How can we avoid deadlock in Java?

In Java, Deadlock is a programming situation in which two or more threads are permanently blocked. A deadlock condition will occur between at least two threads and two or more resources .

How to avoid deadlock

• Avoid nested locks: Deadlock mainly occurs when locks are provided to multiple threads. If we have already provided a lock to one thread, avoid providing locks to other threads.
• Avoid unnecessary locks: We only need to provide locks to those necessary members. Providing locks unnecessarily can lead to deadlocks.
• Use Thread.join(): A deadlock condition occurs when one thread is waiting for another thread to complete. If this happens, we can use Thread.join() and set the maximum execution time.

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);
      <strong>synchronized (obj1)</strong> {
         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);
      <strong>synchronized (obj2)</strong> {
         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@917d8d4
t1 acquired lock on java.lang.Object@917d8d4
t2 acquiring lock on java.lang.Object@5c4b42fb
t2 acquired lock on java.lang.Object@5c4b42fb
t1 released lock on java.lang.Object@917d8d4
t1 acquiring lock on java.lang.Object@5c4b42fb
t1 acquired lock on java.lang.Object@5c4b42fb
t2 released lock on java.lang.Object@5c4b42fb
t2 acquiring lock on java.lang.Object@528cb702
t2 acquired lock on java.lang.Object@528cb702
t1 released lock on java.lang.Object@5c4b42fb
t2 released lock on java.lang.Object@528cb702
t1 finished execution.
t2 finished execution.

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