Deadlock

A Special type of error that you need to avoid that relates specifically to multitasking is Deadlock, which occurs when two threads have a circular dependency on a pair of synchronization objects.
Deadlock is a difficult error to debug for two reasons.

• In general, It occurs only rarely, when the two threads time-slice in just the right way.
• It may involve more than two threads and two synchronization object.

Example: A example of Deadlock.
class A{ synchronized void foo(B b){ String name= Thread.currentThread().getName(); System.out.println(name +" Enter A foo"); try{ Thread.sleep(1000); }catch(Exception e){ System.out.println("A Interrupted"); } System.out.println(name + " Trying to call B.last()"); b.last(); } synchronized void last(){ System.out.println("Inside A.last"); } } class B{ synchronized void bar(A a){ String name= Thread.currentThread().getName(); System.out.println(name +" Enter B.bar"); try{ Thread.sleep(1000); }catch(Exception e){ System.out.println("B Interrupted"); } System.out.println(name + " Trying to call A.last()"); a.last(); } synchronized void last(){ System.out.println("Inside A.last"); } } class Deadlock implements Runnable{ A a=new A(); B b=new B(); Deadlock(){ Thread.currentThread().setName("MainThread"); Thread t= new Thread(this, "RacingThread"); t.start(); a.foo(b); // get lock on a in this thread. System.out.println("Back in main thread."); } public void run(){ b.bar(a); // get lock on b in other thread. System.out.println("Back in Other Thread."); } public static void main(String []manoj){ new Deadlock(); } }
Output: