non-null reference values {@code x} and 160 * {@code y}, this method returns {@code true} if and only 161 * if {@code x} and {@code y} refer to the same object 162 * ({@code x == y} has the value {@code true}). 163 * 164 * In other words, under the reference equality equivalence 165 * relation, each equivalence class only has a single element. 166 * 167 * @apiNote 168 * It is generally necessary to override the {@link #hashCode() hashCode} 169 * method whenever this method is overridden, so as to maintain the 170 * general contract for the {@code hashCode} method, which states 171 * that equal objects must have equal hash codes. 172 * <p>The two-argument {@link java.util.Objects#equals(Object, 173 * Object) Objects.equals} method implements an equivalence relation 174 * on two possibly-null object references. 175 * 176 * @param obj the reference object with which to compare. 177 * @return {@code true} if this object is the same as the obj 178 * argument; {@code false} otherwise. 179 * @see #hashCode() 180 * @see java.util.HashMap 181 */ 182 public boolean equals(Object obj) { 183 return (this == obj); 184 } 185 186 /** 187 * Creates and returns a copy of this object. The precise meaning 188 * of "copy" may depend on the class of the object. The general 189 * intent is that, for any object {@code x}, the expression: 190 * <blockquote> 191 * <pre> 192 * x.clone() != x</pre></blockquote> 193 * will be true, and that the expression: 194 * <blockquote> 195 * <pre> 196 * x.clone().getClass() == x.getClass()</pre></blockquote> 197 * will be {@code true}, but these are not absolute requirements. 198 * While it is typically the case that: 199 * <blockquote> 200 * <pre> 201 * x.clone().equals(x)</pre></blockquote> 202 * will be {@code true}, this is not an absolute requirement. 203 * <p> 204 * By convention, the returned object should be obtained by calling 205 * {@code super.clone}. If a class and all of its superclasses (except 206 * {@code Object}) obey this convention, it will be the case that 207 * {@code x.clone().getClass() == x.getClass()}. 208 * <p> 209 * By convention, the object returned by this method should be independent 210 * of this object (which is being cloned). To achieve this independence, 211 * it may be necessary to modify one or more fields of the object returned 212 * by {@code super.clone} before returning it. Typically, this means 213 * copying any mutable objects that comprise the internal "deep structure" 214 * of the object being cloned and replacing the references to these 215 * objects with references to the copies. If a class contains only 216 * primitive fields or references to immutable objects, then it is usually 217 * the case that no fields in the object returned by {@code super.clone} 218 * need to be modified. 219 * 220 * @implSpec 221 * The method {@code clone} for class {@code Object} performs a 222 * specific cloning operation. First, if the class of this object does 223 * not implement the interface {@code Cloneable}, then a 224 * {@code CloneNotSupportedException} is thrown. Note that all arrays 225 * are considered to implement the interface {@code Cloneable} and that 226 * the return type of the {@code clone} method of an array type {@code T[]} 227 * is {@code T[]} where T is any reference or primitive type. 228 * Otherwise, this method creates a new instance of the class of this 229 * object and initializes all its fields with exactly the contents of 230 * the corresponding fields of this object, as if by assignment; the 231 * contents of the fields are not themselves cloned. Thus, this method 232 * performs a "shallow copy" of this object, not a "deep copy" operation. 233 * <p> 234 * The class {@code Object} does not itself implement the interface 235 * {@code Cloneable}, so calling the {@code clone} method on an object 236 * whose class is {@code Object} will result in throwing an 237 * exception at run time. 238 * 239 * @return a clone of this instance. 240 * @throws CloneNotSupportedException if the object's class does not 241 * support the {@code Cloneable} interface. Subclasses 242 * that override the {@code clone} method can also 243 * throw this exception to indicate that an instance cannot 244 * be cloned. 245 * @see java.lang.Cloneable 246 */ 247 @IntrinsicCandidate 248 protected native Object clone() throws CloneNotSupportedException; 249 250 /** 251 * {@return a string representation of the object} 252 * 253 * Satisfying this method's contract implies a non-{@code null} 254 * result must be returned. 255 * 256 * @apiNote 257 * In general, the 258 * {@code toString} method returns a string that 259 * "textually represents" this object. The result should 260 * be a concise but informative representation that is easy for a 261 * person to read. 262 * It is recommended that all subclasses override this method. 263 * The string output is not necessarily stable over time or across 264 * JVM invocations. 265 * @implSpec 266 * The {@code toString} method for class {@code Object} 267 * returns a string consisting of the name of the class of which the 268 * object is an instance, the at-sign character `{@code @}', and 269 * the unsigned hexadecimal representation of the hash code of the 270 * object. In other words, this method returns a string equal to the 271 * value of: 272 * {@snippet lang=java : 273 * getClass().getName() + '@' + Integer.toHexString(hashCode()) 274 * } 275 * The {@link java.util.Objects#toIdentityString(Object) 276 * Objects.toIdentityString} method returns the string for an 277 * object equal to the string that would be returned if neither 278 * the {@code toString} nor {@code hashCode} methods were 279 * overridden by the object's class. 280 */ 281 public String toString() { 282 return getClass().getName() + "@" + Integer.toHexString(hashCode()); 283 } 284 285 /** 286 * Wakes up a single thread that is waiting on this object's 287 * monitor. If any threads are waiting on this object, one of them 288 * is chosen to be awakened. The choice is arbitrary and occurs at 289 * the discretion of the implementation. A thread waits on an object's 290 * monitor by calling one of the {@code wait} methods. 291 * <p> 292 * The awakened thread will not be able to proceed until the current 293 * thread relinquishes the lock on this object. The awakened thread will 294 * compete in the usual manner with any other threads that might be 295 * actively competing to synchronize on this object; for example, the 296 * awakened thread enjoys no reliable privilege or disadvantage in being 297 * the next thread to lock this object. 298 * <p> 299 * This method should only be called by a thread that is the owner 300 * of this object's monitor. A thread becomes the owner of the 301 * object's monitor in one of three ways: 302 * <ul> 303 * <li>By executing a synchronized instance method of that object. 304 * <li>By executing the body of a {@code synchronized} statement 305 * that synchronizes on the object. 306 * <li>For objects of type {@code Class,} by executing a 307 * static synchronized method of that class. 308 * </ul> 309 * <p> 310 * Only one thread at a time can own an object's monitor. 311 * <div class="preview-block"> 312 * <div class="preview-comment"> 313 * If this object is a {@linkplain Class#isValue() value object}, 314 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown. 315 * </div> 316 * </div> 317 * 318 * @throws IllegalMonitorStateException if the current thread is not 319 * the owner of this object's monitor or 320 * if this object is a {@linkplain Class#isValue() value object}. 321 * @see java.lang.Object#notifyAll() 322 * @see java.lang.Object#wait() 323 */ 324 @IntrinsicCandidate 325 public final native void notify(); 326 327 /** 328 * Wakes up all threads that are waiting on this object's monitor. A 329 * thread waits on an object's monitor by calling one of the 330 * {@code wait} methods. 331 * <p> 332 * The awakened threads will not be able to proceed until the current 333 * thread relinquishes the lock on this object. The awakened threads 334 * will compete in the usual manner with any other threads that might 335 * be actively competing to synchronize on this object; for example, 336 * the awakened threads enjoy no reliable privilege or disadvantage in 337 * being the next thread to lock this object. 338 * <p> 339 * This method should only be called by a thread that is the owner 340 * of this object's monitor. See the {@code notify} method for a 341 * description of the ways in which a thread can become the owner of 342 * a monitor. 343 * 344 * <div class="preview-block"> 345 * <div class="preview-comment"> 346 * If this object is a {@linkplain Class#isValue() value object}, 347 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown. 348 * </div> 349 * </div> 350 * 351 * @throws IllegalMonitorStateException if the current thread is not 352 * the owner of this object's monitor or 353 * if this object is a {@linkplain Class#isValue() value object}. 354 * @see java.lang.Object#notify() 355 * @see java.lang.Object#wait() 356 */ 357 @IntrinsicCandidate 358 public final native void notifyAll(); 359 360 /** 361 * Causes the current thread to wait until it is awakened, typically 362 * by being <em>notified</em> or <em>interrupted</em>. 363 * <p> 364 * In all respects, this method behaves as if {@code wait(0L, 0)} 365 * had been called. See the specification of the {@link #wait(long, int)} method 366 * for details. 367 * 368 * <div class="preview-block"> 369 * <div class="preview-comment"> 370 * If this object is a {@linkplain Class#isValue() value object}, 371 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown. 372 * </div> 373 * </div> 374 * 375 * @throws IllegalMonitorStateException if the current thread is not 376 * the owner of the object's monitor or 377 * if this object is a {@linkplain Class#isValue() value object}. 378 * @throws InterruptedException if any thread interrupted the current thread before or 379 * while the current thread was waiting. The <em>interrupted status</em> of the 380 * current thread is cleared when this exception is thrown. 381 * @see #notify() 382 * @see #notifyAll() 383 * @see #wait(long) 384 * @see #wait(long, int) 385 */ 386 public final void wait() throws InterruptedException { 387 wait(0L); 388 } 389 390 /** 391 * Causes the current thread to wait until it is awakened, typically 392 * by being <em>notified</em> or <em>interrupted</em>, or until a 393 * certain amount of real time has elapsed. 394 * <p> 395 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)} 396 * had been called. See the specification of the {@link #wait(long, int)} method 397 * for details. 398 * 399 * <div class="preview-block"> 400 * <div class="preview-comment"> 401 * If this object is a {@linkplain Class#isValue() value object}, 402 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown. 403 * </div> 404 * </div> 405 * 406 * @param timeoutMillis the maximum time to wait, in milliseconds 407 * @throws IllegalArgumentException if {@code timeoutMillis} is negative 408 * @throws IllegalMonitorStateException if the current thread is not 409 * the owner of the object's monitor or 410 * if this object is a {@linkplain Class#isValue() value object}. 411 * @throws InterruptedException if any thread interrupted the current thread before or 412 * while the current thread was waiting. The <em>interrupted status</em> of the 413 * current thread is cleared when this exception is thrown. 414 * @see #notify() 415 * @see #notifyAll() 416 * @see #wait() 417 * @see #wait(long, int) 418 */ 419 public final void wait(long timeoutMillis) throws InterruptedException { 420 if (timeoutMillis < 0) { 421 throw new IllegalArgumentException("timeout value is negative"); 422 } 423 424 if (Thread.currentThread() instanceof VirtualThread vthread) { 425 try { 426 wait0(timeoutMillis); 427 } catch (InterruptedException e) { 428 // virtual thread's interrupt status needs to be cleared 429 vthread.getAndClearInterrupt(); 430 throw e; 431 } 432 } else { 433 wait0(timeoutMillis); 434 } 435 } 436 437 // final modifier so method not in vtable 438 private final native void wait0(long timeoutMillis) throws InterruptedException; 439 440 /** 441 * Causes the current thread to wait until it is awakened, typically 442 * by being <em>notified</em> or <em>interrupted</em>, or until a 443 * certain amount of real time has elapsed. 444 * <p> 445 * The current thread must own this object's monitor lock. See the 446 * {@link #notify notify} method for a description of the ways in which 447 * a thread can become the owner of a monitor lock. 448 * <p> 449 * This method causes the current thread (referred to here as <var>T</var>) to 450 * place itself in the wait set for this object and then to relinquish any 451 * and all synchronization claims on this object. Note that only the locks 452 * on this object are relinquished; any other objects on which the current 453 * thread may be synchronized remain locked while the thread waits. 454 * <p> 455 * Thread <var>T</var> then becomes disabled for thread scheduling purposes 456 * and lies dormant until one of the following occurs: 457 * <ul> 458 * <li>Some other thread invokes the {@code notify} method for this 459 * object and thread <var>T</var> happens to be arbitrarily chosen as 460 * the thread to be awakened. 461 * <li>Some other thread invokes the {@code notifyAll} method for this 462 * object. 463 * <li>Some other thread {@linkplain Thread#interrupt() interrupts} 464 * thread <var>T</var>. 465 * <li>The specified amount of real time has elapsed, more or less. 466 * The amount of real time, in nanoseconds, is given by the expression 467 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos} 468 * are both zero, then real time is not taken into consideration and the 469 * thread waits until awakened by one of the other causes. 470 * <li>Thread <var>T</var> is awakened spuriously. (See below.) 471 * </ul> 472 * <p> 473 * The thread <var>T</var> is then removed from the wait set for this 474 * object and re-enabled for thread scheduling. It competes in the 475 * usual manner with other threads for the right to synchronize on the 476 * object; once it has regained control of the object, all its 477 * synchronization claims on the object are restored to the status quo 478 * ante - that is, to the situation as of the time that the {@code wait} 479 * method was invoked. Thread <var>T</var> then returns from the 480 * invocation of the {@code wait} method. Thus, on return from the 481 * {@code wait} method, the synchronization state of the object and of 482 * thread {@code T} is exactly as it was when the {@code wait} method 483 * was invoked. 484 * <p> 485 * A thread can wake up without being notified, interrupted, or timing out, a 486 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice, 487 * applications must guard against it by testing for the condition that should 488 * have caused the thread to be awakened, and continuing to wait if the condition 489 * is not satisfied. See the example below. 490 * <p> 491 * For more information on this topic, see section 14.2, 492 * "Condition Queues," in Brian Goetz and others' <cite>Java Concurrency 493 * in Practice</cite> (Addison-Wesley, 2006) or Item 81 in Joshua 494 * Bloch's <cite>Effective Java, Third Edition</cite> (Addison-Wesley, 495 * 2018). 496 * <p> 497 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted} 498 * by any thread before or while it is waiting, then an {@code InterruptedException} 499 * is thrown. The <em>interrupted status</em> of the current thread is cleared when 500 * this exception is thrown. This exception is not thrown until the lock status of 501 * this object has been restored as described above. 502 * 503 * @apiNote 504 * The recommended approach to waiting is to check the condition being awaited in 505 * a {@code while} loop around the call to {@code wait}, as shown in the example 506 * below. Among other things, this approach avoids problems that can be caused 507 * by spurious wakeups. 508 * 509 * {@snippet lang=java : 510 * synchronized (obj) { 511 * while ( <condition does not hold and timeout not exceeded> ) { 512 * long timeoutMillis = ... ; // recompute timeout values 513 * int nanos = ... ; 514 * obj.wait(timeoutMillis, nanos); 515 * } 516 * ... // Perform action appropriate to condition or timeout 517 * } 518 * } 519 * 520 * <div class="preview-block"> 521 * <div class="preview-comment"> 522 * If this object is a {@linkplain Class#isValue() value object}, 523 * it does does not have a monitor, an {@code IllegalMonitorStateException} is thrown. 524 * </div> 525 * </div> 526 * @param timeoutMillis the maximum time to wait, in milliseconds 527 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive 528 * @throws IllegalArgumentException if {@code timeoutMillis} is negative, 529 * or if the value of {@code nanos} is out of range 530 * @throws IllegalMonitorStateException if the current thread is not 531 * the owner of the object's monitor or 532 * if this object is a {@linkplain Class#isValue() value object}. 533 * @throws InterruptedException if any thread interrupted the current thread before or 534 * while the current thread was waiting. The <em>interrupted status</em> of the 535 * current thread is cleared when this exception is thrown. 536 * @see #notify() 537 * @see #notifyAll() 538 * @see #wait() 539 * @see #wait(long) 540 */ 541 public final void wait(long timeoutMillis, int nanos) throws InterruptedException { 542 if (timeoutMillis < 0) { 543 throw new IllegalArgumentException("timeoutMillis value is negative"); 544 } 545 546 if (nanos < 0 || nanos > 999999) { 547 throw new IllegalArgumentException( 548 "nanosecond timeout value out of range"); 549 } 550 551 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) { 552 timeoutMillis++; 553 } 554 555 wait(timeoutMillis); 556 } 557 558 /** 559 * Called by the garbage collector on an object when garbage collection 560 * determines that there are no more references to the object. 561 * A subclass overrides the {@code finalize} method to dispose of 562 * system resources or to perform other cleanup. 563 * <p> 564 * <b>When running in a Java virtual machine in which finalization has been 565 * disabled or removed, the garbage collector will never call 566 * {@code finalize()}. In a Java virtual machine in which finalization is 567 * enabled, the garbage collector might call {@code finalize} only after an 568 * indefinite delay.</b> 569 * <p> 570 * The general contract of {@code finalize} is that it is invoked 571 * if and when the Java virtual 572 * machine has determined that there is no longer any 573 * means by which this object can be accessed by any thread that has 574 * not yet died, except as a result of an action taken by the 575 * finalization of some other object or class which is ready to be 576 * finalized. The {@code finalize} method may take any action, including 577 * making this object available again to other threads; the usual purpose 578 * of {@code finalize}, however, is to perform cleanup actions before 579 * the object is irrevocably discarded. For example, the finalize method 580 * for an object that represents an input/output connection might perform 581 * explicit I/O transactions to break the connection before the object is 582 * permanently discarded. 583 * <p> 584 * The {@code finalize} method of class {@code Object} performs no 585 * special action; it simply returns normally. Subclasses of 586 * {@code Object} may override this definition. 587 * <p> 588 * The Java programming language does not guarantee which thread will 589 * invoke the {@code finalize} method for any given object. It is 590 * guaranteed, however, that the thread that invokes finalize will not 591 * be holding any user-visible synchronization locks when finalize is 592 * invoked. If an uncaught exception is thrown by the finalize method, 593 * the exception is ignored and finalization of that object terminates. 594 * <p> 595 * After the {@code finalize} method has been invoked for an object, no 596 * further action is taken until the Java virtual machine has again 597 * determined that there is no longer any means by which this object can 598 * be accessed by any thread that has not yet died, including possible 599 * actions by other objects or classes which are ready to be finalized, 600 * at which point the object may be discarded. 601 * <p> 602 * The {@code finalize} method is never invoked more than once by a Java 603 * virtual machine for any given object. 604 * <p> 605 * Any exception thrown by the {@code finalize} method causes 606 * the finalization of this object to be halted, but is otherwise 607 * ignored. 608 * 609 * @apiNote 610 * Classes that embed non-heap resources have many options 611 * for cleanup of those resources. The class must ensure that the 612 * lifetime of each instance is longer than that of any resource it embeds. 613 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that 614 * objects remain reachable while resources embedded in the object are in use. 615 * <p> 616 * A subclass should avoid overriding the {@code finalize} method 617 * unless the subclass embeds non-heap resources that must be cleaned up 618 * before the instance is collected. 619 * Finalizer invocations are not automatically chained, unlike constructors. 620 * If a subclass overrides {@code finalize} it must invoke the superclass 621 * finalizer explicitly. 622 * To guard against exceptions prematurely terminating the finalize chain, 623 * the subclass should use a {@code try-finally} block to ensure 624 * {@code super.finalize()} is always invoked. For example, 625 * {@snippet lang="java": 626 * @Override 627 * protected void finalize() throws Throwable { 628 * try { 629 * ... // cleanup subclass state 630 * } finally { 631 * super.finalize(); 632 * } 633 * } 634 * } 635 * 636 * @deprecated Finalization is deprecated and subject to removal in a future 637 * release. The use of finalization can lead to problems with security, 638 * performance, and reliability. 639 * See <a href="https://openjdk.org/jeps/421">JEP 421</a> for 640 * discussion and alternatives. 641 * <p> 642 * Subclasses that override {@code finalize} to perform cleanup should use 643 * alternative cleanup mechanisms and remove the {@code finalize} method. 644 * Use {@link java.lang.ref.Cleaner} and 645 * {@link java.lang.ref.PhantomReference} as safer ways to release resources 646 * when an object becomes unreachable. Alternatively, add a {@code close} 647 * method to explicitly release resources, and implement 648 * {@code AutoCloseable} to enable use of the {@code try}-with-resources 649 * statement. 650 * <p> 651 * This method will remain in place until finalizers have been removed from 652 * most existing code. 653 * 654 * @throws Throwable the {@code Exception} raised by this method 655 * @see java.lang.ref.WeakReference 656 * @see java.lang.ref.PhantomReference 657 * @jls 12.6 Finalization of Class Instances 658 */ 659 @Deprecated(since="9", forRemoval=true) 660 protected void finalize() throws Throwable { } 661 }