Singleton class in java

What is Singleton class

Singleton is a design technique which gaurantees that there will be only instance of a class globally. Such classes are required when we need to create some objects which are memory/ resource extensive and we can't afford many such objects.

Using singleton We can maintain single object per JVM per classloader. Classloader could be different in different hierarchy and in such case we may have more than one instances of singleton but we can avoid it by loading it at appropriate classloader. For example in an ear application there could be multiple web modules and each one of them will have their own singleton instance. Sometimes it may be a need but sometimes it could be a flaw which can be resolved by loading it either at ear level or web module level as per requirement.

Implementing singleton class

There are two different ways to implement singleton in java.

1. Using singleton design pattern

In this pattern we can create singleton either using lazy loading or using eager loading.


In lazy loading, we create the instance when we actually need it. But implementation is little tricky and we need to handle thread safety. Below is the example of lazy loading implementation.

  public class MySingleton{
      private static MySingleton instance = null;
      
      private MySingleton(){}
      
      public static MySingleton getInstance(){
          if(instance==null){
          	synchronised(MySingleton.class){
            	if(instance==null){
              		instance=MySingleton();
                }
            }
          }
          return instance;
      }
  }
  

In eager loading, we create the instance as soon as class is loaded. Implementation is easy here and we don't need to worry about thread safety. But it creates the object whether we need it or notBelow is the example of lazy loading implementation.

  public class MySingleton{
      private static MySingleton instance = MySingleton();
      
      private MySingleton(){}
      
      public static MySingleton getInstance(){

           return instance;
      }
  
  }
  

2. Using enum class

Singleton creation is very easy using enums. Just create an enum class as given below. You can also create parameterized Enum instances by adding the relevant constructor.

  public enum MySingleton {
	
	instance;
	
  }
  

And then you can get the instance by just using below code.

MySingleton i = MySingleton.instance;

Best approach to create singleton

You already saw here that you can create singleton either of the above approaches but there are some differences between them.

With singleton design pattern class creation need to handle synchronization. Also there is a chance that it can be serialized/ deserialized or class variables accessor can be modified using reflection which will destroy the singlton.

In my opinion using Enum for singleton will be the better option as it will automatically handle the above problems.