问为什么这会造成人为的Java代码死锁？

EN

public class Uninterruptible {
    public static void main(String[] args) throws InterruptedException {
        final Object o1 = new Object(); final Object o2 = new Object();

        Thread t1 = new Thread() {
            public void run() {
                try {
                    synchronized(o1) {
                        Thread.sleep(1000);
                        synchronized(o2) {}
                    }
                } catch(InterruptedException e) { System.out.println("t1 interrupted"); }
            }
        };

        Thread t2 = new Thread() {
            public void run() {
                try {
                    synchronized(o2) {
                        Thread.sleep(1000);
                        synchronized(o1) {}
                    }
                } catch(InterruptedException e) { System.out.println("t2 interrupted"); }
            }
        };

        t1.start(); t2.start();
        Thread.sleep(2000);
        t1.interrupt(); t2.interrupt();
        t1.join(); t2.join();

        System.out.println("Donezo!");

    }
}