Singleton

1) Eager Loading

Good for small class definitions:

package com.gs.ilp.innerclasses;

/**
 * 
 * Singleton
 * 
 * @author mohitmalhotra
 *
 */
public class StudentUpdateDAOImpl {

	private static StudentUpdateDAOImpl _instance = new StudentUpdateDAOImpl();

	public static StudentUpdateDAOImpl getInstance() {
		return _instance;
	}

	private StudentUpdateDAOImpl() {

	}

	// instance variable

	public void saveStudent() {
		// making save DB
	}

	public int getTotalStudentCount() {
		return 12;
	}

}

2) Lazy Loading

package com.gs.ilp.innerclasses;

/**
 * 
 * Singleton
 * 
 * @author mohitmalhotra
 *
 */
public class StudentUpdateDAOImpl {

	private static StudentUpdateDAOImpl _instance;

	public static StudentUpdateDAOImpl getInstance() {
		if (null == _instance) {
			_instance = new StudentUpdateDAOImpl();
		}
		return _instance;
	}

	private StudentUpdateDAOImpl() {

	}

	// instance variable

	public void saveStudent() {
		// making save DB
	}

	public int getTotalStudentCount() {
		return 12;
	}

}

Issue: Thread safety

package com.gs.ilp.innerclasses;

/**
 * 
 * Singleton
 * 
 * @author mohitmalhotra
 *
 */
public class StudentUpdateDAOImpl {

	private static StudentUpdateDAOImpl _instance;

	public static StudentUpdateDAOImpl getInstance() {
		if (null == _instance) {
			_instance = new StudentUpdateDAOImpl();
		}
		return _instance;
	}

	private StudentUpdateDAOImpl() {

	}

	// instance variable

	public void saveStudent() {
		// making save DB
	}

	public int getTotalStudentCount() {
		return 12;
	}

}

com.gs.ilp.innerclasses.StudentUpdateDAOImpl@16f1d982
com.gs.ilp.innerclasses.StudentUpdateDAOImpl@3941366b

3) Thread safe - Lazy Loading

package com.gs.ilp.innerclasses;

/**
 * 
 * Singleton
 * 
 * @author mohitmalhotra
 *
 */
public class StudentUpdateDAOImpl {

	private static StudentUpdateDAOImpl _instance;

	public static StudentUpdateDAOImpl getInstance() {
		synchronized (StudentUpdateDAOImpl.class) {
			if (null == _instance) {
				_instance = new StudentUpdateDAOImpl();
			}
		}
		return _instance;
	}

	private StudentUpdateDAOImpl() {

	}

	// instance variable

	public void saveStudent() {
		// making save DB
	}

	public int getTotalStudentCount() {
		return 12;
	}

}

4) Thread safe - Lazy Loading - Starvation : performance issue

Double check locking :

package com.gs.ilp.innerclasses;

/**
 * 
 * Singleton
 * 
 * @author mohitmalhotra
 *
 */
public class StudentUpdateDAOImpl {

	private static volatile StudentUpdateDAOImpl _instance;

	public static StudentUpdateDAOImpl getInstance() {
		if (_instance == null) {
			synchronized (StudentUpdateDAOImpl.class) {
				if (null == _instance) {
					_instance = new StudentUpdateDAOImpl();
				}
			}
		}
		return _instance;
	}

	private StudentUpdateDAOImpl() {

	}

	// instance variable

	public void saveStudent() {
		// making save DB
	}

	public int getTotalStudentCount() {
		return 12;
	}

}