Dinesh on Java

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.

For example, suppose one thread enters the monitor on object X and another thread enters the monitor on object Y. If the thread in X tries to call any synchronized method on Y, it will block as expected. However, if the thread in Y, in turn, tries to call any synchronized method on X, the thread waits forever, because to access X, it would have to release its own lock on Y so that the first thread could complete.

Simple Deadlock

One of the main problems with threading is Deadlock, two threads are both suspended waiting for the other one to do something. The most common cause of deadlock is two threads both acquiring the same set of (two or more) locks, but in a different order. Consider this code:

Object A = new Object();
Object B = new Object();

Thread 1:

synchronized(A)
{                       // <--- preemption
    synchronized(B)
    {   //...
    }
}

Thread 2:

synchronized(B)
{   synchronized(A)
    {   //...
    }
}

Here’s the deadlock scenario:

• Thread 1 acquires A, but is then preempted for some reason.
• Thread 2 wakes up, acquires B, but can’t get A because Thread 1 has it, so is suspended.
• Thread 1 wakes up, tries to acquire B, but can’t because Thread 2 has it, so is suspended.
• Both threads are now suspended forever. They’re deadlocked.

Example of Deadlock in java:

public class DeadLockExample {
  public static void main(String[] args) {
    final String resource1 = "dineshonjava.com";
    final String resource2 = "tutorial";
    // t1 tries to lock resource1 then resource2
    Thread t1 = new Thread() {
      public void run() {
          synchronized (resource1) {
           System.out.println("Thread 1: locked resource 1");

           try { 
              Thread.sleep(100);
           } catch (Exception e) {}

           synchronized (resource2) {
            System.out.println("Thread 1: locked resource 2");
           }
         }
      }
    };

    // t2 tries to lock resource2 then resource1
    Thread t2 = new Thread() {
      public void run() {
        synchronized (resource2) {
          System.out.println("Thread 2: locked resource 2");

          try { 
              Thread.sleep(100);
          } catch (Exception e) {}

          synchronized (resource1) {
            System.out.println("Thread 2: locked resource 1");
          }
        }
      }
    };

    
    t1.start();
    t2.start();
  }
}
        

output:

Because the program has deadlocked, you need to press CTRL-C to end the program. You can see a full thread and monitor cache dump by pressing CTRL-BREAK on a PC .