线程并发-使用字符串对象的锁进行同步

Question

class ZiggyTest{

    public static void main(String[] args){

        RunnableThread rt = new RunnableThread();

        Thread t1 = new Thread(rt);
        Thread t2 = new Thread(rt);
        Thread t3 = new Thread(rt);

        t1.start();
        t2.start();
        t3.setPriority(10);
        t3.start();

        try{
            t3.join();
            t2.join();
            t1.join();

            System.out.println("Joined");
        }catch(InterruptedException e){System.out.println(e);}


        Thread t4 = new Thread(new RunnableThread());
        Thread t5 = new Thread(new RunnableThread());
        Thread t6 = new Thread(new RunnableThread());

        t4.start();
        t5.start();
        t6.start();
    }

}

上面的测试产生以下输出

Thread-0
Thread-0
Thread-0
Thread-0
Thread-0
Thread-2
Thread-2
Thread-2
Thread-2
Thread-2
Thread-1
Thread-1
Thread-1
Thread-1
Thread-1
Joined
Thread-3
Thread-3
Thread-4
Thread-4
Thread-3
Thread-4
Thread-5
Thread-4
Thread-3
Thread-4
Thread-5
Thread-3
Thread-5
Thread-5
Thread-5

我不明白为什么最后三个线程不像前三个线程那样使用String对象作为共享锁。即使最后三个线程使用不同的“RunnableThread”实例，它们是否应该同步，因为在字符串常量池中只有一个“str”副本？

谢谢

编辑

哦..。我忘了包括RunnableThread。

class RunnableThread implements Runnable{

    String str = "HELLO";

    public void run(){
        runMe();
    }

    public synchronized void runMe(){
        for (int a : new int[]{1,2,3,4,5}){
            System.out.println(Thread.currentThread().getName());
        }
    }
}

Answer 1

您的线程不会在任何String对象上同步，更不用说同一个对象了，所以我不确定这个想法从何而来。由于下面这一行中的synchronized关键字，它们在RunnableThread对象本身上同步：

public synchronized void runMe(){

对最后三个线程使用单独的RunnableThread对象意味着它们独立运行。

现在，如果您真的希望锁定全局String对象，则该方法将如下所示

public void runMe(){
    synchronized (str) {
        for (int a : new int[]{1,2,3,4,5}){
            System.out.println(Thread.currentThread().getName());
        }
    }
}

这将是一个好主意，使str final。