Are monitors deadlock free?

Question

If you are using monitors in your concurrent system design, is this sufficient to prevent deadlock? I know that in Java there are many ways to cause deadlock, but few of these examples are actually to do with monitors (most use mutexes).

Answer 1

Deadlock can occur, but you are quite unlikely to have this happen by accident, I spent 10 minutes thinking up a horrible corner case.

In the following code, note that removing "synchronized" will cause it to terminate.

public class Foo implements Runnable {

    private Foo target;
    private int count;

    synchronized void go(int n) {
        try {
            if (n < 1) {
                System.out.println("Got to the bottom.");
            } else {
                Foo f = new Foo();
                f.target = this;
                f.count = n;
                Thread t = new Thread(f);
                t.start();
                t.join();
            }
        } catch (InterruptedException e) {
            // Demo
        }
    }

    @Override
    public void run() {
        target.go(count-1);    
    } 

    public static void main(String[] args) {
        new Foo().go(1);
    }
}