Why main thread not wake up?

Question

The code is below, in the Main method I call a static method:

public class Main {
    public static void main(String[] args) {
        MyFactory.getSomething();
        System.out.println("over"); 
    }
}

In the static method, let the main thread wait, then by demo-thread wake up. But the main thread does not wake up and I don't know why.

import java.util.concurrent.TimeUnit;

public class MyFactory {
    private static Object lock = new Object();

    static{
        init();
    }

    private static void init(){
        new Thread(()->{
            try {
                // make this thread run after main thread
                TimeUnit.SECONDS.sleep(3);
                System.out.println("task run...");
                synchronized (lock){
                    lock.notifyAll();  // notify main thread
                }
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        },"demo-thread").start();

        synchronized (lock){
            try {
                System.out.println("waiting...");
                lock.wait();  // wait
                System.out.println("wake up");
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
    public static Object getSomething(){
        return null;
    }
}

The output is below:

waiting...

My expected should be like this:

waiting...
task run...
wake up
over

Here is the demo code on Github You can download it to run.

---

My try:

I see losts of people say may be a dead lock cause the problem.I don't think so.Because we don't see task run... info in the output. So the demo thread not aquire the lock at all.

When I change the code In MyFactory.java.(Add a new thread to run init method)

from this

static{
  init();
}

to this.

static{
  new Thread(()->init()).start();
}

Every thing work fine.The program not block.

Here is the out put after I change the code .

over
waiting...
task run...
wake up

Back to the original question "Why main thread not wake up"？ I think the JVM didn't load the MyFactory.class completely due to main thread blocking。I don't know if my guess is right???

Answer 1

I'm not an expert in JVM internals but I think the problem lies in here:

static {
    init();
}

I suspect this will be executed when the JVM classloader is loading the class. However this class loading will never finish since you're blocking the classloader itself, and thus execution of the rest of the program will never resume, and thus the other thread is never started.

If you remove the static block and call it directly:

public static void main(String[] args) {
    // ...
    MyFactory.init();
    MyFactory.getSomething();
    // ...
}

You will see that it works properly.

More details under JLS 12.4.2

On a side note, sleeping is not a good idea to synchronize threads.