why does this code produce deadlock? [duplicate]

Question

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");
    }
}

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

Answer 1

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

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:

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.

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...

Upshot: don't do this :)