Singleton object creation

1 ) Singleton with eager loading

import java.io.Serializable;
import java.lang.reflect.Constructor;

public class SingletonTest implements Serializable {

private static final long serialVersionUID = 5287483952638645154L;
private final static SingletonTest INSTANCE = new SingletonTest();

private SingletonTest() {
if (INSTANCE != null) {
throw new IllegalStateException("Inside SingletonTest(): SingletonTest " + "instance already created.");
}
System.out.println("Inside SingletonTest(): Singleton instance is being created.");
}

public static SingletonTest getInstance() {
return INSTANCE;
}

@Override
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}

// This method is called immediately after an object of this class is
// deserialized.
protected Object readResolve() {
// Instead of the object we’re on, return the class variable singleton
return INSTANCE;
}

}

To test this use given main method

public static void main(String[] args) {

//System.out.println(SingletonEnum.INSTANCE);

try {

System.out.println("Inside main(): Getting the singleton instance using getInstance()...");

System.out.println(SingletonTest.getInstance());

Class clazz = SingletonTest.class;

Constructor cons = clazz.getDeclaredConstructor();

cons.setAccessible(true);

SingletonTest s2 = cons.newInstance();

System.out.println(s2);

}

catch (Exception e) {

e.printStackTrace();

}

}

2) Singleton with Lazy Loading

package com;

public class SingletonLazy {

private SingletonLazy() {

}

private static SingletonLazy INSTANCE = null;

public static SingletonLazy getInstance() {

if (INSTANCE == null) {

synchronized (SingletonLazy.class) {

if (INSTANCE == null) {

INSTANCE = new SingletonLazy();

}

}

}

return INSTANCE;

}

}

3) Singleton with Enum

public enum SingletonEnum {

INSTANCE;

private SingletonEnum() {

System.out.println("Here");

}

}