java单例模式

/**
 * 单例模式 ---懒汉式
 * @author YLJ
 *
 */
public class User {
	private static User user = null;
	//效率较低
	public synchronized static User getIntence1(){
		if(null != user){
			user = new User();
		}
		return user;
	}
	//效率较低
	public static User getIntence2(){
		synchronized(User.class){
			if(null != user){
				user = new User();
			}
		}
		return user;
	}
	//双锁 效率高	
	public static User getIntence3(){
		if(null != user){	//存在对象不需要等待，提高效率
			synchronized (User.class) {
				if(null != user){
					user = new User();
				}
			}
		}
		return user;
	}
	private User(){}	
}
/**
 * 单例模式 === 饿汉式
 * @author YLJ
 *
 */
class Role {
	private static Role role = new Role();
	private Role(){}
	public static Role getInstanse(){
		return role;
	}
	
}
/**
 * 单例模式 === 饿汉式2
 * @author YLJ
 *
 */
class Role2 {
	private static class INTER{
		private static Role2 role = new Role2();
	}
	private Role2(){}
	public static Role2 getInstanse(){
		return INTER.role;
	}
	
}