通過DCL雙端檢鎖機制+volatile禁止指令重排，解決單例模式的線程不安全

一、DCL（ double check lock） 雙端檢鎖機制

存在指令重排可能使結果不唯一，不一定線程安全，可通過volatile指令解決

二、併發多線程下存在問題

不加synchronized結果發生改變，直接添加synchronized代碼上鎖的部分過多，不夠靈活

三、解決單例模式線程不安全代碼示例

package com.yfy.multithreading.singleton;

import com.google.common.util.concurrent.ThreadFactoryBuilder;

import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;

/**
 * @author youfyu
 * @date 2020/06/28
 * 單例模式線程不安全情況下，加入雙端檢鎖機制，通過volatile禁止指令重排
 */
public class SingletonDemo {
    //引入guava依賴包，創建線程池
    private static ThreadFactory namedThreadFactory = new ThreadFactoryBuilder().setNameFormat("thread-call-runner-%d").build();

    private static ExecutorService taskExe = new ThreadPoolExecutor(10, 20, 200L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), namedThreadFactory);


    private volatile static SingletonDemo instance = null;

    private SingletonDemo() {
        System.out.println(Thread.currentThread().getName() + "\t 構造方法SingletonDemo()");
    }

    //DCL double check lock 雙端檢鎖機制，不一定線程安全，指令重排可能使結果不唯一
    public static SingletonDemo getInstance() {
        if (instance == null) {
            synchronized (SingletonDemo.class) {
                if (instance == null) {
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }

    public static void main(String[] args) {

        //併發多線程時，不加synchronized結果發生改變，沒有唯一結果
        for (int i = 1; i <= 10; i++) {

            taskExe.execute(() -> SingletonDemo.getInstance());

        }
    }
}