【并发编程】安全发布对象—单例模式升级版（2）

/**
 * 懒汉模式
 * 线程安全
 */
public class LazyMode4 {
    private LazyMode4(){//私有的构造函数}
    private volatile static LazyMode4 instance = null;
    public static LazyMode4 getInstance(){
        if(instance == null){
            synchronized(LazyMode4.class){
                if(instance == null){
                    instance = new LazyMode4();
                }
            }
        }
        return  instance;
    }
}

/**
 * 饿汉模式
 * 线程安全的
 */
public class HungryMode1 {
    private HungryMode1(){//私有的构造函数}
    private static HungryMode1 instance = new HungryMode1();//单例对象
    public static HungryMode1 getInstance(){
        return instance;
    }
}

/**
 * 饿汉模式
 * 单例实例在类装载时进行创建
 */
@ThreadSafe
public class HungryMode2 {
    private HungryMode2(){//私有的构造函数}
    //单例对象
    private static HungryMode2 instance = null;
  //静态代码块
    static {
        instance = new HungryMode2();
    }
    public static HungryMode2 getInstance(){
        return instance;
    }
    public static void main(String[] args) {
//        System.out.println(getInstance());
//        System.out.println(getInstance());
        System.out.println(getInstance().hashCode());
        System.out.println(getInstance().hashCode());
    }
}