java多线程中的synchronized和wait/notify用法总结

自学java也有一段时间了，自我感觉java中的多线程还是比较重要的。至于线程的概念，也没有必要说的那么玄乎，引用马士兵老师的话就很好理解：线程就是程序执行的路径。多线程，也就不难理解了，程序执行的多条路径，它们独立执行，但是又有莫大的联系。synchronized关键字用于同步，也就是对对象进行加锁，防止由于同时访问同一个对象而造成的混乱。wait/notify字面意思是等待和告知，那么是谁等待，等待完了又告知谁呢？下面用一个例子说明。

public class ThreadA {
	public static void main(String[] args) {
		ThreadB b = new ThreadB();
		b.start();//主线程中启动另外一个线程
		System.out.println("b is start....");
		//括号里的b是什么意思,应该很好理解吧
		synchronized(b) {
			try {
				System.out.println("Waiting for b to complete...");
				b.wait();//这一句是什么意思，究竟谁等待?
				System.out.println("ThreadB is Completed. Now back to main thread");
				}catch (InterruptedException e){}
		}
		System.out.println("Total is :" + b.total);
		}
}

class ThreadB extends Thread {
		int total;
		public void run() {
			synchronized(this) {
				System.out.println("ThreadB is running..");
				for (int i=0; i<=100; i++ ) {
					total += i;
				}
				System.out.println("total is " + total);
				notify();
			}
		}
}

运行结果：
b is start....
Waiting for b to complete...
ThreadB is running..
total is 5050
ThreadB is Completed. Now back to main thread
Total is :5050
从程序运行的结果来看就不难理解wait/notify了，wait是让使用wait方法的对象等待，暂时先把对象锁给让出来，给其它持有该锁的对象用，其它对象用完后再告知（notify）等待的那个对象可以继续执行了，整个过程就是这样。wait/notify主要用于一个线程要等待另一个线程执行完后，然后得到执行结果的情况。
呵呵，总体应该表达清楚了吧……

评论

4 楼 namelujl 2012-03-14  

b is start....
ThreadB is running..
Waiting for b to complete...
total is 5050
ThreadB is Completed. Now back to main thread
Total is :5050

3 楼 namelujl 2012-03-14  

package cn.com.zsgc.thread;

/**
*
* @author liujianlinag E-mail:[e-mail]
* @version 1.0
*
*/
public class ThreadB extends Thread {  
    int total;  
    public void run() {  
        synchronized(this) {  
            System.out.println("ThreadB is running..");  
            for (int i=0; i<=100; i++ ) {  
                total += i;  
            }
            try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
            System.out.println("total is " + total);  
              notify();
        }  
    }  
} 

2 楼 namelujl 2012-03-14  

package cn.com.zsgc.thread;

/**
*
* @author liujianlinag E-mail:[e-mail]
* @version 1.0
*
*/
public class ThreadA {  
    public static void main(String[] args) {  
        ThreadB b = new ThreadB();  
        b.start();      
        System.out.println("b is start....");  
        synchronized(b) {
        System.out.println("Waiting for b to complete..."); 
                b.notify();
                try {
b.wait();
} catch (InterruptedException e1) {
e1.printStackTrace();
}
                System.out.println("ThreadB is Completed. Now back to main thread");            
        }  
        System.out.println("Total is :" + b.total);  
        }  
}  

1 楼 namelujl 2012-03-14