synchronized 修飾在 static方法和 非static方法的區別

【問題描述】關於Java中synchronized 用在實例方法和對象方法上面的區別

【問題分析】大家都知道，在Java中，synchronized 是用來表示同步的，我們可以synchronized 來修飾一個方法（實例方法和類方法---注：不知道這樣叫準確不準確，大家理解我的意識就行了）。也可以synchronized 來修飾方法裏面的一個語句塊。

修飾實例方法：

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

修飾類方法（static 方法）：

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

修飾方法裏面語句塊：

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

注意：這裏不能用synchronized修飾方法外面的語句塊（我把他叫做類語句塊），雖然我們可以在方法外面定義語句塊，這樣做會遇到編譯錯誤，這裏涉及到了Java裏面的對象初始化的部分知識。大概的原因就是synchronized鎖住的是對象，當初始化對象的時候，JVM在對象初始化完成之前會調用方法外面的語句塊，這個時候對象還不存在，所以就不存在鎖了。

那麼，在static方法和非static方法前面加synchronized到底有什麼不同呢？

大家都知道，static的方法屬於類方法，它屬於這個Class（注意：這裏的Class不是指Class的某個具體對象），那麼static獲取到的鎖，就是當前調用這個方法的對象所屬的類(Class，而不再是由這個Class產生的某個具體對象了)。而非static方法獲取到的鎖，就是當前調用這個方法的對象的鎖了。所以，他們之間不會產生互斥。

看代碼：

package com.jack.zhang.chapter9.classlock;

/**
 * @author Jack Zhang
 * @version vb1.0
 * @Email [email protected]
 * @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();
    }
}

運行結果是：

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

那當我們想讓所有這個類下面的對象都同步的時候，也就是讓所有這個類下面的對象共用同一把鎖的時候，我們如何辦呢？

看代碼：

package com.jack.zhang.chapter9.classlock;


/**
 * @author Jack Zhang
 * @version vb1.0
 * @Email [email protected]
 * @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();
    }
}

運行結果：

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