Java高级技术系列--回调

1.字面意义上的回调

字面意思上理解回调,就是A调用B,B回过头来再调用A,即是回调.既然是这样,当然就要求A中有B,B中有A.如下:

<li class="alt"><span><span class="keyword">class</span><span>&nbsp;A&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;ask(</span><span class="keyword">final</span><span>&nbsp;B&nbsp;b,&nbsp;</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;b.answer(<span class="keyword">this</span><span>,&nbsp;question);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;processResult(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;answer)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(answer);&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span><span class="keyword">class</span><span>&nbsp;B&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;answer(</span><span class="keyword">final</span><span>&nbsp;A&nbsp;a,&nbsp;</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">if</span><span>&nbsp;(question.equals(</span><span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>))&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a.processResult(<span class="string">"42"</span><span>);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;SyncObjectCallback&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">static</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;main(</span><span class="keyword">final</span><span>&nbsp;String[]&nbsp;args)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B&nbsp;b&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;B();&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A&nbsp;a&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;A();&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a.ask(b,&nbsp;<span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>}&nbsp;&nbsp;</span></li>

2.面向对象的回调

上面的写法中,B的对象只在方法中被传递了.实际上,这个B对象后来又调用了A中的方法,它的作用应该不止局限在一个方法中,而应该是A的一个部分.也就是,上面的写法不够"面向对象",让我们来改造一下:

<li class="alt"><span><span class="keyword">class</span><span>&nbsp;A&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">private</span><span>&nbsp;</span><span class="keyword">final</span><span>&nbsp;B&nbsp;b;&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;A(</span><span class="keyword">final</span><span>&nbsp;B&nbsp;b)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.b&nbsp;=&nbsp;b;&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;ask(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.b.answer(</span><span class="keyword">this</span><span>,&nbsp;question);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;processResult(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;answer)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(answer);&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span><span class="keyword">class</span><span>&nbsp;B&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;answer(</span><span class="keyword">final</span><span>&nbsp;A&nbsp;a,&nbsp;</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">if</span><span>&nbsp;(question.equals(</span><span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>))&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a.processResult(<span class="string">"42"</span><span>);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;SyncOOCallback&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">static</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;main(</span><span class="keyword">final</span><span>&nbsp;String[]&nbsp;args)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B&nbsp;b&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;B();&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A&nbsp;a&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;A(b);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a.ask(<span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li>

3.面向接口的回调
        上面的两个例子,估计没人会承认也是回调吧.因为并没什么卵用.不过这个流程对于理解回调是很重要的.其实回调真正有用的地方,在于它的"预测"能力.

我们扩展想象一下.假设上面例子中的B,为A提供了很多服务之后突然觉醒,想为更多的对象提供服务,这样一来,B就变成了Server.而且还要制定规则.规则是什么呢,就是要Server提供服务可以,对方一定要有一个recvAnswer接口供Server调用才行,这样Server才能把结果传回给Client.具体如何制定规则呢?通过Interface.如下:

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">interface</span><span>&nbsp;IClient&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">void</span><span>&nbsp;recvAnswer(String&nbsp;answer);&nbsp;&nbsp;</span></span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li>

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;Server&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;answer(</span><span class="keyword">final</span><span>&nbsp;IClient&nbsp;client,&nbsp;</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">if</span><span>&nbsp;(question.equals(</span><span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>))&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;calclating();&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;client.recvAnswer(<span class="string">"42"</span><span>);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">private</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;calclating()&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">try</span><span>&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Thread.sleep(<span class="number">5000</span><span>);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;<span class="keyword">catch</span><span>&nbsp;(InterruptedException&nbsp;e)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;e.printStackTrace();&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>}&nbsp;&nbsp;</span></li>

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;ClientSync&nbsp;</span><span class="keyword">implements</span><span>&nbsp;IClient&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">private</span><span>&nbsp;</span><span class="keyword">final</span><span>&nbsp;Server&nbsp;server;&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;ClientSync(</span><span class="keyword">final</span><span>&nbsp;Server&nbsp;server)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.server&nbsp;=&nbsp;server;&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;ask(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.server.answer(</span><span class="keyword">this</span><span>,&nbsp;question);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="annotation">@Override</span><span>&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;recvAnswer(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;answer)&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(answer);&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>}&nbsp;&nbsp;</span></li>

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;SyncInterfaceCallback&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">static</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;main(</span><span class="keyword">final</span><span>&nbsp;String[]&nbsp;args)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Server&nbsp;server&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;Server();&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ClientSync&nbsp;client&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;ClientSync(server);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;client.ask(<span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li>

注意,接口IClient实际上应该是属于Server端的,它是由Server制定的,需要Client来实现的接口,虽然看上去它跟Client很近.

为什么说有"预测"能力呢?想象另一个场景.Server现在是一个底层服务,这个底层服务知道迟早有一天会有高层服务来讨要数据,但是数据如何向上传递呢?底层可以承诺,只有你实现IClient接口,我就会调用其中的recvAnswer方法,把数据传上来.现在底层也可以调用高层的方法,算是有"预测"能力吧?

4.异步回调

上面的调用都是同步的.假设Server计算结果需要较长的时间,你一定希望它能在一个单独的线程中被执行,这是就可以把ask方法的调用用线程包装一下:

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;ClientAsync&nbsp;</span><span class="keyword">implements</span><span>&nbsp;IClient&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">private</span><span>&nbsp;</span><span class="keyword">final</span><span>&nbsp;Server&nbsp;server;&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;ClientAsync(</span><span class="keyword">final</span><span>&nbsp;Server&nbsp;server)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.server&nbsp;=&nbsp;server;&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;ask(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">new</span><span>&nbsp;Thread(</span><span class="keyword">new</span><span>&nbsp;Runnable()&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="annotation">@Override</span><span>&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;run()&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;server.answer(ClientAsync.<span class="keyword">this</span><span>,&nbsp;question);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}).start();&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="annotation">@Override</span><span>&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;recvAnswer(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;answer)&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(answer);&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>}&nbsp;&nbsp;</span></li>

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;AsyncInterfaceCallback&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">static</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;main(</span><span class="keyword">final</span><span>&nbsp;String[]&nbsp;args)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Server&nbsp;server&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;Server();&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ClientAsync&nbsp;client&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;ClientAsync(server);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;client.ask(<span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(<span class="string">"asked&nbsp;!&nbsp;waiting&nbsp;for&nbsp;the&nbsp;answer..."</span><span>);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>}&nbsp;&nbsp;</span></li>

5.线程池异步回调

每次建立新的线程耗费资源巨大,为了重用线程,使用线程池管理异步调用,这时候就要求Client不仅要实现IClient接口,还要同时是一个任务,才能被线程池执行,如下:

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;ClientRunnable&nbsp;</span><span class="keyword">implements</span><span>&nbsp;IClient,&nbsp;Runnable&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">private</span><span>&nbsp;</span><span class="keyword">final</span><span>&nbsp;Server&nbsp;server;&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">private</span><span>&nbsp;</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question;&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">private</span><span>&nbsp;</span><span class="keyword">final</span><span>&nbsp;</span><span class="keyword">int</span><span>&nbsp;id;&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;ClientRunnable(</span><span class="keyword">final</span><span>&nbsp;Server&nbsp;server,&nbsp;</span><span class="keyword">final</span><span>&nbsp;String&nbsp;question,&nbsp;</span><span class="keyword">final</span><span>&nbsp;</span><span class="keyword">int</span><span>&nbsp;id)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.server&nbsp;=&nbsp;server;&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.question&nbsp;=&nbsp;question;&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">this</span><span>.id&nbsp;=&nbsp;id;&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="annotation">@Override</span><span>&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;recvAnswer(</span><span class="keyword">final</span><span>&nbsp;String&nbsp;answer)&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(<span class="string">"clinet&nbsp;"</span><span>&nbsp;+&nbsp;</span><span class="keyword">this</span><span>.id&nbsp;+&nbsp;</span><span class="string">"&nbsp;got&nbsp;answer:&nbsp;"</span><span>&nbsp;+&nbsp;answer);&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="annotation">@Override</span><span>&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;run()&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;server.answer(ClientRunnable.<span class="keyword">this</span><span>,&nbsp;</span><span class="keyword">this</span><span>.question);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>}&nbsp;&nbsp;</span></li>

<li class="alt"><span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;ThreadpoolCallback&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">static</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;main(</span><span class="keyword">final</span><span>&nbsp;String[]&nbsp;args)&nbsp;{&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ExecutorService&nbsp;es&nbsp;=&nbsp;Executors.newCachedThreadPool();&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Server&nbsp;server&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;Server();&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">for</span><span>&nbsp;(</span><span class="keyword">int</span><span>&nbsp;i&nbsp;=&nbsp;</span><span class="number">0</span><span>;&nbsp;i&nbsp;<&nbsp;</span><span class="number">100</span><span>;&nbsp;i++)&nbsp;{&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ClientRunnable&nbsp;cr&nbsp;=&nbsp;<span class="keyword">new</span><span>&nbsp;ClientRunnable(server,&nbsp;</span><span class="string">"What&nbsp;is&nbsp;the&nbsp;answer&nbsp;to&nbsp;life,&nbsp;the&nbsp;universe&nbsp;and&nbsp;everything?"</span><span>,&nbsp;&nbsp;</span></span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;i);&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;es.execute(cr);&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(<span class="string">"client&nbsp;"</span><span>&nbsp;+&nbsp;i&nbsp;+&nbsp;</span><span class="string">"&nbsp;asked&nbsp;!"</span><span>);&nbsp;&nbsp;</span></span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;</span></li><li class=""><span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;es.shutdown();&nbsp;&nbsp;</span></li><li class="alt"><span>&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;</span></li><li class=""><span>}&nbsp;&nbsp;</span></li>

至此,我们就实现了线程池异步回调.