Singleton Class in Java

Java Singleton
  • Singleton pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the java virtual machine.
  • The singleton class must provide a global access point to get the instance of the class.

To implement Singleton pattern, we have different approaches but all of them have following common concepts.
  • Private constructor to restrict instantiation of the class from other classes.
  • Private static variable of the same class that is the only instance of the class.
  • Public static method that returns the instance of the class, this is the global access point for outer world to get the instance of the singleton class.
Below are the different types of singleton designs,
Eager initialization
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.
package com.jithin88.singleton;
public class EagerInitializedSingleton {
private static final EagerInitializedSingleton instance = new EagerInitializedSingleton();
//private constructor to avoid client applications to use constructor
public static EagerInitializedSingleton getInstance(){
private EagerInitializedSingleton(){} return instance; }
}

Lazy Initialization
Lazy initialization method to implement Singleton pattern creates the instance in the global access method. Here is the sample code for creating Singleton class with this approach.
package com.jithin88.singleton;
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 incase 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. In next section, we will see different ways to create a thread-safe singleton class.
Thread Safe Singleton
The easier way to create a thread-safe singleton class is to make the global access method synchronized, so that only one thread can execute this method at a time. General implementation of this approach is like the below class.
package com.jithin88.singleton;
public class ThreadSafeSingleton {
private static ThreadSafeSingleton instance;
private ThreadSafeSingleton(){}
public static synchronized ThreadSafeSingleton getInstance(){
if(instance == null){ instance = new ThreadSafeSingleton(); }
}
return instance; }
Above implementation works fine and provides thread-safety but it reduces the performance because of cost associated with the synchronized method, although we need it only for the first few threads who might create the separate instances . To avoid this extra overhead every time, double checked locking principle is used. In this approach, the synchronized block is used inside the if condition with an additional check to ensure that only one instance of singleton class is created.

Below code snippet provides the double checked locking implementation.
public static ThreadSafeSingleton getInstanceUsingDoubleLocking(){
if(instance == null){
if(instance == null){
synchronized (ThreadSafeSingleton.class) {
}
instance = new ThreadSafeSingleton(); } } return instance;
}

Bill Pugh Singleton Implementation
Prior to Java 5, java memory model had a lot of issues and above approaches used to fail in certain scenarios where too many threads try to get the instance of the Singleton class simultaneously. So Bill Pugh came up with a different approach to create the Singleton class using an inner static helper class. The Bill Pugh Singleton implementation goes like this;
package com.jithin88.singleton;
public class BillPughSingleton {
private BillPughSingleton(){}
private static class SingletonHelper{
private static final BillPughSingleton INSTANCE = new BillPughSingleton();
} public static BillPughSingleton getInstance(){
}
return SingletonHelper.INSTANCE;
}

Notice the private inner static class that contains the instance of the singleton class. When the singleton class is loaded, SingletonHelper class is not loaded into memory and only when someone calls the getInstance method, this class gets loaded and creates the Singleton class instance.

This is the most widely used approach for Singleton class as it doesn’t require synchronization. I am using this approach in many of my projects and it’s easy to understand and implement also.

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