Singleton Class In Java

Q- What is singleton in java ?

Answer:- There should be only one instance allowed for the class in all the situation and we should provide global point of access to that instance.

Number of ways  to create singleton class :- 

• Eager initialization or initialize before use
• Lazy initialization or initialize as and when we need
• Singleton using Inner class
• Singleton with serialization and de serialization
• Using Enum
• A thread-safe singleton pattern in java using Synchronization
• Double-checked locking
• Double-checked locking with volatile keyword
• Thread-safe but not lay initialized
• The ultimate Thread-safe and efficient singleton pattern in Java 

Q- How to write singleton class in java ?
Answer :

Singleton.java

package design.patern.singleton;

public class Singleton {

 private static Singleton instance = null;
 
 // Constructor should be private
 private Singleton() {
 }

 // Create instance of class
 public static Singleton getInstance() {
  if (null == instance) {
   instance = new Singleton();
  }
  return instance;
 }

 public void demoMethod() {
  System.out.println("Print demoMethod");
 }

 public static void demoStaticMethod() {
  System.out.println("Print demoStaticMethod");
 }

}

SingletonMainClass.java

package design.patern.singleton;

public class SingletonMainClass {

 public static void main(String args[]) {
  Singleton inst = Singleton.getInstance();
  System.out.println(inst.hashCode());
  inst.demoMethod();
  Singleton.demoStaticMethod();
 }
}

Write Junit test cases for singletone class 

package design.patern.singleton;
import org.junit.Test;
import static org.junit.Assert.*;
import org.junit.Test;

public class SingletonClassTest {

 @Test
 public void testSingleton() {
  Singleton instance1 = Singleton.getInstance();
  Singleton instance2 = Singleton.getInstance();
  System.out.println("checking singleton objects equality");
  assertEquals(true, instance1 == instance2);
 }
}

  enum based singleton example 

While we create Singleton pattern using Enum, that in only one line of code and Enum instance is thread-safe and guaranteed by JVM. This is the most easy way of writing thread-safe Singleton in Java

public enum SingletonUsingEnum{
    INSTANCE;
}

Let's see complete example and how to use.

public enum SingletonUsingEnum {
    INSTANCE;
 
 private String name;

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }  
}

public class SingletonEnumTest {

 public static void main(String[] args) {

  SingletonUsingEnum singletone = SingletonUsingEnum.INSTANCE;
  singletone.setName("Amby");
  System.out.println(singletone.getName());

  SingletonUsingEnum singletone2 = SingletonUsingEnum.INSTANCE;
  System.out.println("singletone: " + singletone.hashCode() + "<======>singletone2: " + singletone2.hashCode());
  System.out.println("Is same singletone reference : " + singletone.equals(singletone2));
 }
}

Output: As we can see. I have created two instance of singleton but its return same hashCode() and also return true by using equal methods on two different reference.

Amby
singletone: 2018699554<======>singletone2: 2018699554
Is same singletone reference : true

Thread Safe Singleton in Java

package design.patern.singleton;

public class Singleton {

 private static Singleton instance = null;
 private static Object staticObject = new Object();

 // Constructor should be private
 private Singleton() {
 }

 // Create instance of class
 public static Singleton getInstance() {
  Singleton result = instance;
  if (result == null) {
   synchronized (staticObject) {
    result = instance;
    if (result == null)
     instance = result = new Singleton();
   }
  }
  return result;
 }

 public void demoMethod() {
  System.out.println("Print demoMethod");
 }

 public static void demoStaticMethod() {
  System.out.println("Print demoStaticMethod");
 }

}

Q- How to prevent Singleton Class from Serialization, Cloning and Reflection?

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