Java单例模式 双重校验锁 2021

面试经常被问到！！！！！！

package com.company;
 
/**
 * @Author you guess
 * @Date 2021/1/29 09:32
 * @Version 1.0
 * @Desc 懒汉式 双重校验锁
 */
public class SingletonTest {
 
    //变量私有
    private volatile static SingletonTest instance = null;//用volatile static 修饰
 
    //构造方法私有
    private SingletonTest() {
 
    }
 
    //获取实例的方法 公用
    public static SingletonTest getInstance() {//用static修饰方法，类可以直接调用
        if (instance == null) {
            synchronized (SingletonTest.class) {//类锁
                if (instance == null) {
                    instance = new SingletonTest();
                }
            }
        }
        return instance;
    }
 
    public void test1(){
        System.out.println("SingletonTest.test1");
    }
 
}

package com.company;
 
/**
 * @Author you guess
 * @Date 2021/1/29 09:32
 * @Version 1.0
 * @Desc
 */
public class Main32 {
 
    public static void main(String[] args) {
        SingletonTest instance = SingletonTest.getInstance();
        instance.test1();//输出 SingletonTest.test1
    }
}

jdk1.8