java - 多线程程序输出不符合预期

Question

我写了一个小程序，我在类级别字段中存储线程名称并打印它。

public class ThreadClass implements Runnable {

    private String threadName = null;

    @Override
    public void run() {
        System.out.println(" thread name " + threadName);
        System.out.println(" current thread  >>>>>> "
                + Thread.currentThread().getName());
        threadName = Thread.currentThread().getName();
    }
}

我编写了一个测试类，其中我创建了 10 个线程并启动了它们。

public class ThreadController {
    public static void main(String[] args) {
        ThreadClass threadClass = new ThreadClass();
        Thread t1 = new Thread(threadClass, "T1");
        Thread t2 = new Thread(threadClass, "T2");
        Thread t3 = new Thread(threadClass, "T3");
        Thread t4 = new Thread(threadClass, "T4");
        Thread t5 = new Thread(threadClass, "T5");
        Thread t6 = new Thread(threadClass, "T6");
        Thread t7 = new Thread(threadClass, "T7");
        Thread t8 = new Thread(threadClass, "T8");
        Thread t9 = new Thread(threadClass, "T9");
        Thread t10 = new Thread(threadClass, "T10");

        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
        t6.start();
        t7.start();
        t8.start();
        t9.start();
        t10.start();
    }
}

我得到以下输出。

 thread name null
 thread name null
 thread name null
 current thread  >>>>>> T1
 current thread  >>>>>> T6
 current thread  >>>>>> T2
 thread name T2
 thread name T2
 thread name null
 thread name null
 thread name null
 current thread  >>>>>> T4
 current thread  >>>>>> T7
 current thread  >>>>>> T8
 current thread  >>>>>> T9
 thread name T1
 current thread  >>>>>> T5
 thread name T1
 current thread  >>>>>> T3
 current thread  >>>>>> T10

我的疑问是，如果每个线程都创建字段变量的本地副本，那么为什么我并不总是将线程名称设置为 null。抱歉，这听起来像是一个愚蠢的问题，但我正在尝试学习线程。

Answer 1

线程将不会创建其访问的字段的本地副本。在您的实现中，所有线程都试图访问实例 threadClass 中的相同字段，这就是为什么它不总是 null 的原因。

如下更改您的来源，您将获得预期的行为。

Thread t1 = new Thread(new ThreadClass(), "T1");
Thread t2 = new Thread(new ThreadClass(), "T2");
Thread t3 = new Thread(new ThreadClass(), "T3");
Thread t4 = new Thread(new ThreadClass(), "T4");
Thread t5 = new Thread(new ThreadClass(), "T5");
Thread t6 = new Thread(new ThreadClass(), "T6");
Thread t7 = new Thread(new ThreadClass(), "T7");
Thread t8 = new Thread(new ThreadClass(), "T8");
Thread t9 = new Thread(new ThreadClass(), "T9");
Thread t10 = new Thread(new ThreadClass(), "T10");