一、DCL( double check lock) 双端检锁机制
存在指令重排可能使结果不唯一,不一定线程安全,可通过volatile指令解决
二、并发多线程下存在问题
不加synchronized结果发生改变,直接添加synchronized代码上锁的部分过多,不够灵活
三、解决单例模式线程不安全代码示例
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
* 单例模式线程不安全情况下,加入双端检锁机制,通过volatile禁止指令重排
*/
public class SingletonDemo {
//引入guava依赖包,创建线程池
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 构造方法SingletonDemo()");
}
//DCL double check lock 双端检锁机制,不一定线程安全,指令重排可能使结果不唯一
public static SingletonDemo getInstance() {
if (instance == null) {
synchronized (SingletonDemo.class) {
if (instance == null) {
instance = new SingletonDemo();
}
}
}
return instance;
}
public static void main(String[] args) {
//并发多线程时,不加synchronized结果发生改变,没有唯一结果
for (int i = 1; i <= 10; i++) {
taskExe.execute(() -> SingletonDemo.getInstance());
}
}
}