Double Locking in Singleton Pattern in Java

Double Locking in Singleton Pattern in Java

In one of earlier posts, we learned Singleton design pattern in java & Singleton Design Pattern Without Cloning Java.


There is one more issue in this block of code that will come up in a multithreaded environment. In the below code, Thread.sleep is explicitly used in order to test the violation in multithreaded environment

public static SingletonDoubleLocking getSingletonDoubleLocking() {
  if (singletonDoubleLocking == null) {
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
   singletonDoubleLocking = new SingletonDoubleLocking();
  }
  System.out.println("SingletonDoubleLocking - "
    + singletonDoubleLocking);
  return singletonDoubleLocking;
 }

Consider above code, where a method will be called in a multithreaded environment and so multiple threads enter the block and creates multiple instances which will violate the principal of Singleton Design Pattern as shown below :

package com.doubleLock;

public class ClientSingletonDoubleLocking implements Runnable {

 public static void main(String args[]) {
  ClientSingletonDoubleLocking clientSingletonDoubleLocking = new ClientSingletonDoubleLocking();
  int length = 5;
  Thread[] thread = new Thread[length];
  
  for (int index = 0; index < length; index++) {
   thread[index] = new Thread(clientSingletonDoubleLocking);
  }
  
  for (int index = 0; index < length; index++) {
   thread[index].start();
  }
 }

 @Override
 public void run() {
  SingletonDoubleLocking singletonDoubleLocking = SingletonDoubleLocking
    .getSingletonDoubleLocking();
 }
}

To avoid so, we need to perform a double locking to ensure no 2 objects are created in multithreading environment as shown below:

public static synchronized SingletonDoubleLocking getSingletonDoubleLocking() {
  if (singletonDoubleLocking == null) {
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
   singletonDoubleLocking = new SingletonDoubleLocking();
  }
  System.out.println("SingletonDoubleLocking - "
    + singletonDoubleLocking);
  return singletonDoubleLocking;
 }

In the above implementation we are synchronizing the static method which is a lock on an Class object. This will ensure only one thread will enter the synchronize method