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.......................