Java Singleton Pattern

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. Singleton pattern is used for logging, drivers objects, caching and thread pool.

For example, if you have a license for only one connection for your database or your JDBC driver has trouble with multi threading, the Singleton makes sure that only one connection is made or that only one thread can access the connection at a time.

Implementing Singletons

• 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.

Example:

package com.bigbangcode.constructors;

public class MySingleTon {

     

    private static MySingleTon myObj;

    /**

     * Create private constructor

     */

    private MySingleTon(){

         

    }

    /**

     * Create a static method to get instance.

     */

    public static MySingleTon getInstance(){

        if(myObj == null){

            myObj = new MySingleTon();

        }

        return myObj;

    }

     

    public void getSomeThing(){

        // do something here

        System.out.println("I am here....");

    }

     

    public static void main(String a[]){

        MySingleTon st = MySingleTon.getInstance();

        st.getSomeThing();

    }

}

Another example, 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.

package com.bigbangcode.singleton;

public class ThreadSafeSingleton {

    private static ThreadSafeSingleton instance;
    
    private ThreadSafeSingleton(){}
    
    public static synchronized ThreadSafeSingleton getInstance(){
        if(instance == null){
            instance = new ThreadSafeSingleton();
        }
        return instance;
    }
    
}