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java Object解析

發布時間：2024/4/17 编程问答 26 豆豆

生活随笔收集整理的這篇文章主要介紹了 java Object解析小編覺得挺不錯的,現在分享給大家,幫大家做個參考.

java Object是所有對象的根父類，所有對象都直接或間接集成自該類。

java 的Object類也比較簡單，有equals(Object)、toString()、finalize() java方法和hashcode()、clone()、wait()、notify()、notifyAll()、getClass()的native方法。

遠嗎如下：

1 public class Object { 2 3 private static native void registerNatives(); 4 static { 5 registerNatives(); 6 } 7 8 /** 9 * Returns the runtime class of this {@code Object}. The returned 10 * {@code Class} object is the object that is locked by {@code 11 * static synchronized} methods of the represented class. 12 * 13 * The actual result type is {@code Class<? extends |X|>} 14 * where {@code |X|} is the erasure of the static type of the 15 * expression on which {@code getClass} is called. For 16 * example, no cast is required in this code fragment: 17 * 18 * 19 * {@code Number n = 0; } 20 * {@code Class<? extends Number> c = n.getClass(); } 21 * 22 * 23 * @return The {@code Class} object that represents the runtime 24 * class of this object. 25 * @jls 15.8.2 Class Literals 26 */ 27 public final native Class<?> getClass(); 28 29 /** 30 * Returns a hash code value for the object. This method is 31 * supported for the benefit of hash tables such as those provided by 32 * {@link java.util.HashMap}. 33 * 34 * The general contract of {@code hashCode} is: 35 * <ul> 36 * <li>Whenever it is invoked on the same object more than once during 37 * an execution of a Java application, the {@code hashCode} method 38 * must consistently return the same integer, provided no information 39 * used in {@code equals} comparisons on the object is modified. 40 * This integer need not remain consistent from one execution of an 41 * application to another execution of the same application. 42 * <li>If two objects are equal according to the {@code equals(Object)} 43 * method, then calling the {@code hashCode} method on each of 44 * the two objects must produce the same integer result. 45 * <li>It is not required that if two objects are unequal 46 * according to the {@link java.lang.Object#equals(java.lang.Object)} 47 * method, then calling the {@code hashCode} method on each of the 48 * two objects must produce distinct integer results. However, the 49 * programmer should be aware that producing distinct integer results 50 * for unequal objects may improve the performance of hash tables. 51 * </ul> 52 * 53 * As much as is reasonably practical, the hashCode method defined by 54 * class {@code Object} does return distinct integers for distinct 55 * objects. (This is typically implemented by converting the internal 56 * address of the object into an integer, but this implementation 57 * technique is not required by the 58 * Java™ programming language.) 59 * 60 * @return a hash code value for this object. 61 * @see java.lang.Object#equals(java.lang.Object) 62 * @see java.lang.System#identityHashCode 63 */ 64 public native int hashCode(); 65 66 /** 67 * Indicates whether some other object is "equal to" this one. 68 * 69 * The {@code equals} method implements an equivalence relation 70 * on non-null object references: 71 * <ul> 72 * <li>It is reflexive: for any non-null reference value 73 * {@code x}, {@code x.equals(x)} should return 74 * {@code true}. 75 * <li>It is symmetric: for any non-null reference values 76 * {@code x} and {@code y}, {@code x.equals(y)} 77 * should return {@code true} if and only if 78 * {@code y.equals(x)} returns {@code true}. 79 * <li>It is transitive: for any non-null reference values 80 * {@code x}, {@code y}, and {@code z}, if 81 * {@code x.equals(y)} returns {@code true} and 82 * {@code y.equals(z)} returns {@code true}, then 83 * {@code x.equals(z)} should return {@code true}. 84 * <li>It is consistent: for any non-null reference values 85 * {@code x} and {@code y}, multiple invocations of 86 * {@code x.equals(y)} consistently return {@code true} 87 * or consistently return {@code false}, provided no 88 * information used in {@code equals} comparisons on the 89 * objects is modified. 90 * <li>For any non-null reference value {@code x}, 91 * {@code x.equals(null)} should return {@code false}. 92 * </ul> 93 * 94 * The {@code equals} method for class {@code Object} implements 95 * the most discriminating possible equivalence relation on objects; 96 * that is, for any non-null reference values {@code x} and 97 * {@code y}, this method returns {@code true} if and only 98 * if {@code x} and {@code y} refer to the same object 99 * ({@code x == y} has the value {@code true}). 100 * 101 * Note that it is generally necessary to override the {@code hashCode} 102 * method whenever this method is overridden, so as to maintain the 103 * general contract for the {@code hashCode} method, which states 104 * that equal objects must have equal hash codes. 105 * 106 * @param obj the reference object with which to compare. 107 * @return {@code true} if this object is the same as the obj 108 * argument; {@code false} otherwise. 109 * @see #hashCode() 110 * @see java.util.HashMap 111 */ 112 public boolean equals(Object obj) { 113 return (this == obj); 114 } 115 116 /** 117 * Creates and returns a copy of this object. The precise meaning 118 * of "copy" may depend on the class of the object. The general 119 * intent is that, for any object {@code x}, the expression: 120 * <blockquote> 121 * <pre> 122 * x.clone() != x</pre></blockquote> 123 * will be true, and that the expression: 124 * <blockquote> 125 * <pre> 126 * x.clone().getClass() == x.getClass()</pre></blockquote> 127 * will be {@code true}, but these are not absolute requirements. 128 * While it is typically the case that: 129 * <blockquote> 130 * <pre> 131 * x.clone().equals(x)</pre></blockquote> 132 * will be {@code true}, this is not an absolute requirement. 133 * 134 * By convention, the returned object should be obtained by calling 135 * {@code super.clone}. If a class and all of its superclasses (except 136 * {@code Object}) obey this convention, it will be the case that 137 * {@code x.clone().getClass() == x.getClass()}. 138 * 139 * By convention, the object returned by this method should be independent 140 * of this object (which is being cloned). To achieve this independence, 141 * it may be necessary to modify one or more fields of the object returned 142 * by {@code super.clone} before returning it. Typically, this means 143 * copying any mutable objects that comprise the internal "deep structure" 144 * of the object being cloned and replacing the references to these 145 * objects with references to the copies. If a class contains only 146 * primitive fields or references to immutable objects, then it is usually 147 * the case that no fields in the object returned by {@code super.clone} 148 * need to be modified. 149 * 150 * The method {@code clone} for class {@code Object} performs a 151 * specific cloning operation. First, if the class of this object does 152 * not implement the interface {@code Cloneable}, then a 153 * {@code CloneNotSupportedException} is thrown. Note that all arrays 154 * are considered to implement the interface {@code Cloneable} and that 155 * the return type of the {@code clone} method of an array type {@code T[]} 156 * is {@code T[]} where T is any reference or primitive type. 157 * Otherwise, this method creates a new instance of the class of this 158 * object and initializes all its fields with exactly the contents of 159 * the corresponding fields of this object, as if by assignment; the 160 * contents of the fields are not themselves cloned. Thus, this method 161 * performs a "shallow copy" of this object, not a "deep copy" operation. 162 * 163 * The class {@code Object} does not itself implement the interface 164 * {@code Cloneable}, so calling the {@code clone} method on an object 165 * whose class is {@code Object} will result in throwing an 166 * exception at run time. 167 * 168 * @return a clone of this instance. 169 * @throws CloneNotSupportedException if the object's class does not 170 * support the {@code Cloneable} interface. Subclasses 171 * that override the {@code clone} method can also 172 * throw this exception to indicate that an instance cannot 173 * be cloned. 174 * @see java.lang.Cloneable 175 */ 176 protected native Object clone() throws CloneNotSupportedException; 177 178 /** 179 * Returns a string representation of the object. In general, the 180 * {@code toString} method returns a string that 181 * "textually represents" this object. The result should 182 * be a concise but informative representation that is easy for a 183 * person to read. 184 * It is recommended that all subclasses override this method. 185 * 186 * The {@code toString} method for class {@code Object} 187 * returns a string consisting of the name of the class of which the 188 * object is an instance, the at-sign character `{@code @}', and 189 * the unsigned hexadecimal representation of the hash code of the 190 * object. In other words, this method returns a string equal to the 191 * value of: 192 * <blockquote> 193 * <pre> 194 * getClass().getName() + '@' + Integer.toHexString(hashCode()) 195 * </pre></blockquote> 196 * 197 * @return a string representation of the object. 198 */ 199 public String toString() { 200 return getClass().getName() + "@" + Integer.toHexString(hashCode()); 201 } 202 203 /** 204 * Wakes up a single thread that is waiting on this object's 205 * monitor. If any threads are waiting on this object, one of them 206 * is chosen to be awakened. The choice is arbitrary and occurs at 207 * the discretion of the implementation. A thread waits on an object's 208 * monitor by calling one of the {@code wait} methods. 209 * 210 * The awakened thread will not be able to proceed until the current 211 * thread relinquishes the lock on this object. The awakened thread will 212 * compete in the usual manner with any other threads that might be 213 * actively competing to synchronize on this object; for example, the 214 * awakened thread enjoys no reliable privilege or disadvantage in being 215 * the next thread to lock this object. 216 * 217 * This method should only be called by a thread that is the owner 218 * of this object's monitor. A thread becomes the owner of the 219 * object's monitor in one of three ways: 220 * <ul> 221 * <li>By executing a synchronized instance method of that object. 222 * <li>By executing the body of a {@code synchronized} statement 223 * that synchronizes on the object. 224 * <li>For objects of type {@code Class,} by executing a 225 * synchronized static method of that class. 226 * </ul> 227 * 228 * Only one thread at a time can own an object's monitor. 229 * 230 * @throws IllegalMonitorStateException if the current thread is not 231 * the owner of this object's monitor. 232 * @see java.lang.Object#notifyAll() 233 * @see java.lang.Object#wait() 234 */ 235 public final native void notify(); 236 237 /** 238 * Wakes up all threads that are waiting on this object's monitor. A 239 * thread waits on an object's monitor by calling one of the 240 * {@code wait} methods. 241 * 242 * The awakened threads will not be able to proceed until the current 243 * thread relinquishes the lock on this object. The awakened threads 244 * will compete in the usual manner with any other threads that might 245 * be actively competing to synchronize on this object; for example, 246 * the awakened threads enjoy no reliable privilege or disadvantage in 247 * being the next thread to lock this object. 248 * 249 * This method should only be called by a thread that is the owner 250 * of this object's monitor. See the {@code notify} method for a 251 * description of the ways in which a thread can become the owner of 252 * a monitor. 253 * 254 * @throws IllegalMonitorStateException if the current thread is not 255 * the owner of this object's monitor. 256 * @see java.lang.Object#notify() 257 * @see java.lang.Object#wait() 258 */ 259 public final native void notifyAll(); 260 261 /** 262 * Causes the current thread to wait until either another thread invokes the 263 * {@link java.lang.Object#notify()} method or the 264 * {@link java.lang.Object#notifyAll()} method for this object, or a 265 * specified amount of time has elapsed. 266 * 267 * The current thread must own this object's monitor. 268 * 269 * This method causes the current thread (call it <var>T</var>) to 270 * place itself in the wait set for this object and then to relinquish 271 * any and all synchronization claims on this object. Thread <var>T</var> 272 * becomes disabled for thread scheduling purposes and lies dormant 273 * until one of four things happens: 274 * <ul> 275 * <li>Some other thread invokes the {@code notify} method for this 276 * object and thread <var>T</var> happens to be arbitrarily chosen as 277 * the thread to be awakened. 278 * <li>Some other thread invokes the {@code notifyAll} method for this 279 * object. 280 * <li>Some other thread {@linkplain Thread#interrupt() interrupts} 281 * thread <var>T</var>. 282 * <li>The specified amount of real time has elapsed, more or less. If 283 * {@code timeout} is zero, however, then real time is not taken into 284 * consideration and the thread simply waits until notified. 285 * </ul> 286 * The thread <var>T</var> is then removed from the wait set for this 287 * object and re-enabled for thread scheduling. It then competes in the 288 * usual manner with other threads for the right to synchronize on the 289 * object; once it has gained control of the object, all its 290 * synchronization claims on the object are restored to the status quo 291 * ante - that is, to the situation as of the time that the {@code wait} 292 * method was invoked. Thread <var>T</var> then returns from the 293 * invocation of the {@code wait} method. Thus, on return from the 294 * {@code wait} method, the synchronization state of the object and of 295 * thread {@code T} is exactly as it was when the {@code wait} method 296 * was invoked. 297 * 298 * A thread can also wake up without being notified, interrupted, or 299 * timing out, a so-called spurious wakeup. While this will rarely 300 * occur in practice, applications must guard against it by testing for 301 * the condition that should have caused the thread to be awakened, and 302 * continuing to wait if the condition is not satisfied. In other words, 303 * waits should always occur in loops, like this one: 304 * <pre> 305 * synchronized (obj) { 306 * while (<condition does not hold>) 307 * obj.wait(timeout); 308 * ... // Perform action appropriate to condition 309 * } 310 * </pre> 311 * (For more information on this topic, see Section 3.2.3 in Doug Lea's 312 * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley, 313 * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming 314 * Language Guide" (Addison-Wesley, 2001). 315 * 316 * If the current thread is {@linkplain java.lang.Thread#interrupt() 317 * interrupted} by any thread before or while it is waiting, then an 318 * {@code InterruptedException} is thrown. This exception is not 319 * thrown until the lock status of this object has been restored as 320 * described above. 321 * 322 * 323 * Note that the {@code wait} method, as it places the current thread 324 * into the wait set for this object, unlocks only this object; any 325 * other objects on which the current thread may be synchronized remain 326 * locked while the thread waits. 327 * 328 * This method should only be called by a thread that is the owner 329 * of this object's monitor. See the {@code notify} method for a 330 * description of the ways in which a thread can become the owner of 331 * a monitor. 332 * 333 * @param timeout the maximum time to wait in milliseconds. 334 * @throws IllegalArgumentException if the value of timeout is 335 * negative. 336 * @throws IllegalMonitorStateException if the current thread is not 337 * the owner of the object's monitor. 338 * @throws InterruptedException if any thread interrupted the 339 * current thread before or while the current thread 340 * was waiting for a notification. The interrupted 341 * status of the current thread is cleared when 342 * this exception is thrown. 343 * @see java.lang.Object#notify() 344 * @see java.lang.Object#notifyAll() 345 */ 346 public final native void wait(long timeout) throws InterruptedException; 347 348 /** 349 * Causes the current thread to wait until another thread invokes the 350 * {@link java.lang.Object#notify()} method or the 351 * {@link java.lang.Object#notifyAll()} method for this object, or 352 * some other thread interrupts the current thread, or a certain 353 * amount of real time has elapsed. 354 * 355 * This method is similar to the {@code wait} method of one 356 * argument, but it allows finer control over the amount of time to 357 * wait for a notification before giving up. The amount of real time, 358 * measured in nanoseconds, is given by: 359 * <blockquote> 360 * <pre> 361 * 1000000*timeout+nanos</pre></blockquote> 362 * 363 * In all other respects, this method does the same thing as the 364 * method {@link #wait(long)} of one argument. In particular, 365 * {@code wait(0, 0)} means the same thing as {@code wait(0)}. 366 * 367 * The current thread must own this object's monitor. The thread 368 * releases ownership of this monitor and waits until either of the 369 * following two conditions has occurred: 370 * <ul> 371 * <li>Another thread notifies threads waiting on this object's monitor 372 * to wake up either through a call to the {@code notify} method 373 * or the {@code notifyAll} method. 374 * <li>The timeout period, specified by {@code timeout} 375 * milliseconds plus {@code nanos} nanoseconds arguments, has 376 * elapsed. 377 * </ul> 378 * 379 * The thread then waits until it can re-obtain ownership of the 380 * monitor and resumes execution. 381 * 382 * As in the one argument version, interrupts and spurious wakeups are 383 * possible, and this method should always be used in a loop: 384 * <pre> 385 * synchronized (obj) { 386 * while (<condition does not hold>) 387 * obj.wait(timeout, nanos); 388 * ... // Perform action appropriate to condition 389 * } 390 * </pre> 391 * This method should only be called by a thread that is the owner 392 * of this object's monitor. See the {@code notify} method for a 393 * description of the ways in which a thread can become the owner of 394 * a monitor. 395 * 396 * @param timeout the maximum time to wait in milliseconds. 397 * @param nanos additional time, in nanoseconds range 398 * 0-999999. 399 * @throws IllegalArgumentException if the value of timeout is 400 * negative or the value of nanos is 401 * not in the range 0-999999. 402 * @throws IllegalMonitorStateException if the current thread is not 403 * the owner of this object's monitor. 404 * @throws InterruptedException if any thread interrupted the 405 * current thread before or while the current thread 406 * was waiting for a notification. The interrupted 407 * status of the current thread is cleared when 408 * this exception is thrown. 409 */ 410 public final void wait(long timeout, int nanos) throws InterruptedException { 411 if (timeout < 0) { 412 throw new IllegalArgumentException("timeout value is negative"); 413 } 414 415 if (nanos < 0 || nanos > 999999) { 416 throw new IllegalArgumentException( 417 "nanosecond timeout value out of range"); 418 } 419 420 if (nanos > 0) { 421 timeout++; 422 } 423 424 wait(timeout); 425 } 426 427 /** 428 * Causes the current thread to wait until another thread invokes the 429 * {@link java.lang.Object#notify()} method or the 430 * {@link java.lang.Object#notifyAll()} method for this object. 431 * In other words, this method behaves exactly as if it simply 432 * performs the call {@code wait(0)}. 433 * 434 * The current thread must own this object's monitor. The thread 435 * releases ownership of this monitor and waits until another thread 436 * notifies threads waiting on this object's monitor to wake up 437 * either through a call to the {@code notify} method or the 438 * {@code notifyAll} method. The thread then waits until it can 439 * re-obtain ownership of the monitor and resumes execution. 440 * 441 * As in the one argument version, interrupts and spurious wakeups are 442 * possible, and this method should always be used in a loop: 443 * <pre> 444 * synchronized (obj) { 445 * while (<condition does not hold>) 446 * obj.wait(); 447 * ... // Perform action appropriate to condition 448 * } 449 * </pre> 450 * This method should only be called by a thread that is the owner 451 * of this object's monitor. See the {@code notify} method for a 452 * description of the ways in which a thread can become the owner of 453 * a monitor. 454 * 455 * @throws IllegalMonitorStateException if the current thread is not 456 * the owner of the object's monitor. 457 * @throws InterruptedException if any thread interrupted the 458 * current thread before or while the current thread 459 * was waiting for a notification. The interrupted 460 * status of the current thread is cleared when 461 * this exception is thrown. 462 * @see java.lang.Object#notify() 463 * @see java.lang.Object#notifyAll() 464 */ 465 public final void wait() throws InterruptedException { 466 wait(0); 467 } 468 469 /** 470 * Called by the garbage collector on an object when garbage collection 471 * determines that there are no more references to the object. 472 * A subclass overrides the {@code finalize} method to dispose of 473 * system resources or to perform other cleanup. 474 * 475 * The general contract of {@code finalize} is that it is invoked 476 * if and when the Java™ virtual 477 * machine has determined that there is no longer any 478 * means by which this object can be accessed by any thread that has 479 * not yet died, except as a result of an action taken by the 480 * finalization of some other object or class which is ready to be 481 * finalized. The {@code finalize} method may take any action, including 482 * making this object available again to other threads; the usual purpose 483 * of {@code finalize}, however, is to perform cleanup actions before 484 * the object is irrevocably discarded. For example, the finalize method 485 * for an object that represents an input/output connection might perform 486 * explicit I/O transactions to break the connection before the object is 487 * permanently discarded. 488 * 489 * The {@code finalize} method of class {@code Object} performs no 490 * special action; it simply returns normally. Subclasses of 491 * {@code Object} may override this definition. 492 * 493 * The Java programming language does not guarantee which thread will 494 * invoke the {@code finalize} method for any given object. It is 495 * guaranteed, however, that the thread that invokes finalize will not 496 * be holding any user-visible synchronization locks when finalize is 497 * invoked. If an uncaught exception is thrown by the finalize method, 498 * the exception is ignored and finalization of that object terminates. 499 * 500 * After the {@code finalize} method has been invoked for an object, no 501 * further action is taken until the Java virtual machine has again 502 * determined that there is no longer any means by which this object can 503 * be accessed by any thread that has not yet died, including possible 504 * actions by other objects or classes which are ready to be finalized, 505 * at which point the object may be discarded. 506 * 507 * The {@code finalize} method is never invoked more than once by a Java 508 * virtual machine for any given object. 509 * 510 * Any exception thrown by the {@code finalize} method causes 511 * the finalization of this object to be halted, but is otherwise 512 * ignored. 513 * 514 * @throws Throwable the {@code Exception} raised by this method 515 * @see java.lang.ref.WeakReference 516 * @see java.lang.ref.PhantomReference 517 * @jls 12.6 Finalization of Class Instances 518 */ 519 protected void finalize() throws Throwable { } 520 }

? ? 二、native源碼查看，源碼路徑：Object類對應openjdk\jdk\src\share\native\java\lang\Object.c

1 #include <stdio.h> 2 #include <signal.h> 3 #include <limits.h> 4 5 #include "jni.h" 6 #include "jni_util.h" 7 #include "jvm.h" 8 9 #include "java_lang_Object.h" 10 11 static JNINativeMethod methods[] = { 12 {"hashCode", "()I", (void *)&JVM_IHashCode}, 13 {"wait", "(J)V", (void *)&JVM_MonitorWait}, 14 {"notify", "()V", (void *)&JVM_MonitorNotify}, 15 {"notifyAll", "()V", (void *)&JVM_MonitorNotifyAll}, 16 {"clone", "()Ljava/lang/Object;", (void *)&JVM_Clone}, 17 }; 18 19 JNIEXPORT void JNICALL 20 Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls) 21 { 22 (*env)->RegisterNatives(env, cls, 23 methods, sizeof(methods)/sizeof(methods[0])); 24 } 25 26 JNIEXPORT jclass JNICALL 27 Java_java_lang_Object_getClass(JNIEnv *env, jobject this) 28 { 29 if (this == NULL) { 30 JNU_ThrowNullPointerException(env, NULL); 31 return 0; 32 } else { 33 return (*env)->GetObjectClass(env, this); 34 } 35 }

其中final方法有wait（）、notify（）、notifyAll（）、getClass（），也即同步的方法和getClass；wait和notify的相關方法必須在同步塊或方法中調用；

拋異常的又clone（） throws CloneNotSupportedException，也就是克隆必須實現cloneable（標記性接口）或重寫clone（）方法就可以使用系統默認的clone，默認是淺克隆。深復制和淺復制區別看原因用和新引用是否指向同一個對象；

? ? ? ? ? ? ? ? ? 3個wait()方法 throws InterruptException，中斷異常，即可以相應中斷；

對象比較，equals（Object obj）默認實現是this==obj，需要重寫，兩個相等的對象必須具有相同的hashcode值，不相等的兩個對象的hashcode可以相等;

toString（）默認實現是getClass.getName()+"@"+Integer.toHexString(hashcode)。

另外，java值傳遞和引用傳遞問題：

? ? java的參數傳遞其實是按值傳遞的，按值傳遞值的拷貝。按引用傳遞其實是傳遞的引用的地址。

? ? java基本類型都是按值進行傳遞的，對象類型和數組都是按引用傳遞的

? ? ? ? ? ?String x = "google com";

? ? ? ? ? ?change(x);

? ? ? ? ? ?log.d("linghu", "=======x:"+x+"========");

? ? ? ? ? ?

? ? ? ? ? ? void change(String x){

? ? ? ? ? ? ? ? ? ? ? x="alibaba com";

? ? ? ? ? ?}

? ? ? ? ? ?結果依舊是=======x:google com========, String是final const類型的

轉載于:https://www.cnblogs.com/linghu-java/p/10021510.html

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