Deadlock in Java Multithreading

By Ayush R Sachan

Deadlock in Java multithreading is a special type of error. It occurs when two threads have circular dependency on a pair of synchronized objects. Consider the following case:

Two threads enter the monitor on objects X and Y respectively. If the thread in X calls synchronized method on Y, it will block it. However, when thread in Y calls synchronized method in X, it would have to release its own lock on Y so that first thread could complete. Hence the thread waits forever. This is called a deadlock. It may include more than two threads and synchronized objects also.

Deadlocks are difficult to debug because:

• It occurs when two threads time-slice in just the right way
• It might contain more than two threads and more than two synchronized objects. This means that ot can occur through a more convoluted sequence of events.

Demonstration of deadlock in Java Multithreading

class A
{
  synchronized void f1(B b)
  {
    System.out.println(name+" entered A.f1");
    try
    {
      Thread.sleep(1000);
    }catch(Exception e){
      System.out.println("A interrupted ");
    }
    System.out.println(name+" trying to call B.last() ");
    b.last();
  }
  synchronized void last()
  {
    System.out.println("Inside A.last");
  }
}

class B
{
  synchronized void f2(A a)
  {
    String name=Thread.currentThread().getName();
    System.out.println(name+" entered B.f2");
    try{
      Thread.sleep(1000);

    }catch(Exception e){
      System.out.println("B interrupted");
    }
    System.out.println(name+" trying to call A.last()");
      a.last();

    }
    synchronized void last()
    {
      System.out.println("Inside A.last");
    }
}

class deadlock implements Runnable{
  A a=new A();
  B b =new B();
  deadlock(){
    Thread.currentThread().setName("MainThread");
    Thread t = new Thread(this, "RunningThread");
    t.start();
    a.f1(b);
    System.out.println("Back in main thread"); 
  }
  public void run(){
    b.f2(a);
    System.out.println("Back in other thread");
  }
   public static void main(String args[]){
   	new deadlock();
   }
}

Output:

MainThread entered A.f1
RunningThread entered B.f2
MainThread trying to call B.last()
RuuningThread trying to call A.last()