java回调机制及其实现

1. 什么是回调函数

回调函数，顾名思义，用于回调的函数。回调函数只是一个功能片段，由用户按照回调函数调用约定来实现的一个函数。回调函数是一个工作流的一部分，由工作流来决定函数的调用（回调）时机。回调函数包含下面几个特性：

  (1)属于工作流的一个部分；
  (2)必须按照工作流指定的调用约定来申明（定义）；
    (3)他的调用时机由工作流决定，回调函数的实现者不能直接调用回调函数来实现工作流的功能；

2.  回调机制

回调机制是一种常见的设计模式，它把工作流内的某个供，按照约定的接口暴露给外部使用者，为外部使用者提供数据，或要求外部使用者提供数据。

3.java的回调机制

软件模块之间总是存在这一定的接口，从调用方式上，可以把他们分为三类：同步调用、回调、异步调用。

同步调用：一种阻塞式调用，调用方要等待对方执行完毕才能返回，它是一种单向调用；

回调：一种双向调用模式，也就是说，被调用方在接口被调用时也会调用对方的接口

异步调用：一种类似消息或事件的机制，不过它的调用方向刚好相反，接口的服务在收到某种讯息或发生某种事件时，会主动通知客户方（即调用客户方的接口）。

回调和异步调用的关系非常紧密：使用回调来实现异步消息的注册，通过异步调用来实现消息的通知。

4.回调的java例子

例1：

<span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">class</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">CallbackExample</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">static</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> main</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">String</span><span class="pun" style="color: rgb(102, 102, 0);">[]</span><span class="pln" style="color: rgb(0, 0, 0);"> args</span><span class="pun" style="color: rgb(102, 102, 0);">)</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="typ" style="color: rgb(102, 0, 102);">FooBar</span><span class="pln" style="color: rgb(0, 0, 0);"> fooBar </span><span class="pun" style="color: rgb(102, 102, 0);">=</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">FooBar</span><span class="pun" style="color: rgb(102, 102, 0);">();</span><span class="pln" style="color: rgb(0, 0, 0);">fooBar</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">setCallback</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">ICallback</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> postExec</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="typ" style="color: rgb(102, 0, 102);">System</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="kwd" style="color: rgb(0, 0, 136);">out</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">println</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="str" style="color: rgb(0, 136, 0);">"在CallbackExample类中实现但不能被CallbackExample的对象调用，而由FooBar对象调用"</span><span class="pun" style="color: rgb(102, 102, 0);">);</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">});</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">interface</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">ICallback</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="com" style="color: rgb(136, 0, 0);">//需要回调的方法</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> postExec</span><span class="pun" style="color: rgb(102, 102, 0);">();</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">class</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">FooBar</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">private</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">ICallback</span><span class="pln" style="color: rgb(0, 0, 0);"> callback</span><span class="pun" style="color: rgb(102, 102, 0);">;</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> setCallback</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">ICallback</span><span class="pln" style="color: rgb(0, 0, 0);"> callback</span><span class="pun" style="color: rgb(102, 102, 0);">)</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">this</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">callback </span><span class="pun" style="color: rgb(102, 102, 0);">=</span><span class="pln" style="color: rgb(0, 0, 0);"> callback</span><span class="pun" style="color: rgb(102, 102, 0);">;</span><span class="pln" style="color: rgb(0, 0, 0);">doSth</span><span class="pun" style="color: rgb(102, 102, 0);">();</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> doSth</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);">callback</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">postExec</span><span class="pun" style="color: rgb(102, 102, 0);">();</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span>

例2：

<span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">class</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">CallbackFetcherExample</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">static</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> main</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">String</span><span class="pun" style="color: rgb(102, 102, 0);">[]</span><span class="pln" style="color: rgb(0, 0, 0);"> args</span><span class="pun" style="color: rgb(102, 102, 0);">)</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">Worker</span><span class="pun" style="color: rgb(102, 102, 0);">().</span><span class="pln" style="color: rgb(0, 0, 0);">doWork</span><span class="pun" style="color: rgb(102, 102, 0);">();</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">interface</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">FetchCallback</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> onData</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">Data</span><span class="pln" style="color: rgb(0, 0, 0);"> data</span><span class="pun" style="color: rgb(102, 102, 0);">);</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> onError</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">Throwable</span><span class="pln" style="color: rgb(0, 0, 0);"> cause</span><span class="pun" style="color: rgb(102, 102, 0);">);</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">interface</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">Fetcher</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> fetchData</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">FetchCallback</span><span class="pln" style="color: rgb(0, 0, 0);"> callback</span><span class="pun" style="color: rgb(102, 102, 0);">);</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">class</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">Worker</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="typ" style="color: rgb(102, 0, 102);">Data</span><span class="pln" style="color: rgb(0, 0, 0);"> data </span><span class="pun" style="color: rgb(102, 102, 0);">=</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">Data</span><span class="pun" style="color: rgb(102, 102, 0);">();</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> doWork</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="typ" style="color: rgb(102, 0, 102);">Fetcher</span><span class="pln" style="color: rgb(0, 0, 0);"> fetcher </span><span class="pun" style="color: rgb(102, 102, 0);">=</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">Fetcher</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> fetchData</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">FetchCallback</span><span class="pln" style="color: rgb(0, 0, 0);"> callback</span><span class="pun" style="color: rgb(102, 102, 0);">)</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="typ" style="color: rgb(102, 0, 102);">System</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="kwd" style="color: rgb(0, 0, 136);">out</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">println</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="str" style="color: rgb(0, 136, 0);">"start fetch data..."</span><span class="pun" style="color: rgb(102, 102, 0);">);</span><span class="pln" style="color: rgb(0, 0, 0);">callback</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">onData</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="pln" style="color: rgb(0, 0, 0);">data</span><span class="pun" style="color: rgb(102, 102, 0);">);</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">};</span><span class="pln" style="color: rgb(0, 0, 0);">fetcher</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">fetchData</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="kwd" style="color: rgb(0, 0, 136);">new</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">FetchCallback</span><span class="pun" style="color: rgb(102, 102, 0);">()</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> onData</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">Data</span><span class="pln" style="color: rgb(0, 0, 0);"> data</span><span class="pun" style="color: rgb(102, 102, 0);">)</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="com" style="color: rgb(136, 0, 0);">// call if data is fetched without error</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="typ" style="color: rgb(102, 0, 102);">System</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="kwd" style="color: rgb(0, 0, 136);">out</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">println</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="str" style="color: rgb(0, 136, 0);">"Data received: "</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">+</span><span class="pln" style="color: rgb(0, 0, 0);"> data</span><span class="pun" style="color: rgb(102, 102, 0);">);</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">public</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="kwd" style="color: rgb(0, 0, 136);">void</span><span class="pln" style="color: rgb(0, 0, 0);"> onError</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="typ" style="color: rgb(102, 0, 102);">Throwable</span><span class="pln" style="color: rgb(0, 0, 0);"> cause</span><span class="pun" style="color: rgb(102, 102, 0);">)</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="com" style="color: rgb(136, 0, 0);">//call if error is received during fetch</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="typ" style="color: rgb(102, 0, 102);">System</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">err</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">println</span><span class="pun" style="color: rgb(102, 102, 0);">(</span><span class="str" style="color: rgb(0, 136, 0);">"An error accour: "</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">+</span><span class="pln" style="color: rgb(0, 0, 0);"> cause</span><span class="pun" style="color: rgb(102, 102, 0);">.</span><span class="pln" style="color: rgb(0, 0, 0);">getMessage</span><span class="pun" style="color: rgb(102, 102, 0);">());</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">});</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="kwd" style="color: rgb(0, 0, 136);">class</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="typ" style="color: rgb(102, 0, 102);">Data</span><span class="pln" style="color: rgb(0, 0, 0);"> </span><span class="pun" style="color: rgb(102, 102, 0);">{</span><span class="pln" style="color: rgb(0, 0, 0);"></span><span class="pun" style="color: rgb(102, 102, 0);">}</span><div class="save_code tracking-ad" data-mod="popu_249" style="position: absolute; height: 60px; right: 30px; top: 5px; font-size: 14px; color: rgb(255, 255, 255); cursor: pointer; z-index: 2;"><a target=_blank 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