jdk之object源码理解

　　Java所有类都继承与Object，本文谈谈我对object源码的理解，如果有错的，请看官多多批评指正。

　　1、registerNatives()

   private static native void registerNatives();
    static {
        registerNatives();
    }

　　注册本地方法，静态块内运行，将object内native方法都加载上，native修饰表示这个方法涉及JNI，注册本地方法，用C++写的程序。

　　2、getClass()

 //@return The {@code Class} object that represents the runtime
 class of this object.

public final native Class<?> getClass();

　　　native方法，调用此方法，返回运行时类名

　　3、hashCode　

    / 
　　*<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 <em>not</em> 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.


　　* @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();

　　native方法，返回对象的hashCode码。hashCode返回的并不一定是对象的内存地址，具体取决于运行时库和JVM中方法具体C++实现。

如果两个对象object.equal()相等，那么他们的hashCode一定相等。
如果两个对象通过object.equal()比较，结果不相等，这两个对象分别调用hashCode可以返回两个不相同的整数。
如果对象没有重写equals()，或者重写equals中比较的属性没有被改变，则调用多次hashCode()，其返回值一定是同一个数值，不会变。

　　4、equals()

 public boolean equals(Object obj) {
        return (this == obj);
    }

　　从上面看出来，Object的equals()比较的是对象的地址

　　5、clone()

     / * @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;

　　native()方法，表示对对象的深复制。

　　如： Object A = new Object(); Object B = A;表示浅复制A,此处产生了一个对象A;

　　　　 Object C = new Object(); Object D = C.clone() 或者 Object D = new Object;表示深复制C,此处产生了两个对象C和D;　

　　6、toString()

    / * @return  a string representation of the object.*/
    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

　　返回类名+16进制的hashCode

　　7、notify()、notifyAll()、多用于同步代码块、同步方法中。用于唤醒wait()的线程。

public final native void notify();

public final native void notifyAll();

public final native void wait(long timeout) throws InterruptedException;

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);
}

public final void wait() throws InterruptedException {
    wait(0);
}

　　wait()线程需要先抛出中断异常，wait(long timeout, int nanos)timout毫秒单位，nanos纳秒，但是在JVM中只判断有值时，timeout加一毫秒，有点搞笑！

　　wait(0)表示一直wait(),直至被notify。

　　8、finalize()

 protected void finalize() throws Throwable { }

　　　　垃圾回收器在认为该对象是垃圾对象的时候会调用该方法。子类可以通过重写该方法来达到资源释放的目的。在方法调用过程中出现的异常会被忽略且方法调用会被终止。

任何对象的该方法只会被调用一次

　　总结，Object类中，equal()、toString()、finalize()可以被重写，equal()重写时一般都会对hashCode修改。