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Thread,ThreadPool,Task, 到async await 的基本使用方法和理解 - 常山造纸农 - 博客园

Excerpt

很久以前的一个面试场景： 面试官：说说你对JavaScript闭包的理解吧？ 我：嗯，平时都是前端工程师在写JS，我们一般只管写后端代码。 面试官：你是后端程序员啊，好吧，那问问你多线程编程的问题吧。 我：一般没用到多线程。 面试官:……………………….. （面试

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很久以前的一个面试场景：

面试官：说说你对JavaScript闭包的理解吧？

我：嗯，平时都是前端工程师在写JS，我们一般只管写后端代码。

面试官：你是后端程序员啊，好吧，那问问你多线程编程的问题吧。

我：一般没用到多线程。

面试官:……………………….. （面试结束）

 好了，哈哈一笑后，我们来看看 Thread,ThreadPool,Task, async, await 的基本使用方法。 

1.Thread

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);
    </span><span>var</span> thread = <span>new</span> Thread(<span>new</span><span> ThreadStart(ThreadTest));
    thread.Start();

    System.Console.WriteLine(</span><span>"</span><span>主线程结束</span><span>"</span><span>);
    System.Console.ReadLine();
}

</span><span>private</span> <span>static</span> <span>void</span><span> ThreadTest()
{
    System.Console.WriteLine(</span><span>"</span><span>开始执行子线程.... </span><span>"</span><span>);
    Thread.Sleep(</span><span>100</span><span>);
}</span>

执行结果：

 

上面的代码，大家应该都很好理解，通过new Thread 来创建一个子线程，然后.Start() 开始执行。

我们F12 ThreadStart 看到 public delegate void ThreadStart();  是一个无参数无返回值的委托，那么，如果要在线程中执行一个有参数的方法怎么办了？

OK，我们看Thread的构造函数

ParameterizedThreadStart 是什么？按字面上意思就是带参数的ThreadStart，继续F12看看它

果然是可以带一个object的参数。

改造一下刚才的代码：

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);
    </span><span>var</span> thread = <span>new</span> Thread(<span>new</span><span> ParameterizedThreadStart(ThreadTest));
    thread.Start(</span><span>10</span><span>);

    System.Console.WriteLine(</span><span>"</span><span>主线程结束</span><span>"</span><span>);
    System.Console.ReadLine();
}

</span><span>private</span> <span>static</span> <span>void</span> ThreadTest(<span>object</span><span> p)
{
    System.Console.WriteLine(</span><span>"</span><span>开始执行子线程.... 参数：{0} </span><span>"</span><span>, p);
    Thread.Sleep(</span><span>100</span><span>);
}</span>&nbsp;

执行结果：

（当然还可以用ThreadStart（（）=>{ }） 直接用lambda表达式的方式，这里就不写示例代码了 ）  

 看到上面的执行结果，子线程因为Thread.Sleep(100) ，所以每次都最后才打印出输出结果，那么你可能会疑问，如果我想等子线程执行完，我再执行主线程后面的代码，怎么办？

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);
    </span><span>var</span> thread = <span>new</span> Thread(<span>new</span><span> ParameterizedThreadStart(ThreadTest));
    thread.Start(</span><span>10</span><span>);
    thread.Join();

    System.Console.WriteLine(</span><span>"</span><span>主线程结束</span><span>"</span><span>);
    System.Console.ReadLine();
}<br>
</span><span>private</span> <span>static</span> <span>void</span> ThreadTest(<span>object</span><span> p)
{
    System.Console.WriteLine(</span><span>"</span><span>开始执行子线程.... 参数：{0} </span><span>"</span><span>, p);
    Thread.Sleep(</span><span>100</span><span>);
}</span>

注意看， 加了这句 thread.Join() ，管他什么意思，我们先看看执行结果吧！

OK，是不是明白Join()的意义了？ 

2.ThreadPool

 为什么有了Thread还要出现ThreadPool了？

 如果你的代码设计了大量使用Thread，那么有可能会超过系统最大的线程数导致崩溃，而且每次创建和销毁线程也是很耗资源，ThreadPool就可以帮你提高代码效率并管理你的线程。

 这不是重点，今天重点是学习它的基础使用方法。 

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);
    ThreadPool.QueueUserWorkItem(</span><span>new</span> WaitCallback(ThreadTest), <span>1</span><span>);

    System.Console.WriteLine(</span><span>"</span><span>主线程结束</span><span>"</span><span>);
    System.Console.ReadLine();
}

</span><span>private</span> <span>static</span> <span>void</span> ThreadTest(<span>object</span><span> p)
{
    System.Console.WriteLine(</span><span>"</span><span>开始执行子线程.... 参数：{0} </span><span>"</span><span>, p);
    Thread.Sleep(</span><span>100</span><span>);
}</span>

 先看看WaitCallback的定义

一个带参数的委托，这就要求它的委托方法必须带一个object的参数了。

ThreadPool静态类通过QueueUserWorkItem()方法将工作函数排入线程池,它不需要我们主动的.Start(),那么他能不能Join()了？

我们点一下就知道了，它既不要你手动Start也没有Join这样的方法。 

好了，简单学习Thread和ThreadPool后，发现他们构造函数中都是没有返回值的委托，如果我们要在主线程中获取子线程执行方法的返回值，怎么办？ Task闪亮登场了！ 

3.Task

        <span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
        {
            System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);
            </span><span>//</span><span> new Task 创建方式-不带参数
            </span><span>//</span><span>Task task = new Task(ThreadTest);
            </span><span>//</span><span>task.Start();

            </span><span>//</span><span> new Task 创建方式-带参数
            </span><span>//</span><span>Task task=new Task(() =&gt;  ThreadTest(10));

            </span><span>//</span><span>Task.Factory 创建方式-不带参数
            </span><span>//</span><span>Task task = Task.Factory.StartNew(ThreadTest);
            </span><span>//</span><span>task.Start();

            </span><span>//</span><span>Task.Factory 创建方式-带参数
            </span><span>//</span><span>Task task = Task.Factory.StartNew(() =&gt; ThreadTest(10));
            </span><span>//</span><span>task.Start();</span>
<span>
            Task task </span>= Task.Run(() =&gt;<span> ThreadTest());
            </span><span>//</span><span>Task task = Task.Run(() =&gt; ThreadTest(10));</span>
<span>
            System.Console.WriteLine(</span><span>"</span><span>主线程结束</span><span>"</span><span>);
            System.Console.ReadLine();<br>        }</span>&nbsp;

Task 的三种创建线程的方法，Task.Run() 不需要手动Start() 。其他两种方式是需要手动Start()。 他们没有Join()方法，取而代之的是Wait()

我们用Run()方法为例，看Task如何获取方法的返回值。 

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);

    Task</span>&lt;<span>int</span>&gt; task = Task.Run(() =&gt; ThreadTest(<span>10</span><span>));
    </span><span>var</span> result =<span> task.Result;

    System.Console.WriteLine(</span><span>"</span><span>主线程结束,result={0}</span><span>"</span><span>, result);
    System.Console.ReadLine();
}

</span><span>private</span> <span>static</span> <span>int</span> ThreadTest(<span>int</span><span> i)
{
    Thread.Sleep(</span><span>100</span><span>);
    System.Console.WriteLine(</span><span>"</span><span>子线程开始</span><span>"</span><span>);
    </span><span>return</span> i * <span>100</span><span>;
}</span>

执行结果：

 通过task.Result 我们获取到了在子线程中ThreadTest方法的返回值。有没有注意，主线程是等子线程执行完之后才打印最后输出的！ task.Result 除了拿到返回值外，是不是和Wait()类似？

看到这里，你肯定会想到，这样另起线程去跑耗时作业和我们平时普通写法有什么区别？效率上会高很多吗？我们来测试看看！ 

常规方法： 

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    DateTime dt1 </span>=<span> DateTime.Now;
    </span><span>int</span> count = <span>0</span><span>;
    </span><span>for</span> (<span>int</span> i = <span>0</span>; i &lt; <span>10</span>; i++<span>)
    {
        Thread.Sleep(</span><span>10</span><span>);
        count </span>+=<span> i;
    }
    System.Console.WriteLine(</span><span>"</span><span>执行完成，耗时=</span><span>"</span> + (DateTime.Now -<span> dt1).TotalMilliseconds);
    System.Console.ReadLine();
}</span>

 

Task方法：

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    DateTime dt1 </span>=<span> DateTime.Now;
    Task</span>&lt;<span>int</span>&gt; task = Task.Run(() =&gt;<span>
     {
         </span><span>int</span> count = <span>0</span><span>;
         </span><span>for</span> (<span>int</span> i = <span>0</span>; i &lt; <span>10</span>; i++<span>)
         {
             Thread.Sleep(</span><span>10</span><span>);
             count </span>+=<span> i;
         }
         </span><span>return</span><span> count;
     });

    </span><span>var</span> result =<span> task.Result;

    System.Console.WriteLine(</span><span>"</span><span>执行完成，耗时=</span><span>"</span> + (DateTime.Now -<span> dt1).TotalMilliseconds);
    System.Console.ReadLine();
}</span>

这就很尴尬了，用Task反而执行时间更长！！！  是不是我的打开方式不对？  

4.async  await

async是修饰一个异步方法的关键字。有两种返回类型（void 或者 Task）

await必须是在async修饰的异步方法体内，await后面必须是一个异步方法或者Task。表示异步等待后面方法的结果。

1.返回void的使用方法

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);
    </span><span>for</span> (<span>int</span> i = <span>0</span>; i &lt; <span>10</span>; i++<span>)
    {
        ThreadTest(i);
    }
    System.Console.WriteLine(</span><span>"</span><span>主线程执行完成</span><span>"</span><span>);
    System.Console.ReadLine();
}

</span><span>private</span> <span>static</span> <span>async</span> <span>void</span> ThreadTest(<span>int</span><span> i)
{
    </span><span>await</span> Task.Run(() =&gt;<span>
    {
        Thread.Sleep(</span><span>10</span><span>);
        System.Console.WriteLine(</span><span>"</span><span>子线程开始,i=</span><span>"</span> +<span> i);
    });
}</span>

执行结果

 2.返回Task的使用方法 

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    System.Console.WriteLine(</span><span>"</span><span>主线程开始</span><span>"</span><span>);
    </span><span>var</span> task =<span> ThreadTest();
    System.Console.WriteLine(</span><span>"</span><span>主线程执行完成,result=</span><span>"</span>+<span> task.Result);
    System.Console.ReadLine();
}
</span><span>private</span> <span>static</span> <span>async</span> Task&lt;<span>int</span>&gt;<span> ThreadTest()
{
    </span><span>var</span> count = <span>0</span><span>;
    </span><span>await</span> Task.Run(() =&gt;<span>
    {
        </span><span>for</span> (<span>int</span> i = <span>0</span>; i &lt; <span>10</span>; i++<span>)
        {
            Thread.Sleep(</span><span>10</span><span>);
            count </span>+=<span> i;
            System.Console.WriteLine(</span><span>"</span><span>count=</span><span>"</span>+<span> count);
        }
    });
    </span><span>return</span><span> count;
}</span>

返回的是Task，那么像得到它的返回值，肯定也是通过.Result了，我们肯定有疑问了，这样和直接写Task有什么区别? 只是为了更加方便和美观吗？

接下来我们来测试下执行效率！ 

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    DateTime dt1 </span>=<span> DateTime.Now;
    </span><span>var</span> t =<span> ThreadTest().Result;
    System.Console.WriteLine(</span><span>"</span><span>执行完成，耗时=</span><span>"</span> + (DateTime.Now - dt1).TotalMilliseconds + <span>"</span><span>  count=</span><span>"</span> +<span> t);
    System.Console.ReadLine();
}

</span><span>private</span> <span>static</span> <span>async</span> Task&lt;<span>int</span>&gt;<span> ThreadTest()
{
    </span><span>var</span> count = <span>0</span><span>;
    </span><span>await</span> Task.Run(() =&gt;<span>
    {
        </span><span>for</span> (<span>int</span> i = <span>0</span>; i &lt; <span>10</span>; i++<span>)
        {
            Thread.Sleep(</span><span>10</span><span>);
            count </span>+=<span> i;
        }
    });
    </span><span>return</span><span> count;
}</span>

  

 执行效率和之前没什么区别，不知道这种测试方式是否合理？跪求大神们分享赐教！

 今天就写到这里，关于 async  await 和Task区别，async  await 线程阻塞问题，后面再来仔细研究。

========================================================================================================

分割线

========================================================================================================

昨天这篇博客发布后，收到大神的批评和指教，非常感谢！

读了这篇文章后，才恍然大悟。 文章链接：https://msdn.microsoft.com/zh-cn/magazine/jj991977.aspx

分析下昨天测试“性能”的实例代码的使用错误:

1.盲目使用task.Result来获取最终结果，这样导致主线程阻塞，都是等待子线程执行完毕。这样的时间差比没有太多意义。

2.都是在一个Task.Run()中模拟一个耗时操作，内部循环Thread.Sleep(10)。这样把耗时操作搬到一个子线程去做，就算快也能快到哪里去了，完全没有体现出多线程的优越性。

 下面是修改后的测试代码，再看看async await给程序带来的性能：

 1.普通Task，通过task.Result获取返回值。

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    DateTime dt1 </span>=<span> DateTime.Now;

    </span><span>for</span> (<span>int</span> i = <span>0</span>; i &lt;= <span>50</span>; i++<span>)
    {
        </span><span>var</span> re = Task.Run(() =&gt;<span>
        {
            Thread.Sleep(</span><span>10</span><span>);
            </span><span>return</span><span> i;
        }).Result;

        System.Console.WriteLine(</span><span>"</span><span>result=</span><span>"</span> +<span> re);

        </span><span>if</span> (i == <span>50</span><span>)
            System.Console.WriteLine(</span><span>"</span><span>执行完成，耗时=</span><span>"</span> + (DateTime.Now -<span> dt1).TotalMilliseconds);
    }
    System.Console.ReadLine();
}</span>

  2.使用async await

<span>private</span> <span>static</span> <span>void</span> Main(<span>string</span><span>[] args)
{
    DateTime dt1 </span>=<span> DateTime.Now;

    </span><span>for</span> (<span>int</span> i = <span>0</span>; i &lt;= <span>50</span>; i++<span>)
    {
        </span><span>var</span> task =<span> ThreadTest(dt1, i);
    }

    System.Console.ReadLine();
}

</span><span>private</span> <span>static</span> <span>async</span> Task&lt;<span>int</span>&gt; ThreadTest(DateTime dt1, <span>int</span><span> i)
{
    </span><span>int</span> re = <span>await</span> Task.Run(() =&gt;<span>
    {
        Thread.Sleep(</span><span>10</span><span>);
        </span><span>return</span><span> i;
    });

    System.Console.WriteLine(</span><span>"</span><span>result=</span><span>"</span> +<span> re);

    </span><span>if</span> (i == <span>50</span><span>)
        System.Console.WriteLine(</span><span>"</span><span>执行完成，耗时=</span><span>"</span> + (DateTime.Now -<span> dt1).TotalMilliseconds);

    </span><span>return</span><span> re;
}</span>

 

 async await 真正体现了它的性能所在 。

总结：

1.不要盲目使用task.Result

2.async await的意义（或者说和Task的区别）在于不阻塞线程的情况下获取返回值。

本文博客园地址:http://www.cnblogs.com/struggle999/p/6933376.html