Java中synchronized 修饰在static方法和非static方法的区别

【问题描述】关于Java中synchronized 用在实例方法和对象方法上面的区别

【问题分析】大家都知道，在Java中，synchronized 是用来表示同步的，我们可以synchronized 来修饰一个方法（实例方法和类方法---注：不知道这样叫准确不准确，大家理解我的意识就行了）。也可以synchronized 来修饰方法里面的一个语句块。

修饰实例方法：

public synchronized void x() throws InterruptedException
    {
        for (int i = 0; i < 10; i++)
        {
            Thread.sleep(1000);
            System.out.println("x.......................");
        }
    }

修饰类方法（static 方法）：

public static synchronized void staticX() throws InterruptedException
    {
        for (int i = 0; i < 10; i++)
        {
            Thread.sleep(1000);
            System.out.println("staticX.......................");
        }
    }

修饰方法里面语句块：

public static void staticX() throws InterruptedException
    {
        synchronized (locks)
        {
            for (int i = 0; i < 10; i++)
            {
                Thread.sleep(1000);
                System.out.println("staticX.......................");
            }
        }
    }

注意：这里不能用synchronized修饰方法外面的语句块（我把他叫做类语句块），虽然我们可以在方法外面定义语句块，这样做会遇到编译错误，这里涉及到了Java里面的对象初始化的部分知识。大概的原因就是synchronized锁住的是对象，当初始化对象的时候，JVM在对象初始化完成之前会调用方法外面的语句块，这个时候对象还不存在，所以就不存在锁了。

那么，在static方法和非static方法前面加synchronized到底有什么不同呢？

大家都知道，static的方法属于类方法，它属于这个Class（注意：这里的Class不是指Class的某个具体对象），那么static获取到的锁，就是当前调用这个方法的对象所属的类(Class，而不再是由这个Class产生的某个具体对象了)。而非static方法获取到的锁，就是当前调用这个方法的对象的锁了。所以，他们之间不会产生互斥。

看代码：

package com.jack.zhang.chapter9.classlock;

/**
 * @author Jack Zhang
 * @version vb1.0
 * @Email virgoboy2004@163.com
 * @Date 2012-5-20
 */
public class Test
{
    public static synchronized void staticX() throws InterruptedException
    {
        for (int i = 0; i < 10; i++)
        {
            Thread.sleep(1000);
            System.out.println("staticX.......................");
        }
    }

    public synchronized void x() throws InterruptedException
    {
        for (int i = 0; i < 10; i++)
        {
            Thread.sleep(1000);
            System.out.println("x.......................");
        }
    }

    public static void main(String[] args)
    {
        final Test test1 = new Test();
        Thread thread = new Thread(new Runnable()
        {
            public void run()
            {
                try
                {
                    test1.x();
                }
                catch (InterruptedException e)
                {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }, "a");

        Thread thread1 = new Thread(new Runnable()
        {
            public void run()
            {
                try
                {
                    Test.staticX();
                }
                catch (InterruptedException e)
                {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }, "b");

        thread1.start();
        thread.start();
    }
}

运行结果是：

staticX.......................
x.......................
x.......................
staticX.......................
staticX.......................
x.......................
x.......................
staticX.......................
x.......................
staticX.......................
staticX.......................
x.......................
x.......................
staticX.......................
x.......................
staticX.......................
x.......................
staticX.......................
x.......................
staticX.......................

那当我们想让所有这个类下面的对象都同步的时候，也就是让所有这个类下面的对象共用同一把锁的时候，我们如何办呢？

看代码：

package com.jack.zhang.chapter9.classlock;


/**
 * @author Jack Zhang
 * @version vb1.0
 * @Email virgoboy2004@163.com
 * @Date 2012-5-20
 */
public class Test
{
    public final static Byte[] locks = new Byte[0];

    public static void staticX() throws InterruptedException
    {
        synchronized (locks)
        {
            for (int i = 0; i < 10; i++)
            {
                Thread.sleep(1000);
                System.out.println("staticX.......................");
            }
        }
    }

    public void x() throws InterruptedException
    {
        synchronized (locks)
        {
            for (int i = 0; i < 10; i++)
            {
                Thread.sleep(1000);
                System.out.println("x.......................");
            }
        }
    }

    public static void main(String[] args)
    {
        final Test test1 = new Test();
        final Test test2 = new Test();
        Thread thread = new Thread(new Runnable()
        {
            public void run()
            {
                try
                {
                    test1.x();
                }
                catch (InterruptedException e)
                {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }, "a");

        Thread thread1 = new Thread(new Runnable()
        {
            public void run()
            {
                try
                {
                    Test.staticX();
                }
                catch (InterruptedException e)
                {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }, "b");

        thread1.start();
        thread.start();
    }
}

运行结果：

staticX.......................
staticX.......................
staticX.......................
staticX.......................
staticX.......................
staticX.......................
staticX.......................
staticX.......................
staticX.......................
staticX.......................
x.......................
x.......................
x.......................
x.......................
x.......................
x.......................
x.......................
x.......................
x.......................
x.......................

http://blog.csdn.net/virgoboy2004/article/details/7585182