单例模式的各种实现

package com.fast.DesignPattern.singleton;

/**
 * 单例模式的各种实现
 * @author LUSHUIFA242
 *
 */
public class UserDao {
	
    private UserDao(){};
    
    private static UserDao userDao=null;  
    
    public static UserDao getInstance(){
    	if (userDao == null) {    
    		userDao = new UserDao();  
        }    
        return userDao;  
    }
    
    /**
     * 加同步处理
     * @return
     */
    public static synchronized UserDao getInstance2(){
    	if (userDao == null) {    
    		userDao = new UserDao();  
        }    
        return userDao;  
    }
    
    /**
     * 双重检查锁定
     * @return
     */
    public static UserDao getInstance3() {  
        if (userDao == null) {    
            synchronized (UserDao.class) {    
               if (userDao == null) {    
            	   userDao = new UserDao();   
               }    
            }    
        }    
        return userDao;   
    } 
    
    //============================静态内部类处理开始====================================
    /**
     * 静态内部类处理
     * @author LUSHUIFA242
     *
     */
	private static class LazyHolder {
		private static final UserDao INSTANCE = new UserDao();
	}
	
	
	public static UserDao getInstance4() {  
        return LazyHolder.INSTANCE;   
    } 
	//============================静态内部类处理结束====================================
	
	//============================枚举内部类处理开始====================================
	public static UserDao getInstance5(){
        return Singleton.INSTANCE.getInstance();
    }
	
	private static enum Singleton{
		
        INSTANCE;
        
        private UserDao singleton;
        //JVM会保证此方法绝对只调用一次
        private Singleton(){
            singleton = new UserDao();
        }
        public UserDao getInstance(){
            return singleton;
        }
    }
	//============================枚举内部类处理结束====================================
	
    
    public static void main(String[] args) {
    	System.out.println(UserDao.getInstance());
    	System.out.println(UserDao.getInstance2());
    	System.out.println(UserDao.getInstance3());
    	System.out.println(UserDao.getInstance4());
    	System.out.println(UserDao.getInstance5());
	}
}