java - 简单的Java多线程

Question

我运行了以下 java 代码来测试多线程。我想让每个线程将索引变量增加 1。但是以下不起作用。

public class start {
    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Thread[] threadList = new Thread[20];

        for (int i = 0; i < 20; i++) {
            threadList[i] = new worker();
            threadList[i].start();
        }
    }
}

public class worker extends Thread {
    private static int index = 0;

    public static synchronized int getIndex() {
        return index++;
    }

    public void run() {
        index = getIndex();
        System.out.println("Thread Name: "+Thread.currentThread().getName() + "index: "+index);
    }
}

它给了我以下结果:

Thread Name: Thread-0index: 0
Thread Name: Thread-2index: 0
Thread Name: Thread-4index: 0
Thread Name: Thread-1index: 0
Thread Name: Thread-6index: 0
Thread Name: Thread-8index: 0
Thread Name: Thread-5index: 0
Thread Name: Thread-7index: 0
Thread Name: Thread-3index: 0
Thread Name: Thread-9index: 0
Thread Name: Thread-13index: 0
Thread Name: Thread-11index: 0
Thread Name: Thread-12index: 0
Thread Name: Thread-10index: 0
Thread Name: Thread-14index: 0
Thread Name: Thread-17index: 0
Thread Name: Thread-15index: 0
Thread Name: Thread-19index: 0
Thread Name: Thread-16index: 0
Thread Name: Thread-18index: 0

索引值没有改变。如何解决这个问题？

Answer 1

此分配正在重置 index 的值(并且不同步):

index = getIndex();

覆盖增量运算符的效果。一种可能的解决方案是将 getIndex() 的结果存储在局部变量中:

final int i = getIndex();
System.out.println("Thread Name: "                  +
                   Thread.currentThread().getName() +
                   "index: "                        +
                   i);

<小时/>

"implements Runnable" vs. "extends Thread"