从安全和效率角度，实现一个单例;

package com.Isay.learn;

/**

* @author isay_me

* @date 2019/10/22

* @time 11:32

* @target:

*/

public class Singleton {

private static Singleton instance;

private Singleton(){}

public static Singleton getInstance(){

if (instance == null){

synchronized (Singleton.class){

if (instance == null){

instance = new Singleton();

}

}

}

return instance;

}

}