The DeadlockDetector class allows you to programmatically detect basic deadlocks in your Java code. You can output deadlocks using the following code (note that printing a thread dump using the ThreadDump class will automatically attempt to find any deadlocks).

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DeadlockDetector.printDeadlocks(System.out);

There are various types of deadlock in concurrent systems, broadly speaking with regard to Java, they can be categorised as

• Java monitor cyclic locking dependency
• Java Lock cyclic locking dependency
• External resource based dependency
• Live lock

The DeadlockDecector class can only spot Java monitor cyclic locking problems. It's implementation is basically the same as that used by jconsole and jstack and suffers the same limitations. Java 1.6 versions of jstack and jconsole can additionally detect Lock based cyclic problems. The types of deadlock it can detect can be illustrated in the example below.

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 public void potentialDeadlock() {
     new Kidnapper().start();
     new Negotiator().start();
 }

 public class Kidnapper extends Thread {
     public void run() {
         synchronized (nibbles) {
             synchronized (cash) {
                 take(cash);
             }
         }
     }
 }

 public class Negotiator extends Thread {
     public void run() {
         synchronized (cash) {
             synchronized (nibbles) {
                 take(nibbles);
             }
         }
     }
 }

Here, the Kidnapper is unwilling to release poor Nibbles the Cat until he has the Cash but our Negotiator is unwilling to part with the Cash until he has poor Nibbles back in his arms. The deadlock detector displays this woeful situation as follows.

Deadlock detected
=================

"Negotiator-Thread-1":
  waiting to lock Monitor of com.google.code.tempusfugit.concurrency.DeadlockDetectorTest$Cat@ce4a8a
  which is held by "Kidnapper-Thread-0"

"Kidnapper-Thread-0":
  waiting to lock Monitor of com.google.code.tempusfugit.concurrency.DeadlockDetectorTest$Cash@7fc8b2
  which is held by "Negotiator-Thread-1"

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