A simple example showing a typical Java thread deadlock. 2 threads are spawned: thread-0 first locks lock1 and then lock2; thread-1 first locks lock2 and then lock1. Each thread sleeps a bit before acquiring the second lock, to increase the chance of deadlock.

package test.concurrent;
public class DeadLockTest {
   private static long sleepMillis;
   private final Object lock1 = new Object();
   private final Object lock2 = new Object();

   public static void main(String[] args) {
       sleepMillis = Long.parseLong(args[0]);
       DeadLockTest test = new DeadLockTest();
       test.doTest();
   }

   private void doTest() {
       Thread t1 = new Thread(new Runnable() {
           public void run() {
               lock12();
           }
       });
       Thread t2 = new Thread(new Runnable() {
           public void run() {
               lock21();
           }
       });
       t1.start();
       t2.start();
   }

   private void lock12() {
       synchronized (lock1) {
           sleep();
           synchronized (lock2) {
               sleep();
           }
       }
   }

   private void lock21() {
       synchronized (lock2) {
           sleep();
           synchronized (lock1) {
               sleep();
           }
       }
   }

   private void sleep() {
       try {
           Thread.sleep(sleepMillis);
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
   }
}

To run the deadlock test with sleep time 1 millisecond:

java -cp . test.concurrent.DeadLockTest 1

The process seems to hang for ever. To get the thread dump, press Ctrl-\ (Ctrl-break on Windows). Thread dump contains the following deadlock diagnositc:

Found one Java-level deadlock:
=============================
"Thread-1":
 waiting to lock monitor 1017fcfb0 (object 7f31050b0, a java.lang.Object),
 which is held by "Thread-0"
"Thread-0":
 waiting to lock monitor 1017fe360 (object 7f31050c0, a java.lang.Object),
 which is held by "Thread-1"

Java stack information for the threads listed above:
===================================================
"Thread-1":
   at test.concurrent.DeadLockTest.lock21(DeadLockTest.java:44)
   - waiting to lock <7f31050b0> (a java.lang.Object)
   - locked <7f31050c0> (a java.lang.Object)
   at test.concurrent.DeadLockTest.access$1(DeadLockTest.java:40)
   at test.concurrent.DeadLockTest$2.run(DeadLockTest.java:23)
   at java.lang.Thread.run(Thread.java:680)
"Thread-0":
   at test.concurrent.DeadLockTest.lock12(DeadLockTest.java:35)
   - waiting to lock <7f31050c0> (a java.lang.Object)
   - locked <7f31050b0> (a java.lang.Object)
   at test.concurrent.DeadLockTest.access$0(DeadLockTest.java:31)
   at test.concurrent.DeadLockTest$1.run(DeadLockTest.java:18)
   at java.lang.Thread.run(Thread.java:680)

Found 1 deadlock.