对class的同步注意事项

package web;

 

/**

 * synchronized锁住了整个静态类

 * @author zengst

 *

 */

class aaa2{

 

private static int a=2;

 

//定义了私有构造函数，外部无法通过该构造函数构造实例，除非重载

 

private aaa2(){}

 

public static void getInstance(){

if(a > 0){

System.out.println(Thread.currentThread().getName()+"getInstance");

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

e.printStackTrace();

}

synchronized(aaa2.class){

System.out.println(Thread.currentThread().getName());

System.out.println(a);

a--;

System.out.println(a);

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println(Thread.currentThread().getName());

}

}

}

 

}

 

public class StaticSharedTest {

 

public static void main(String args[]){

 

zengst2 z = new zengst2();

Thread t1 = new Thread(z);

Thread t2 = new Thread(z);

Thread t3 = new Thread(z);

t1.start();

t2.start();

t3.start();

}

}

 

class zengst2 implements Runnable{

 

public void run() {

aaa2.getInstance();

}

}

 

运行结果：

Thread-0getInstance

Thread-1getInstance

Thread-2getInstance

Thread-0

2

1

Thread-0

Thread-2

1

0

Thread-2

Thread-1

0

-1

Thread-1

 

分析，这里出现了-1.是没有按照预期的。

原因是因为，三个线程都进入了a>0的判断，虽然紧接着实现了对整个class的同步。