单例模式线程安全问题

饿汉式：

package com.atjava.test;

public class Single {

    private static Single single;

    private Single(){

    }

    public static Single getSingle() {
        return single;
    }
}

　　懒汉式：

package com.atjava.test;

public class SingleHungry {
    private static SingleHungry singleHungry = null;

    private SingleHungry(){

    }

    public static SingleHungry getSingleHungry() {
        if(singleHungry == null){
            synchronized (SingleHungry.class){
                if(singleHungry == null){
                    singleHungry = new SingleHungry();
                }
            }
        }
        return singleHungry;
    }
}
声明为静态的可以通过类直接调用，实例对象是private，构造方法是private，这样其他类不能直接创建实例对象必须通过getInstace方法