为什么这段代码会产生死锁? [英] why does this code produce deadlock?

问题描述

class A {
    static final int i;
    static {
        i = 128;

        Thread t = new Thread() {
            public void run() {
                System.out.println("i=" + i);
            }
        };
        t.start();
        try {
           t.join();
        } catch (InterruptedException e) {
           Thread.currentThread().interrupt();
        }
    }
}
public class MainTesting {


    public static void main(String[] args) {
        A a = new A();
        System.out.println("finish");
    }
}

我永远不会得到finish得到打印并得到i的值. 为什么会这样?

I never get finish get printed and value of i. Why is it so?

推荐答案

您从线程1(主"线程)开始，并开始为A类执行静态初始化程序.

You start off on thread 1 (the "main" thread), and start executing the static initializer for the A class.

然后在该静态初始化程序中启动一个新线程(2)，该线程使用A类中的某些内容.这意味着根据

Within that static initializer, you then start a new thread (2), which uses something within the A class. That means that thread 2 needs to wait until the A class has finished initilaizing before it will proceed, as per section 12.4.2 of the JLS:

如果C的Class对象指示其他线程正在对C进行初始化，则释放LC并阻塞当前线程，直到得知正在进行的初始化已完成，然后重复此步骤.

If the Class object for C indicates that initialization is in progress for C by some other thread, then release LC and block the current thread until informed that the in-progress initialization has completed, at which time repeat this step.

但是，A的静态初始化程序要等到 完成之前，直到线程2完成(通过调用join())，这会导致死锁:静态初始化程序要等到线程2才能完成已经完成，线程2在静态初始化程序完成之前无法完成...

However, your static initializer for A waits until thread 2 has completed (by calling join()) before it completes, leading to deadlock: the static initializer can't complete until thread 2 has completed, and thread 2 can't complete until the static initializer has completed...

结果:不要这样做:)

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