Java基础1----Object类解析 / 憋错料

1.Object简介

众所周知，Object类是Java所有类的万类之源，所有Java类都是继承之Object类，而默认就直接忽略了extends Object这段代码。

2.Object类的源码

话不多说，源码先贴为敬，源码如下：

 1 package java.lang;
 2
 3 public class Object {
 4     //本地方法,通过JNI调用
 5     private static native void registerNatives();
 6     static {//对象初始化时调用
 7         registerNatives();
 8     }
 9     //返回object在运行时的类对象
10     public final native Class<?> getClass();
11     //获取object对象的hashcode
12     public native int hashCode();
13     //比较对象的内存地址
14     public boolean equals(Object obj) {
15         return (this == obj);
16     }
17     //本地的clone方法，用于对象的copy
18     protected native Object clone() throws CloneNotSupportedException;
19     //返回对象的字符串
20     public String toString() {
21         return getClass().getName() + "@" + Integer.toHexString(hashCode());
22     }
23
24     //唤醒等待此对象锁的单个线程
25     public final native void notify();
26     //唤醒等待此对象锁的所有线程
27     public final native void notifyAll();
28     //放弃对象锁，等待指定时间
29     public final native void wait(long timeout) throws InterruptedException;
30
31     public final void wait(long timeout, int nanos) throws InterruptedException {
32         if (timeout < 0) {
33             throw new IllegalArgumentException("timeout value is negative");
34         }
35
36         if (nanos < 0 || nanos > 999999) {
37             throw new IllegalArgumentException(
38                                 "nanosecond timeout value out of range");
39         }
40
41         if (nanos > 0) {
42             timeout++;
43         }
44
45         wait(timeout);
46     }
47
48
49     public final void wait() throws InterruptedException {
50         wait(0);
51     }
52     //用于垃圾回收
53     protected void finalize() throws Throwable { }
54 }

3.Object类的方法

Object类方法如下

getClass()

hashCode()

equals(Object obj)

Object clone()

toString()

notify()

notifyAll()

wait()

wait(long timeout)

wait(long timeout,int nanos)

finalize()

4.注意事项

/* * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */
package java.lang;
/** * Class {@code Object} is the root of the class hierarchy. * Every class has {@code Object} as a superclass. All objects, * including arrays, implement the methods of this class. * * @author unascribed * @see java.lang.Class * @since JDK1.0 */public class Object {
private static native void registerNatives(); static { registerNatives(); }
/** * Returns the runtime class of this {@code Object}. The returned * {@code Class} object is the object that is locked by {@code * static synchronized} methods of the represented class. * * The actual result type is {@code Class<? extends |X|>} * where {@code |X|} is the erasure of the static type of the * expression on which {@code getClass} is called. For * example, no cast is required in this code fragment: * * * {@code Number n = 0; } * {@code Class<? extends Number> c = n.getClass(); } * * * @return The {@code Class} object that represents the runtime * class of this object. * @jls 15.8.2 Class Literals */ public final native Class<?> getClass();
/** * Returns a hash code value for the object. This method is * supported for the benefit of hash tables such as those provided by * {@link java.util.HashMap}. * * The general contract of {@code hashCode} is: * <ul> * <li>Whenever it is invoked on the same object more than once during * an execution of a Java application, the {@code hashCode} method * must consistently return the same integer, provided no information * used in {@code equals} comparisons on the object is modified. * This integer need not remain consistent from one execution of an * application to another execution of the same application. * <li>If two objects are equal according to the {@code equals(Object)} * method, then calling the {@code hashCode} method on each of * the two objects must produce the same integer result. * <li>It is not required that if two objects are unequal * according to the {@link java.lang.Object#equals(java.lang.Object)} * method, then calling the {@code hashCode} method on each of the * two objects must produce distinct integer results. However, the * programmer should be aware that producing distinct integer results * for unequal objects may improve the performance of hash tables. * </ul> * * As much as is reasonably practical, the hashCode method defined by * class {@code Object} does return distinct integers for distinct * objects. (This is typically implemented by converting the internal * address of the object into an integer, but this implementation * technique is not required by the * Java™ programming language.) * * @return a hash code value for this object. * @see java.lang.Object#equals(java.lang.Object) * @see java.lang.System#identityHashCode */ public native int hashCode();
/** * Indicates whether some other object is "equal to" this one. * * The {@code equals} method implements an equivalence relation * on non-null object references: * <ul> * <li>It is reflexive: for any non-null reference value * {@code x}, {@code x.equals(x)} should return * {@code true}. * <li>It is symmetric: for any non-null reference values * {@code x} and {@code y}, {@code x.equals(y)} * should return {@code true} if and only if * {@code y.equals(x)} returns {@code true}. * <li>It is transitive: for any non-null reference values * {@code x}, {@code y}, and {@code z}, if * {@code x.equals(y)} returns {@code true} and * {@code y.equals(z)} returns {@code true}, then * {@code x.equals(z)} should return {@code true}. * <li>It is consistent: for any non-null reference values * {@code x} and {@code y}, multiple invocations of * {@code x.equals(y)} consistently return {@code true} * or consistently return {@code false}, provided no * information used in {@code equals} comparisons on the * objects is modified. * <li>For any non-null reference value {@code x}, * {@code x.equals(null)} should return {@code false}. * </ul> * * The {@code equals} method for class {@code Object} implements * the most discriminating possible equivalence relation on objects; * that is, for any non-null reference values {@code x} and * {@code y}, this method returns {@code true} if and only * if {@code x} and {@code y} refer to the same object * ({@code x == y} has the value {@code true}). * * Note that it is generally necessary to override the {@code hashCode} * method whenever this method is overridden, so as to maintain the * general contract for the {@code hashCode} method, which states * that equal objects must have equal hash codes. * * @param obj the reference object with which to compare. * @return {@code true} if this object is the same as the obj * argument; {@code false} otherwise. * @see #hashCode() * @see java.util.HashMap */ public boolean equals(Object obj) { return (this == obj); }
/** * Creates and returns a copy of this object. The precise meaning * of "copy" may depend on the class of the object. The general * intent is that, for any object {@code x}, the expression: * <blockquote> * <pre> * x.clone() != x</pre></blockquote> * will be true, and that the expression: * <blockquote> * <pre> * x.clone().getClass() == x.getClass()</pre></blockquote> * will be {@code true}, but these are not absolute requirements. * While it is typically the case that: * <blockquote> * <pre> * x.clone().equals(x)</pre></blockquote> * will be {@code true}, this is not an absolute requirement. * * By convention, the returned object should be obtained by calling * {@code super.clone}. If a class and all of its superclasses (except * {@code Object}) obey this convention, it will be the case that * {@code x.clone().getClass() == x.getClass()}. * * By convention, the object returned by this method should be independent * of this object (which is being cloned). To achieve this independence, * it may be necessary to modify one or more fields of the object returned * by {@code super.clone} before returning it. Typically, this means * copying any mutable objects that comprise the internal "deep structure" * of the object being cloned and replacing the references to these * objects with references to the copies. If a class contains only * primitive fields or references to immutable objects, then it is usually * the case that no fields in the object returned by {@code super.clone} * need to be modified. * * The method {@code clone} for class {@code Object} performs a * specific cloning operation. First, if the class of this object does * not implement the interface {@code Cloneable}, then a * {@code CloneNotSupportedException} is thrown. Note that all arrays * are considered to implement the interface {@code Cloneable} and that * the return type of the {@code clone} method of an array type {@code T[]} * is {@code T[]} where T is any reference or primitive type. * Otherwise, this method creates a new instance of the class of this * object and initializes all its fields with exactly the contents of * the corresponding fields of this object, as if by assignment; the * contents of the fields are not themselves cloned. Thus, this method * performs a "shallow copy" of this object, not a "deep copy" operation. * * The class {@code Object} does not itself implement the interface * {@code Cloneable}, so calling the {@code clone} method on an object * whose class is {@code Object} will result in throwing an * exception at run time. * * @return a clone of this instance. * @throws CloneNotSupportedException if the object‘s class does not * support the {@code Cloneable} interface. Subclasses * that override the {@code clone} method can also * throw this exception to indicate that an instance cannot * be cloned. * @see java.lang.Cloneable */ protected native Object clone() throws CloneNotSupportedException;
/** * Returns a string representation of the object. In general, the * {@code toString} method returns a string that * "textually represents" this object. The result should * be a concise but informative representation that is easy for a * person to read. * It is recommended that all subclasses override this method. * * The {@code toString} method for class {@code Object} * returns a string consisting of the name of the class of which the * object is an instance, the at-sign character `{@code @}‘, and * the unsigned hexadecimal representation of the hash code of the * object. In other words, this method returns a string equal to the * value of: * <blockquote> * <pre> * getClass().getName() + ‘@‘ + Integer.toHexString(hashCode()) * </pre></blockquote> * * @return a string representation of the object. */ public String toString() { return getClass().getName() + "@" + Integer.toHexString(hashCode()); }
/** * Wakes up a single thread that is waiting on this object‘s * monitor. If any threads are waiting on this object, one of them * is chosen to be awakened. The choice is arbitrary and occurs at * the discretion of the implementation. A thread waits on an object‘s * monitor by calling one of the {@code wait} methods. * * The awakened thread will not be able to proceed until the current * thread relinquishes the lock on this object. The awakened thread will * compete in the usual manner with any other threads that might be * actively competing to synchronize on this object; for example, the * awakened thread enjoys no reliable privilege or disadvantage in being * the next thread to lock this object. * * This method should only be called by a thread that is the owner * of this object‘s monitor. A thread becomes the owner of the * object‘s monitor in one of three ways: * <ul> * <li>By executing a synchronized instance method of that object. * <li>By executing the body of a {@code synchronized} statement * that synchronizes on the object. * <li>For objects of type {@code Class,} by executing a * synchronized static method of that class. * </ul> * * Only one thread at a time can own an object‘s monitor. * * @throws IllegalMonitorStateException if the current thread is not * the owner of this object‘s monitor. * @see java.lang.Object#notifyAll() * @see java.lang.Object#wait() */ public final native void notify();
/** * Wakes up all threads that are waiting on this object‘s monitor. A * thread waits on an object‘s monitor by calling one of the * {@code wait} methods. * * The awakened threads will not be able to proceed until the current * thread relinquishes the lock on this object. The awakened threads * will compete in the usual manner with any other threads that might * be actively competing to synchronize on this object; for example, * the awakened threads enjoy no reliable privilege or disadvantage in * being the next thread to lock this object. * * This method should only be called by a thread that is the owner * of this object‘s monitor. See the {@code notify} method for a * description of the ways in which a thread can become the owner of * a monitor. * * @throws IllegalMonitorStateException if the current thread is not * the owner of this object‘s monitor. * @see java.lang.Object#notify() * @see java.lang.Object#wait() */ public final native void notifyAll();
/** * Causes the current thread to wait until either another thread invokes the * {@link java.lang.Object#notify()} method or the * {@link java.lang.Object#notifyAll()} method for this object, or a * specified amount of time has elapsed. * * The current thread must own this object‘s monitor. * * This method causes the current thread (call it <var>T</var>) to * place itself in the wait set for this object and then to relinquish * any and all synchronization claims on this object. Thread <var>T</var> * becomes disabled for thread scheduling purposes and lies dormant * until one of four things happens: * <ul> * <li>Some other thread invokes the {@code notify} method for this * object and thread <var>T</var> happens to be arbitrarily chosen as * the thread to be awakened. * <li>Some other thread invokes the {@code notifyAll} method for this * object. * <li>Some other thread {@linkplain Thread#interrupt() interrupts} * thread <var>T</var>. * <li>The specified amount of real time has elapsed, more or less. If * {@code timeout} is zero, however, then real time is not taken into * consideration and the thread simply waits until notified. * </ul> * The thread <var>T</var> is then removed from the wait set for this * object and re-enabled for thread scheduling. It then competes in the * usual manner with other threads for the right to synchronize on the * object; once it has gained control of the object, all its * synchronization claims on the object are restored to the status quo * ante - that is, to the situation as of the time that the {@code wait} * method was invoked. Thread <var>T</var> then returns from the * invocation of the {@code wait} method. Thus, on return from the * {@code wait} method, the synchronization state of the object and of * thread {@code T} is exactly as it was when the {@code wait} method * was invoked. * * A thread can also wake up without being notified, interrupted, or * timing out, a so-called spurious wakeup. While this will rarely * occur in practice, applications must guard against it by testing for * the condition that should have caused the thread to be awakened, and * continuing to wait if the condition is not satisfied. In other words, * waits should always occur in loops, like this one: * <pre> * synchronized (obj) { * while (<condition does not hold>) * obj.wait(timeout); * ... // Perform action appropriate to condition * } * </pre> * (For more information on this topic, see Section 3.2.3 in Doug Lea‘s * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley, * 2000), or Item 50 in Joshua Bloch‘s "Effective Java Programming * Language Guide" (Addison-Wesley, 2001). * * If the current thread is {@linkplain java.lang.Thread#interrupt() * interrupted} by any thread before or while it is waiting, then an * {@code InterruptedException} is thrown. This exception is not * thrown until the lock status of this object has been restored as * described above. * * * Note that the {@code wait} method, as it places the current thread * into the wait set for this object, unlocks only this object; any * other objects on which the current thread may be synchronized remain * locked while the thread waits. * * This method should only be called by a thread that is the owner * of this object‘s monitor. See the {@code notify} method for a * description of the ways in which a thread can become the owner of * a monitor. * * @param timeout the maximum time to wait in milliseconds. * @throws IllegalArgumentException if the value of timeout is * negative. * @throws IllegalMonitorStateException if the current thread is not * the owner of the object‘s monitor. * @throws InterruptedException if any thread interrupted the * current thread before or while the current thread * was waiting for a notification. The interrupted * status of the current thread is cleared when * this exception is thrown. * @see java.lang.Object#notify() * @see java.lang.Object#notifyAll() */ public final native void wait(long timeout) throws InterruptedException;
/** * Causes the current thread to wait until another thread invokes the * {@link java.lang.Object#notify()} method or the * {@link java.lang.Object#notifyAll()} method for this object, or * some other thread interrupts the current thread, or a certain * amount of real time has elapsed. * * This method is similar to the {@code wait} method of one * argument, but it allows finer control over the amount of time to * wait for a notification before giving up. The amount of real time, * measured in nanoseconds, is given by: * <blockquote> * <pre> * 1000000*timeout+nanos</pre></blockquote> * * In all other respects, this method does the same thing as the * method {@link #wait(long)} of one argument. In particular, * {@code wait(0, 0)} means the same thing as {@code wait(0)}. * * The current thread must own this object‘s monitor. The thread * releases ownership of this monitor and waits until either of the * following two conditions has occurred: * <ul> * <li>Another thread notifies threads waiting on this object‘s monitor * to wake up either through a call to the {@code notify} method * or the {@code notifyAll} method. * <li>The timeout period, specified by {@code timeout} * milliseconds plus {@code nanos} nanoseconds arguments, has * elapsed. * </ul> * * The thread then waits until it can re-obtain ownership of the * monitor and resumes execution. * * As in the one argument version, interrupts and spurious wakeups are * possible, and this method should always be used in a loop: * <pre> * synchronized (obj) { * while (<condition does not hold>) * obj.wait(timeout, nanos); * ... // Perform action appropriate to condition * } * </pre> * This method should only be called by a thread that is the owner * of this object‘s monitor. See the {@code notify} method for a * description of the ways in which a thread can become the owner of * a monitor. * * @param timeout the maximum time to wait in milliseconds. * @param nanos additional time, in nanoseconds range * 0-999999. * @throws IllegalArgumentException if the value of timeout is * negative or the value of nanos is * not in the range 0-999999. * @throws IllegalMonitorStateException if the current thread is not * the owner of this object‘s monitor. * @throws InterruptedException if any thread interrupted the * current thread before or while the current thread * was waiting for a notification. The interrupted * status of the current thread is cleared when * this exception is thrown. */ public final void wait(long timeout, int nanos) throws InterruptedException { if (timeout < 0) { throw new IllegalArgumentException("timeout value is negative"); }
if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException( "nanosecond timeout value out of range"); }
if (nanos > 0) { timeout++; }
wait(timeout); }
/** * Causes the current thread to wait until another thread invokes the * {@link java.lang.Object#notify()} method or the * {@link java.lang.Object#notifyAll()} method for this object. * In other words, this method behaves exactly as if it simply * performs the call {@code wait(0)}. * * The current thread must own this object‘s monitor. The thread * releases ownership of this monitor and waits until another thread * notifies threads waiting on this object‘s monitor to wake up * either through a call to the {@code notify} method or the * {@code notifyAll} method. The thread then waits until it can * re-obtain ownership of the monitor and resumes execution. * * As in the one argument version, interrupts and spurious wakeups are * possible, and this method should always be used in a loop: * <pre> * synchronized (obj) { * while (<condition does not hold>) * obj.wait(); * ... // Perform action appropriate to condition * } * </pre> * This method should only be called by a thread that is the owner * of this object‘s monitor. See the {@code notify} method for a * description of the ways in which a thread can become the owner of * a monitor. * * @throws IllegalMonitorStateException if the current thread is not * the owner of the object‘s monitor. * @throws InterruptedException if any thread interrupted the * current thread before or while the current thread * was waiting for a notification. The interrupted * status of the current thread is cleared when * this exception is thrown. * @see java.lang.Object#notify() * @see java.lang.Object#notifyAll() */ public final void wait() throws InterruptedException { wait(0); }
/** * Called by the garbage collector on an object when garbage collection * determines that there are no more references to the object. * A subclass overrides the {@code finalize} method to dispose of * system resources or to perform other cleanup. * * The general contract of {@code finalize} is that it is invoked * if and when the Java™ virtual * machine has determined that there is no longer any * means by which this object can be accessed by any thread that has * not yet died, except as a result of an action taken by the * finalization of some other object or class which is ready to be * finalized. The {@code finalize} method may take any action, including * making this object available again to other threads; the usual purpose * of {@code finalize}, however, is to perform cleanup actions before * the object is irrevocably discarded. For example, the finalize method * for an object that represents an input/output connection might perform * explicit I/O transactions to break the connection before the object is * permanently discarded. * * The {@code finalize} method of class {@code Object} performs no * special action; it simply returns normally. Subclasses of * {@code Object} may override this definition. * * The Java programming language does not guarantee which thread will * invoke the {@code finalize} method for any given object. It is * guaranteed, however, that the thread that invokes finalize will not * be holding any user-visible synchronization locks when finalize is * invoked. If an uncaught exception is thrown by the finalize method, * the exception is ignored and finalization of that object terminates. * * After the {@code finalize} method has been invoked for an object, no * further action is taken until the Java virtual machine has again * determined that there is no longer any means by which this object can * be accessed by any thread that has not yet died, including possible * actions by other objects or classes which are ready to be finalized, * at which point the object may be discarded. * * The {@code finalize} method is never invoked more than once by a Java * virtual machine for any given object. * * Any exception thrown by the {@code finalize} method causes * the finalization of this object to be halted, but is otherwise * ignored. * * @throws Throwable the {@code Exception} raised by this method * @see java.lang.ref.WeakReference * @see java.lang.ref.PhantomReference * @jls 12.6 Finalization of Class Instances */ protected void finalize() throws Throwable { }}

原文地址：https://www.cnblogs.com/jackion5/p/9451817.html

时间： 2024-11-06 11:25:49

Java基础1----Object类解析

1.Object简介

2.Object类的源码

3.Object类的方法

4.注意事项

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