1.单例模式分为两种,饿汉模式和懒汉模式.以下是饿汉模式:

public class SingleTonHungry {private static SingleTonHungry singleTonHungry = new SingleTonHungry();private SingleTonHungry() {}public static SingleTonHungry getInstance() {return singleTonHungry;}
}

2.懒汉模式下,需要考虑多线程安全问题,首先是使用synchronized加锁,然后是使用双if判断是否存在实例,最后需要使用volatile保证内存可见性和指令重排序问题.

public class SingleTonLazy {private static volatile SingleTonLazy singleTonLazy = null;private static Object lock = new Object();private SingleTonLazy() {}public static SingleTonLazy getInstance() {if (singleTonLazy == null) {synchronized (lock) {if (singleTonLazy == null) {singleTonLazy = new SingleTonLazy();}}}return singleTonLazy;}
}