How do I avoid deadlocks

In this Q&A, we'll go over what's a deadlock and how to avoid it.

Deadlock is a condition when two or more threads are blocked on each other and waiting forever.  This typically happens as below:

- Thread T1 obtains a write lock on object O1.  T1 then tries to obtain a write lock on object O2.

- Thread T2 obtains a write lock on object O2.  T2 then tries to obtain a write lock on object O1.

See sample code below.

Deadlock can be avoided as below:

1. Avoid creating exclusive locks on multiple objects. Where possible create exclusive lock on a single object
2. If #1 is not feasible, obtain locks in the same order and not in a different order
3. Where possible use Locks and use shared locks instead of exclusive locks.
4. When obtaining exclusive lock set a timeout with retry if the lock cannot be obtained within the set time.

package com.javahowdoi.thread;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class DeadlockDemo {

    private static class DeadlockTask implements Runnable {

        final Object s1 , s2;

        boolean order;

        DeadlockTask(Object s1, Object s2, boolean order) {

            this.s1 = s1; this.s2 = s2; this.order = order;

        }

        @Override

        public void run() {

            if(order)

            {

                synchronized(s1) {

                    System.out.println("Synchronized on s1.  About to wait for s2");

                    synchronized(s2) {

                        System.out.println("Got lock on s2.  Avoided deadlock ");

                    }

                }

            }

            else

            {

                // Note here that the order of synchronization is different

                // This can lead to deadlock is string objects s1 and s2 are shared

                synchronized(s2) {

                    System.out.println("Synchronized on s2.  About to wait for s1");

                    synchronized(s1) {

                        System.out.println("Got lock on s1.  Avoided deadlock ");

                    }

                }

            }

        }

    }

    public static void main(String[] args ) {

        ExecutorService es = Executors.newFixedThreadPool(2);

        String s1 ="s1", s2 =  "s2";

        es.submit(new DeadlockTask(s1, s2, true));

        es.submit(new DeadlockTask(s1, s2, false));

        es.shutdown();

    }

}