1 /* 2 * Copyright (c) 1994, 2022, 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.IntrinsicCandidate; 29 import jdk.internal.access.SharedSecrets; 30 31 import java.util.Objects; 32 33 /** 34 * Class {@code Object} is the root of the class hierarchy. 35 * Every class has {@code Object} as a superclass. All objects, 36 * including arrays, implement the methods of this class. 37 * 38 * @see java.lang.Class 39 * @since 1.0 40 */ 41 public class Object { 42 43 /** 44 * Constructs a new object. 45 */ 46 @IntrinsicCandidate 47 public Object() {} 48 49 /** 50 * Returns the runtime class of this {@code Object}. The returned 51 * {@code Class} object is the object that is locked by {@code 52 * static synchronized} methods of the represented class. 53 * 54 * <p><b>The actual result type is {@code Class<? extends |X|>} 55 * where {@code |X|} is the erasure of the static type of the 56 * expression on which {@code getClass} is called.</b> For 57 * example, no cast is required in this code fragment:</p> 58 * 59 * <p> 60 * {@code Number n = 0; }<br> 61 * {@code Class<? extends Number> c = n.getClass(); } 62 * </p> 63 * 64 * @return The {@code Class} object that represents the runtime 65 * class of this object. 66 * @jls 15.8.2 Class Literals 67 */ 68 @IntrinsicCandidate 69 public final native Class<?> getClass(); 70 71 /** 72 * Returns a hash code value for the object. This method is 73 * supported for the benefit of hash tables such as those provided by 74 * {@link java.util.HashMap}. 75 * <p> 76 * The general contract of {@code hashCode} is: 77 * <ul> 78 * <li>Whenever it is invoked on the same object more than once during 79 * an execution of a Java application, the {@code hashCode} method 80 * must consistently return the same integer, provided no information 81 * used in {@code equals} comparisons on the object is modified. 82 * This integer need not remain consistent from one execution of an 83 * application to another execution of the same application. 84 * <li>If two objects are equal according to the {@link 85 * equals(Object) equals} method, then calling the {@code 86 * hashCode} method on each of the two objects must produce the 87 * same integer result. 88 * <li>It is <em>not</em> required that if two objects are unequal 89 * according to the {@link equals(Object) equals} method, then 90 * calling the {@code hashCode} method on each of the two objects 91 * must produce distinct integer results. However, the programmer 92 * should be aware that producing distinct integer results for 93 * unequal objects may improve the performance of hash tables. 94 * </ul> 95 * 96 * @implSpec 97 * As far as is reasonably practical, the {@code hashCode} method defined 98 * by class {@code Object} returns distinct integers for distinct objects. 99 * 100 * @return a hash code value for this object. 101 * @see java.lang.Object#equals(java.lang.Object) 102 * @see java.lang.System#identityHashCode 103 */ 104 @IntrinsicCandidate 105 public native int hashCode(); 106 107 /** 108 * Indicates whether some other object is "equal to" this one. 109 * <p> 110 * The {@code equals} method implements an equivalence relation 111 * on non-null object references: 112 * <ul> 113 * <li>It is <i>reflexive</i>: for any non-null reference value 114 * {@code x}, {@code x.equals(x)} should return 115 * {@code true}. 116 * <li>It is <i>symmetric</i>: for any non-null reference values 117 * {@code x} and {@code y}, {@code x.equals(y)} 118 * should return {@code true} if and only if 119 * {@code y.equals(x)} returns {@code true}. 120 * <li>It is <i>transitive</i>: for any non-null reference values 121 * {@code x}, {@code y}, and {@code z}, if 122 * {@code x.equals(y)} returns {@code true} and 123 * {@code y.equals(z)} returns {@code true}, then 124 * {@code x.equals(z)} should return {@code true}. 125 * <li>It is <i>consistent</i>: for any non-null reference values 126 * {@code x} and {@code y}, multiple invocations of 127 * {@code x.equals(y)} consistently return {@code true} 128 * or consistently return {@code false}, provided no 129 * information used in {@code equals} comparisons on the 130 * objects is modified. 131 * <li>For any non-null reference value {@code x}, 132 * {@code x.equals(null)} should return {@code false}. 133 * </ul> 134 * 135 * <p> 136 * An equivalence relation partitions the elements it operates on 137 * into <i>equivalence classes</i>; all the members of an 138 * equivalence class are equal to each other. Members of an 139 * equivalence class are substitutable for each other, at least 140 * for some purposes. 141 * 142 * @implSpec 143 * The {@code equals} method for class {@code Object} implements 144 * the most discriminating possible equivalence relation on objects; 145 * that is, for any non-null reference values {@code x} and 146 * {@code y}, this method returns {@code true} if and only 147 * if {@code x} and {@code y} refer to the same object 148 * ({@code x == y} has the value {@code true}). 149 * 150 * In other words, under the reference equality equivalence 151 * relation, each equivalence class only has a single element. 152 * 153 * @apiNote 154 * It is generally necessary to override the {@link hashCode hashCode} 155 * method whenever this method is overridden, so as to maintain the 156 * general contract for the {@code hashCode} method, which states 157 * that equal objects must have equal hash codes. 158 * 159 * @param obj the reference object with which to compare. 160 * @return {@code true} if this object is the same as the obj 161 * argument; {@code false} otherwise. 162 * @see #hashCode() 163 * @see java.util.HashMap 164 */ 165 public boolean equals(Object obj) { 166 return (this == obj); 167 } 168 169 /** 170 * Creates and returns a copy of this object. The precise meaning 171 * of "copy" may depend on the class of the object. The general 172 * intent is that, for any object {@code x}, the expression: 173 * <blockquote> 174 * <pre> 175 * x.clone() != x</pre></blockquote> 176 * will be true, and that the expression: 177 * <blockquote> 178 * <pre> 179 * x.clone().getClass() == x.getClass()</pre></blockquote> 180 * will be {@code true}, but these are not absolute requirements. 181 * While it is typically the case that: 182 * <blockquote> 183 * <pre> 184 * x.clone().equals(x)</pre></blockquote> 185 * will be {@code true}, this is not an absolute requirement. 186 * <p> 187 * By convention, the returned object should be obtained by calling 188 * {@code super.clone}. If a class and all of its superclasses (except 189 * {@code Object}) obey this convention, it will be the case that 190 * {@code x.clone().getClass() == x.getClass()}. 191 * <p> 192 * By convention, the object returned by this method should be independent 193 * of this object (which is being cloned). To achieve this independence, 194 * it may be necessary to modify one or more fields of the object returned 195 * by {@code super.clone} before returning it. Typically, this means 196 * copying any mutable objects that comprise the internal "deep structure" 197 * of the object being cloned and replacing the references to these 198 * objects with references to the copies. If a class contains only 199 * primitive fields or references to immutable objects, then it is usually 200 * the case that no fields in the object returned by {@code super.clone} 201 * need to be modified. 202 * 203 * @implSpec 204 * The method {@code clone} for class {@code Object} performs a 205 * specific cloning operation. First, if the class of this object does 206 * not implement the interface {@code Cloneable}, then a 207 * {@code CloneNotSupportedException} is thrown. Note that all arrays 208 * are considered to implement the interface {@code Cloneable} and that 209 * the return type of the {@code clone} method of an array type {@code T[]} 210 * is {@code T[]} where T is any reference or primitive type. 211 * Otherwise, this method creates a new instance of the class of this 212 * object and initializes all its fields with exactly the contents of 213 * the corresponding fields of this object, as if by assignment; the 214 * contents of the fields are not themselves cloned. Thus, this method 215 * performs a "shallow copy" of this object, not a "deep copy" operation. 216 * <p> 217 * The class {@code Object} does not itself implement the interface 218 * {@code Cloneable}, so calling the {@code clone} method on an object 219 * whose class is {@code Object} will result in throwing an 220 * exception at run time. 221 * 222 * @return a clone of this instance. 223 * @throws CloneNotSupportedException if the object's class does not 224 * support the {@code Cloneable} interface. Subclasses 225 * that override the {@code clone} method can also 226 * throw this exception to indicate that an instance cannot 227 * be cloned. 228 * @see java.lang.Cloneable 229 */ 230 @IntrinsicCandidate 231 protected native Object clone() throws CloneNotSupportedException; 232 233 /** 234 * Returns a string representation of the object. 235 * @apiNote 236 * In general, the 237 * {@code toString} method returns a string that 238 * "textually represents" this object. The result should 239 * be a concise but informative representation that is easy for a 240 * person to read. 241 * It is recommended that all subclasses override this method. 242 * The string output is not necessarily stable over time or across 243 * JVM invocations. 244 * @implSpec 245 * If this object is an instance of an identity class, then 246 * the {@code toString} method for class {@code Object} 247 * returns a string consisting of the name of the class of which the 248 * object is an instance, the at-sign character `{@code @}', and 249 * the unsigned hexadecimal representation of the hash code of the 250 * object. In other words, this method returns a string equal to the 251 * value of: 252 * <blockquote> 253 * <pre> 254 * getClass().getName() + '@' + Integer.toHexString(hashCode()) 255 * </pre></blockquote> 256 * <p> 257 * If this object is an instance of a primitive class, then 258 * the {@code toString} method returns a string which contains 259 * the name of the primitive class, and string representations of 260 * all its fields. The precise format produced by this method 261 * is unspecified and subject to change. 262 * 263 * @return a string representation of the object. 264 */ 265 public String toString() { 266 return getClass().getName() + "@" + Integer.toHexString(hashCode()); 267 } 268 269 /** 270 * Wakes up a single thread that is waiting on this object's 271 * monitor. If any threads are waiting on this object, one of them 272 * is chosen to be awakened. The choice is arbitrary and occurs at 273 * the discretion of the implementation. A thread waits on an object's 274 * monitor by calling one of the {@code wait} methods. 275 * <p> 276 * The awakened thread will not be able to proceed until the current 277 * thread relinquishes the lock on this object. The awakened thread will 278 * compete in the usual manner with any other threads that might be 279 * actively competing to synchronize on this object; for example, the 280 * awakened thread enjoys no reliable privilege or disadvantage in being 281 * the next thread to lock this object. 282 * <p> 283 * This method should only be called by a thread that is the owner 284 * of this object's monitor. A thread becomes the owner of the 285 * object's monitor in one of three ways: 286 * <ul> 287 * <li>By executing a synchronized instance method of that object. 288 * <li>By executing the body of a {@code synchronized} statement 289 * that synchronizes on the object. 290 * <li>For objects of type {@code Class,} by executing a 291 * static synchronized method of that class. 292 * </ul> 293 * <p> 294 * Only one thread at a time can own an object's monitor. 295 * 296 * @throws IllegalMonitorStateException if the current thread is not 297 * the owner of this object's monitor. 298 * @see java.lang.Object#notifyAll() 299 * @see java.lang.Object#wait() 300 */ 301 @IntrinsicCandidate 302 public final native void notify(); 303 304 /** 305 * Wakes up all threads that are waiting on this object's monitor. A 306 * thread waits on an object's monitor by calling one of the 307 * {@code wait} methods. 308 * <p> 309 * The awakened threads will not be able to proceed until the current 310 * thread relinquishes the lock on this object. The awakened threads 311 * will compete in the usual manner with any other threads that might 312 * be actively competing to synchronize on this object; for example, 313 * the awakened threads enjoy no reliable privilege or disadvantage in 314 * being the next thread to lock this object. 315 * <p> 316 * This method should only be called by a thread that is the owner 317 * of this object's monitor. See the {@code notify} method for a 318 * description of the ways in which a thread can become the owner of 319 * a monitor. 320 * 321 * @throws IllegalMonitorStateException if the current thread is not 322 * the owner of this object's monitor. 323 * @see java.lang.Object#notify() 324 * @see java.lang.Object#wait() 325 */ 326 @IntrinsicCandidate 327 public final native void notifyAll(); 328 329 /** 330 * Causes the current thread to wait until it is awakened, typically 331 * by being <em>notified</em> or <em>interrupted</em>. 332 * <p> 333 * In all respects, this method behaves as if {@code wait(0L, 0)} 334 * had been called. See the specification of the {@link #wait(long, int)} method 335 * for details. 336 * 337 * @throws IllegalMonitorStateException if the current thread is not 338 * the owner of the object's monitor 339 * @throws InterruptedException if any thread interrupted the current thread before or 340 * while the current thread was waiting. The <em>interrupted status</em> of the 341 * current thread is cleared when this exception is thrown. 342 * @see #notify() 343 * @see #notifyAll() 344 * @see #wait(long) 345 * @see #wait(long, int) 346 */ 347 public final void wait() throws InterruptedException { 348 wait(0L); 349 } 350 351 /** 352 * Causes the current thread to wait until it is awakened, typically 353 * by being <em>notified</em> or <em>interrupted</em>, or until a 354 * certain amount of real time has elapsed. 355 * <p> 356 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)} 357 * had been called. See the specification of the {@link #wait(long, int)} method 358 * for details. 359 * 360 * @param timeoutMillis the maximum time to wait, in milliseconds 361 * @throws IllegalArgumentException if {@code timeoutMillis} is negative 362 * @throws IllegalMonitorStateException if the current thread is not 363 * the owner of the object's monitor 364 * @throws InterruptedException if any thread interrupted the current thread before or 365 * while the current thread was waiting. The <em>interrupted status</em> of the 366 * current thread is cleared when this exception is thrown. 367 * @see #notify() 368 * @see #notifyAll() 369 * @see #wait() 370 * @see #wait(long, int) 371 */ 372 public final native void wait(long timeoutMillis) throws InterruptedException; 373 374 /** 375 * Causes the current thread to wait until it is awakened, typically 376 * by being <em>notified</em> or <em>interrupted</em>, or until a 377 * certain amount of real time has elapsed. 378 * <p> 379 * The current thread must own this object's monitor lock. See the 380 * {@link #notify notify} method for a description of the ways in which 381 * a thread can become the owner of a monitor lock. 382 * <p> 383 * This method causes the current thread (referred to here as <var>T</var>) to 384 * place itself in the wait set for this object and then to relinquish any 385 * and all synchronization claims on this object. Note that only the locks 386 * on this object are relinquished; any other objects on which the current 387 * thread may be synchronized remain locked while the thread waits. 388 * <p> 389 * Thread <var>T</var> then becomes disabled for thread scheduling purposes 390 * and lies dormant until one of the following occurs: 391 * <ul> 392 * <li>Some other thread invokes the {@code notify} method for this 393 * object and thread <var>T</var> happens to be arbitrarily chosen as 394 * the thread to be awakened. 395 * <li>Some other thread invokes the {@code notifyAll} method for this 396 * object. 397 * <li>Some other thread {@linkplain Thread#interrupt() interrupts} 398 * thread <var>T</var>. 399 * <li>The specified amount of real time has elapsed, more or less. 400 * The amount of real time, in nanoseconds, is given by the expression 401 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos} 402 * are both zero, then real time is not taken into consideration and the 403 * thread waits until awakened by one of the other causes. 404 * <li>Thread <var>T</var> is awakened spuriously. (See below.) 405 * </ul> 406 * <p> 407 * The thread <var>T</var> is then removed from the wait set for this 408 * object and re-enabled for thread scheduling. It competes in the 409 * usual manner with other threads for the right to synchronize on the 410 * object; once it has regained control of the object, all its 411 * synchronization claims on the object are restored to the status quo 412 * ante - that is, to the situation as of the time that the {@code wait} 413 * method was invoked. Thread <var>T</var> then returns from the 414 * invocation of the {@code wait} method. Thus, on return from the 415 * {@code wait} method, the synchronization state of the object and of 416 * thread {@code T} is exactly as it was when the {@code wait} method 417 * was invoked. 418 * <p> 419 * A thread can wake up without being notified, interrupted, or timing out, a 420 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice, 421 * applications must guard against it by testing for the condition that should 422 * have caused the thread to be awakened, and continuing to wait if the condition 423 * is not satisfied. See the example below. 424 * <p> 425 * For more information on this topic, see section 14.2, 426 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency 427 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua 428 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley, 429 * 2008). 430 * <p> 431 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted} 432 * by any thread before or while it is waiting, then an {@code InterruptedException} 433 * is thrown. The <em>interrupted status</em> of the current thread is cleared when 434 * this exception is thrown. This exception is not thrown until the lock status of 435 * this object has been restored as described above. 436 * 437 * @apiNote 438 * The recommended approach to waiting is to check the condition being awaited in 439 * a {@code while} loop around the call to {@code wait}, as shown in the example 440 * below. Among other things, this approach avoids problems that can be caused 441 * by spurious wakeups. 442 * 443 * <pre>{@code 444 * synchronized (obj) { 445 * while (<condition does not hold> and <timeout not exceeded>) { 446 * long timeoutMillis = ... ; // recompute timeout values 447 * int nanos = ... ; 448 * obj.wait(timeoutMillis, nanos); 449 * } 450 * ... // Perform action appropriate to condition or timeout 451 * } 452 * }</pre> 453 * 454 * @param timeoutMillis the maximum time to wait, in milliseconds 455 * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive 456 * @throws IllegalArgumentException if {@code timeoutMillis} is negative, 457 * or if the value of {@code nanos} is out of range 458 * @throws IllegalMonitorStateException if the current thread is not 459 * the owner of the object's monitor 460 * @throws InterruptedException if any thread interrupted the current thread before or 461 * while the current thread was waiting. The <em>interrupted status</em> of the 462 * current thread is cleared when this exception is thrown. 463 * @see #notify() 464 * @see #notifyAll() 465 * @see #wait() 466 * @see #wait(long) 467 */ 468 public final void wait(long timeoutMillis, int nanos) throws InterruptedException { 469 if (timeoutMillis < 0) { 470 throw new IllegalArgumentException("timeoutMillis value is negative"); 471 } 472 473 if (nanos < 0 || nanos > 999999) { 474 throw new IllegalArgumentException( 475 "nanosecond timeout value out of range"); 476 } 477 478 if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) { 479 timeoutMillis++; 480 } 481 482 wait(timeoutMillis); 483 } 484 485 /** 486 * Called by the garbage collector on an object when garbage collection 487 * determines that there are no more references to the object. 488 * A subclass overrides the {@code finalize} method to dispose of 489 * system resources or to perform other cleanup. 490 * <p> 491 * <b>When running in a Java virtual machine in which finalization has been 492 * disabled or removed, the garbage collector will never call 493 * {@code finalize()}. In a Java virtual machine in which finalization is 494 * enabled, the garbage collector might call {@code finalize} only after an 495 * indefinite delay.</b> 496 * <p> 497 * The general contract of {@code finalize} is that it is invoked 498 * if and when the Java virtual 499 * machine has determined that there is no longer any 500 * means by which this object can be accessed by any thread that has 501 * not yet died, except as a result of an action taken by the 502 * finalization of some other object or class which is ready to be 503 * finalized. The {@code finalize} method may take any action, including 504 * making this object available again to other threads; the usual purpose 505 * of {@code finalize}, however, is to perform cleanup actions before 506 * the object is irrevocably discarded. For example, the finalize method 507 * for an object that represents an input/output connection might perform 508 * explicit I/O transactions to break the connection before the object is 509 * permanently discarded. 510 * <p> 511 * The {@code finalize} method of class {@code Object} performs no 512 * special action; it simply returns normally. Subclasses of 513 * {@code Object} may override this definition. 514 * <p> 515 * The Java programming language does not guarantee which thread will 516 * invoke the {@code finalize} method for any given object. It is 517 * guaranteed, however, that the thread that invokes finalize will not 518 * be holding any user-visible synchronization locks when finalize is 519 * invoked. If an uncaught exception is thrown by the finalize method, 520 * the exception is ignored and finalization of that object terminates. 521 * <p> 522 * After the {@code finalize} method has been invoked for an object, no 523 * further action is taken until the Java virtual machine has again 524 * determined that there is no longer any means by which this object can 525 * be accessed by any thread that has not yet died, including possible 526 * actions by other objects or classes which are ready to be finalized, 527 * at which point the object may be discarded. 528 * <p> 529 * The {@code finalize} method is never invoked more than once by a Java 530 * virtual machine for any given object. 531 * <p> 532 * Any exception thrown by the {@code finalize} method causes 533 * the finalization of this object to be halted, but is otherwise 534 * ignored. 535 * 536 * @apiNote 537 * Classes that embed non-heap resources have many options 538 * for cleanup of those resources. The class must ensure that the 539 * lifetime of each instance is longer than that of any resource it embeds. 540 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that 541 * objects remain reachable while resources embedded in the object are in use. 542 * <p> 543 * A subclass should avoid overriding the {@code finalize} method 544 * unless the subclass embeds non-heap resources that must be cleaned up 545 * before the instance is collected. 546 * Finalizer invocations are not automatically chained, unlike constructors. 547 * If a subclass overrides {@code finalize} it must invoke the superclass 548 * finalizer explicitly. 549 * To guard against exceptions prematurely terminating the finalize chain, 550 * the subclass should use a {@code try-finally} block to ensure 551 * {@code super.finalize()} is always invoked. For example, 552 * <pre>{@code @Override 553 * protected void finalize() throws Throwable { 554 * try { 555 * ... // cleanup subclass state 556 * } finally { 557 * super.finalize(); 558 * } 559 * } 560 * }</pre> 561 * 562 * @deprecated Finalization is deprecated and subject to removal in a future 563 * release. The use of finalization can lead to problems with security, 564 * performance, and reliability. 565 * See <a href="https://openjdk.java.net/jeps/421">JEP 421</a> for 566 * discussion and alternatives. 567 * <p> 568 * Subclasses that override {@code finalize} to perform cleanup should use 569 * alternative cleanup mechanisms and remove the {@code finalize} method. 570 * Use {@link java.lang.ref.Cleaner} and 571 * {@link java.lang.ref.PhantomReference} as safer ways to release resources 572 * when an object becomes unreachable. Alternatively, add a {@code close} 573 * method to explicitly release resources, and implement 574 * {@code AutoCloseable} to enable use of the {@code try}-with-resources 575 * statement. 576 * <p> 577 * This method will remain in place until finalizers have been removed from 578 * most existing code. 579 * 580 * @throws Throwable the {@code Exception} raised by this method 581 * @see java.lang.ref.WeakReference 582 * @see java.lang.ref.PhantomReference 583 * @jls 12.6 Finalization of Class Instances 584 */ 585 @Deprecated(since="9", forRemoval=true) 586 protected void finalize() throws Throwable { } 587 }