单例设计模式

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方式一：饿汉式 简单直接，不用

class Single{    private static final Single s = new Single();    private Single(){}    public static Single getSingle(){        return s;    }}

方式二：懒汉式

（1）存在安全问题

class Single_{    private static  Single_ s = null;    private Single_(){}    public static Single_ getSingle_(){        if(s == null){            s = new Single_();        }        return s;    }}

（2）安全，但有效率问题

class Single_1{    private static  Single_1 s = null;    private Single_1(){}    public static Single_1 getSingle_1(){        synchronized(Single_1.class){           //加锁            if(s == null){                s = new Single_1();            }        }        return s;    }}

(3) 安全、效率高

class Single_2{    private static  Single_2 s = null;    private Single_2(){}    public static Single_2 getSingle_2(){        if(s == null){                          //再加一层循环，解决效率问题            synchronized(Single_2.class){       //加锁                if(s == null){                    s = new Single_2();                }            }        }        return s;    }}

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