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Deadlock in Java

Deadlock in Java is a part of multithreading. Deadlock can occur in a situation when a thread is waiting for an object lock, that is acquired by another thread and second thread is waiting for an object lock that is acquired by first thread. Since, both threads are waiting for each other to release the lock, the condition is called deadlock.

Deadlock in Java

Example of Deadlock in Java


Thread 1: locked resource 1
        Thread 2: locked resource 2

More Complicated Deadlocks

A deadlock may also include more than two threads. The reason is that it can be difficult to detect a deadlock. Here is an example in which four threads have deadlocked:

Thread 1 locks A, waits for B

Thread 2 locks B, waits for C

Thread 3 locks C, waits for D

Thread 4 locks D, waits for A

Thread 1 waits for thread 2, thread 2 waits for thread 3, thread 3 waits for thread 4, and thread 4 waits for thread 1.

How to avoid deadlock?

A solution for a problem is found at its roots. In deadlock it is the pattern of accessing the resources A and B, is the main issue. To solve the issue we will have to simply re-order the statements where the code is accessing shared resources.


In block 1
In block 2

In the above code, class DeadlockSolved solves the deadlock kind of situation. It will help in avoiding deadlocks, and if encountered, in resolving them.

How to Avoid Deadlock in Java?

Deadlocks cannot be completely resolved. But we can avoid them by following basic rules mentioned below:

  1. Avoid Nested Locks: We must avoid giving locks to multiple threads, this is the main reason for a deadlock condition. It normally happens when you give locks to multiple threads.
  2. Avoid Unnecessary Locks: The locks should be given to the important threads. Giving locks to the unnecessary threads that cause the deadlock condition.
  3. Using Thread Join: A deadlock usually happens when one thread is waiting for the other to finish. In this case, we can use join with a maximum time that a thread will take.

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