懒汉式【单例模式】

懒汉式

public class Singleton {
	private Singleton(){
	}
	private static Singleton singleton = null;
	public static Singleton getSingleton(){
		if(singleton == null){
			singletion = new Singleton();
		}
		return singleton;
	}
}

上述懒汉式会有线程安全问题
优化

public class Singleton{
	private Singleton(){
	}
	//保证可见性，防止指令重排
	private volatile static Singleton singleton = null;
	public static Singleton getSingleton(){
	//提升效率，对象已经存在就不再加锁判断
		if(singleton == null){
		//加同步锁
			synchronized (Singleton.class){
			//检查对象是否存在，不存在创建对象
				if(singleton == null){
					singleton = new Singleton();
				}
			}
		}
		return singleton;
	}
}