How can we avoid a deadlock in Java?

In Java, a deadlock is a programming situation where two or more threads are blocked forever. A deadlock condition will occur with at least two threads and two or more resources.

How To Avoid Deadlock

• Avoid Nested Locks − A deadlock mainly happens when we give locks to multiple threads. Avoid giving a lock to multiple threads if we already have given to one.

• Avoid Unnecessary Locks − We can have a lock only those members which are required. Having a lock unnecessarily can lead to a deadlock.

• Using Thread.join() − A deadlock condition appears when one thread is waiting other to finish. If this condition occurs we can use Thread.join() with the maximum time the execution will take.

Example

public class DeadlockTest {
   public static void main(String[] args) throws InterruptedException {
      Object obj1 = new Object();
      Object obj2 = new Object();
      Object obj3 = new Object();
      Thread t1 = new Thread(new SyncThread(obj1, obj2), "t1");
      Thread t2 = new Thread(new SyncThread(obj2, obj3), "t2");
      t1.start();
      Thread.sleep(2000);
      t2.start();
      Thread.sleep(2000);
   }
}
class SyncThread implements Runnable {
   private Object obj1;
   private Object obj2;
   public SyncThread(Object o1, Object o2){
      this.obj1=o1;
      this.obj2=o2;
   }
   @Override
   public void run() {
      String name = Thread.currentThread().getName();
      System.out.println(name + " acquiring lock on " + obj1);
      synchronized (obj1) {
         System.out.println(name + " acquired lock on " + obj1);
         work();
      }
      System.out.println(name + " released lock on " + obj1);
      System.out.println(name + " acquiring lock on " + obj2);
      synchronized (obj2) {
         System.out.println(name + " acquired lock on " + obj2);
         work();
      }
      System.out.println(name + " released lock on " + obj2);
      System.out.println(name + " finished execution.");
   }
   private void work() {
      try {
         Thread.sleep(5000);
      } catch (InterruptedException ie) {
         ie.printStackTrace();
      }
   }
}

Output

t1 acquiring lock on java.lang.Object@27a54c50
t1 acquired lock on java.lang.Object@27a54c50
t2 acquiring lock on java.lang.Object@2ee6a5aa
t2 acquired lock on java.lang.Object@2ee6a5aa
t1 released lock on java.lang.Object@27a54c50
t1 acquiring lock on java.lang.Object@2ee6a5aa
t2 released lock on java.lang.Object@2ee6a5aa
t2 acquiring lock on java.lang.Object@20214e06
t1 acquired lock on java.lang.Object@2ee6a5aa
t2 acquired lock on java.lang.Object@20214e06
t1 released lock on java.lang.Object@2ee6a5aa
t2 released lock on java.lang.Object@20214e06
t1 finished execution.
t2 finished execution.