單例模式真的是單例嗎?

我們知道單例模式分爲餓漢模式（直接創建對象）和懶漢模式（延遲加載）
單例模式-懶漢模式在併發的情況下，會導致創建出多個對象的情況。
如下圖所示，單例模式-懶漢模式創建了兩個實例

下面是代碼

package com.design.mode.single;

/**
 * @description:
 * @author: HYW
 * @create: 2020-01-03 08:45
 */
public class Single {
    private static Single single;

    private Single(){};

    public static Single getInstance(){

        if(single == null){
            single=new Single();
        }
        return  single;
    }


    String name;

    int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

測試類

package com.design.mode.single;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Executor;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * @description:
 * @author: HYW
 * @create: 2020-01-03 08:48
 */
public class Demo {

    public static void main(String[] args) {
        ThreadPoolExecutor  pool=new ThreadPoolExecutor(10,20,1000,
                TimeUnit.MILLISECONDS,  new ArrayBlockingQueue<Runnable>(20));
        for(int i=0;i<15;i++) {

            pool.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    Single single=Single.getInstance();
                    System.out.println(single);
                }
            });
            System.out.println("線程池中線程數目："+pool.getPoolSize()+"，隊列中等待執行的任務數目："+
                    pool.getQueue().size()+"，已執行玩別的任務數目："+pool.getCompletedTaskCount());
        }
        pool.shutdown();

    }
}

問題原因，如果第一次創建實例時，多個線程同時執行任務，就會導致這段代碼被多次執行

 if(single == null){
            single=new Single();
}

解決方式
1.雙重鎖 volatile + synchronized

volatile private static Single single;

    private Single(){};

    synchronized public static Single getInstance(){

        if(single == null){
            single=new Single();
        }
        return  single;
    }

2.餓漢模式

private static Single single=new Single();

這樣保證了類加載時創建一次