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 registerNatives()41 private static native void registerNatives(); 42 static { registerNatives()43 registerNatives(); 44 } 45 46 /** 47 * Constructs a new object. 48 */ 49 @HotSpotIntrinsicCandidate Object()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 getClass()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 hashCode()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 */ equals(Object obj)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 clone()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 */ toString()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 notify()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 notifyAll()307 public final native void notifyAll(); 308 309 /** 310 * Causes the current thread to wait until it is awakened, typically 311 * by being <em>notified</em> or <em>interrupted</em>. 312 * <p> 313 * In all respects, this method behaves as if {@code wait(0L, 0)} 314 * had been called. See the specification of the {@link #wait(long, int)} method 315 * for details. 316 * 317 * @throws IllegalMonitorStateException if the current thread is not 318 * the owner of the object's monitor 319 * @throws InterruptedException if any thread interrupted the current thread before or 320 * while the current thread was waiting. The <em>interrupted status</em> of the 321 * current thread is cleared when this exception is thrown. 322 * @see #notify() 323 * @see #notifyAll() 324 * @see #wait(long) 325 * @see #wait(long, int) 326 */ wait()327 public final void wait() throws InterruptedException { 328 wait(0L); 329 } 330 331 /** 332 * Causes the current thread to wait until it is awakened, typically 333 * by being <em>notified</em> or <em>interrupted</em>, or until a 334 * certain amount of real time has elapsed. 335 * <p> 336 * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)} 337 * had been called. See the specification of the {@link #wait(long, int)} method 338 * for details. 339 * 340 * @param timeoutMillis the maximum time to wait, in milliseconds 341 * @throws IllegalArgumentException if {@code timeoutMillis} is negative 342 * @throws IllegalMonitorStateException if the current thread is not 343 * the owner of the object's monitor 344 * @throws InterruptedException if any thread interrupted the current thread before or 345 * while the current thread was waiting. The <em>interrupted status</em> of the 346 * current thread is cleared when this exception is thrown. 347 * @see #notify() 348 * @see #notifyAll() 349 * @see #wait() 350 * @see #wait(long, int) 351 */ wait(long timeoutMillis)352 public final native void wait(long timeoutMillis) throws InterruptedException; 353 354 /** 355 * Causes the current thread to wait until it is awakened, typically 356 * by being <em>notified</em> or <em>interrupted</em>, or until a 357 * certain amount of real time has elapsed. 358 * <p> 359 * The current thread must own this object's monitor lock. See the 360 * {@link #notify notify} method for a description of the ways in which 361 * a thread can become the owner of a monitor lock. 362 * <p> 363 * This method causes the current thread (referred to here as <var>T</var>) to 364 * place itself in the wait set for this object and then to relinquish any 365 * and all synchronization claims on this object. Note that only the locks 366 * on this object are relinquished; any other objects on which the current 367 * thread may be synchronized remain locked while the thread waits. 368 * <p> 369 * Thread <var>T</var> then becomes disabled for thread scheduling purposes 370 * and lies dormant until one of the following occurs: 371 * <ul> 372 * <li>Some other thread invokes the {@code notify} method for this 373 * object and thread <var>T</var> happens to be arbitrarily chosen as 374 * the thread to be awakened. 375 * <li>Some other thread invokes the {@code notifyAll} method for this 376 * object. 377 * <li>Some other thread {@linkplain Thread#interrupt() interrupts} 378 * thread <var>T</var>. 379 * <li>The specified amount of real time has elapsed, more or less. 380 * The amount of real time, in nanoseconds, is given by the expression 381 * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos} 382 * are both zero, then real time is not taken into consideration and the 383 * thread waits until awakened by one of the other causes. 384 * <li>Thread <var>T</var> is awakened spuriously. (See below.) 385 * </ul> 386 * <p> 387 * The thread <var>T</var> is then removed from the wait set for this 388 * object and re-enabled for thread scheduling. It competes in the 389 * usual manner with other threads for the right to synchronize on the 390 * object; once it has regained control of the object, all its 391 * synchronization claims on the object are restored to the status quo 392 * ante - that is, to the situation as of the time that the {@code wait} 393 * method was invoked. Thread <var>T</var> then returns from the 394 * invocation of the {@code wait} method. Thus, on return from the 395 * {@code wait} method, the synchronization state of the object and of 396 * thread {@code T} is exactly as it was when the {@code wait} method 397 * was invoked. 398 * <p> 399 * A thread can wake up without being notified, interrupted, or timing out, a 400 * so-called <em>spurious wakeup</em>. While this will rarely occur in practice, 401 * applications must guard against it by testing for the condition that should 402 * have caused the thread to be awakened, and continuing to wait if the condition 403 * is not satisfied. See the example below. 404 * <p> 405 * For more information on this topic, see section 14.2, 406 * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency 407 * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua 408 * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley, 409 * 2008). 410 * <p> 411 * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted} 412 * by any thread before or while it is waiting, then an {@code InterruptedException} 413 * is thrown. The <em>interrupted status</em> of the current thread is cleared when 414 * this exception is thrown. This exception is not thrown until the lock status of 415 * this object has been restored as described above. 416 * 417 * @apiNote 418 * The recommended approach to waiting is to check the condition being awaited in 419 * a {@code while} loop around the call to {@code wait}, as shown in the example 420 * below. Among other things, this approach avoids problems that can be caused 421 * by spurious wakeups. 422 * 423 * <pre>{@code 424 * synchronized (obj) { 425 * while (<condition does not hold> and <timeout not exceeded>) { 426 * long timeoutMillis = ... ; // recompute timeout values 427 * int nanos = ... ; 428 * obj.wait(timeoutMillis, nanos); 429 * } 430 * ... // Perform action appropriate to condition or timeout 431 * } 432 * }</pre> 433 * 434 * @param timeoutMillis the maximum time to wait, in milliseconds 435 * @param nanos additional time, in nanoseconds, in the range range 0-999999 inclusive 436 * @throws IllegalArgumentException if {@code timeoutMillis} is negative, 437 * or if the value of {@code nanos} is out of range 438 * @throws IllegalMonitorStateException if the current thread is not 439 * the owner of the object's monitor 440 * @throws InterruptedException if any thread interrupted the current thread before or 441 * while the current thread was waiting. The <em>interrupted status</em> of the 442 * current thread is cleared when this exception is thrown. 443 * @see #notify() 444 * @see #notifyAll() 445 * @see #wait() 446 * @see #wait(long) 447 */ wait(long timeoutMillis, int nanos)448 public final void wait(long timeoutMillis, int nanos) throws InterruptedException { 449 if (timeoutMillis < 0) { 450 throw new IllegalArgumentException("timeoutMillis value is negative"); 451 } 452 453 if (nanos < 0 || nanos > 999999) { 454 throw new IllegalArgumentException( 455 "nanosecond timeout value out of range"); 456 } 457 458 if (nanos > 0) { 459 timeoutMillis++; 460 } 461 462 wait(timeoutMillis); 463 } 464 465 /** 466 * Called by the garbage collector on an object when garbage collection 467 * determines that there are no more references to the object. 468 * A subclass overrides the {@code finalize} method to dispose of 469 * system resources or to perform other cleanup. 470 * <p> 471 * The general contract of {@code finalize} is that it is invoked 472 * if and when the Java™ virtual 473 * machine has determined that there is no longer any 474 * means by which this object can be accessed by any thread that has 475 * not yet died, except as a result of an action taken by the 476 * finalization of some other object or class which is ready to be 477 * finalized. The {@code finalize} method may take any action, including 478 * making this object available again to other threads; the usual purpose 479 * of {@code finalize}, however, is to perform cleanup actions before 480 * the object is irrevocably discarded. For example, the finalize method 481 * for an object that represents an input/output connection might perform 482 * explicit I/O transactions to break the connection before the object is 483 * permanently discarded. 484 * <p> 485 * The {@code finalize} method of class {@code Object} performs no 486 * special action; it simply returns normally. Subclasses of 487 * {@code Object} may override this definition. 488 * <p> 489 * The Java programming language does not guarantee which thread will 490 * invoke the {@code finalize} method for any given object. It is 491 * guaranteed, however, that the thread that invokes finalize will not 492 * be holding any user-visible synchronization locks when finalize is 493 * invoked. If an uncaught exception is thrown by the finalize method, 494 * the exception is ignored and finalization of that object terminates. 495 * <p> 496 * After the {@code finalize} method has been invoked for an object, no 497 * further action is taken until the Java virtual machine has again 498 * determined that there is no longer any means by which this object can 499 * be accessed by any thread that has not yet died, including possible 500 * actions by other objects or classes which are ready to be finalized, 501 * at which point the object may be discarded. 502 * <p> 503 * The {@code finalize} method is never invoked more than once by a Java 504 * virtual machine for any given object. 505 * <p> 506 * Any exception thrown by the {@code finalize} method causes 507 * the finalization of this object to be halted, but is otherwise 508 * ignored. 509 * 510 * @apiNote 511 * Classes that embed non-heap resources have many options 512 * for cleanup of those resources. The class must ensure that the 513 * lifetime of each instance is longer than that of any resource it embeds. 514 * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that 515 * objects remain reachable while resources embedded in the object are in use. 516 * <p> 517 * A subclass should avoid overriding the {@code finalize} method 518 * unless the subclass embeds non-heap resources that must be cleaned up 519 * before the instance is collected. 520 * Finalizer invocations are not automatically chained, unlike constructors. 521 * If a subclass overrides {@code finalize} it must invoke the superclass 522 * finalizer explicitly. 523 * To guard against exceptions prematurely terminating the finalize chain, 524 * the subclass should use a {@code try-finally} block to ensure 525 * {@code super.finalize()} is always invoked. For example, 526 * <pre>{@code @Override 527 * protected void finalize() throws Throwable { 528 * try { 529 * ... // cleanup subclass state 530 * } finally { 531 * super.finalize(); 532 * } 533 * } 534 * }</pre> 535 * 536 * @deprecated The finalization mechanism is inherently problematic. 537 * Finalization can lead to performance issues, deadlocks, and hangs. 538 * Errors in finalizers can lead to resource leaks; there is no way to cancel 539 * finalization if it is no longer necessary; and no ordering is specified 540 * among calls to {@code finalize} methods of different objects. 541 * Furthermore, there are no guarantees regarding the timing of finalization. 542 * The {@code finalize} method might be called on a finalizable object 543 * only after an indefinite delay, if at all. 544 * 545 * Classes whose instances hold non-heap resources should provide a method 546 * to enable explicit release of those resources, and they should also 547 * implement {@link AutoCloseable} if appropriate. 548 * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference} 549 * provide more flexible and efficient ways to release resources when an object 550 * becomes unreachable. 551 * 552 * @throws Throwable the {@code Exception} raised by this method 553 * @see java.lang.ref.WeakReference 554 * @see java.lang.ref.PhantomReference 555 * @jls 12.6 Finalization of Class Instances 556 */ 557 @Deprecated(since="9") finalize()558 protected void finalize() throws Throwable { } 559 } 560