Wednesday, December 14, 2011

Singleton Pattern: Am I implementing it Correctly? - Part II

In my last post, I discussed implementation of Singleton Pattern in a Single Threaded Environment. There was a very good discussion on whether about very existence of this pattern as well. Actually it is an ever going debate, few people think that it is an anti pattern and few think it is a simple solution of a common problem.

My intent of this post (and previous also) is to discuss on how to implement it, rather than whether to use it!!! So, let's start discussing Singleton Pattern in a Multi-threaded environment.

Singleton Pattern in Multi-Thread environment:

We have seen using static factory to implement Singleton Pattern in a single thread environment and also discussed problems associated with it. When in Multi-threaded environment, there are other problems as well.

What will happen if two threads simultaneously try to access static factory method (getInstance() method)? It will result in two instance of class, though we required only one instance. Clearly, singleton pattern is broken.

This problem is not only associated with Java, but also to other languages as well, like C++.  A naive solution is to synchronize the static factory method, so that at any given time, only one thread can execute getInstance() method. So, even if we ignore the problems arising from reflection (see previous post) and want to use singleton pattern, then it would be used in this way:
public class SingletonInMultiThread {
 private static SingletonInMultiThread singletonInstance; 
 private SingletonInMultiThread(){} 
 public SingletonInMultiThread getInstance(){
  if(singletonInstance == null) singletonInstance = new SingletonInMultiThread();
  return singletonInstance;

Well, there is no problem (other than we discussed for reflection) in above code, but there is an overhead of synchronization every time, as we need synchronization only at the time of creation.There are two ways to resolve this problem.

One is to instantiate the variable at the time of declaration and remove synchronization from static factory method.To resolve this synchronization
public class SingletonInMultiThread {
 private static SingletonInMultiThread singletonInstance = new SingletonInMultiThread(); 
 private SingletonInMultiThread(){} 
 public SingletonInMultiThread getInstance(){  
  return singletonInstance;
Another solution is to use infamous Double Checked Locking (Just google it and can find thousands of articles saying same thing), but it is also broken, see DCL.

But, Good News is that with Java 5 and using volatile for instance variable, you can seal this DCL broking :)

So, we have seen how we can use Singleton Pattern in a multi-thread environment, but with above discussion, there are still few issues:
  • We ignore the breaking of Singleton Pattern by reflection
  • It is quite cumbersome to implement Singleton Pattern in multi-threaded environment

As I said in previous post, starting from Java 5, a much easier way to control number of instances is to use ENUM. With all the advantages we discussed earlier, it is threadsafe!!!

Whenever you need to control number of instances of a type, it is better to consider Enum. Enums are special classes whose number of instances are controlled by JVM itself!!!


Sandeep said...

Thanks for the information

Devendra said...

i liked both of ur posts on singleton patterns and they are very useful...

Anand Kumar said...

You can get complete fully singleton working code here -