Java语言中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.......................