Singleton Pattern

The singleton pattern is one of the simplest design patterns: it involves only one class which is responsible to instantiate itself, to make sure it creates not more than one instance; in the same time it provides a global point of access to that instance. In this case the same instance can be used from everywhere, being impossible to invoke directly the constructor each time.

In eager initialization, the instance of Singleton Class is created at the time of class loading, this is the easiest method to create a singleton class but it has a drawback that instance is created even though client application might not be using it.

public class EagerSingleton {
    private static volatile EagerSingleton instance = new EagerSingleton();
 
    // private constructor
    private EagerSingleton() {
    }
 
    public static EagerSingleton getInstance() {
        return instance;
    }
}

Static block initialization implementation is similar to eager initialization, except that instance of class is created in the static block that provides option for exception handling.

public class StaticBlockSingleton {
 
    private static StaticBlockSingleton instance;
     
    private StaticBlockSingleton(){}
     
    //static block initialization for exception handling
    static{
        try{
            instance = new StaticBlockSingleton();
        }catch(Exception e){
            throw new RuntimeException("Exception occured in creating singleton instance");
        }
    }
     
    public static StaticBlockSingleton getInstance(){
        return instance;
    }
}

Lazy initialization method to implement Singleton pattern creates the instance in the global access method.

public class LazyInitializedSingleton {
 
    private static LazyInitializedSingleton instance;
     
    private LazyInitializedSingleton(){}
 
    public static LazyInitializedSingleton getInstance(){
        if(instance == null){
            instance = new LazyInitializedSingleton();
        }
        return instance;
    }
}

The above implementation works fine in case of single threaded environment but when it comes to multithreaded systems, it can cause issues if multiple threads are inside the if loop at the same time. It will destroy the singleton pattern and both threads will get the different instances of singleton class.