java反射简单例子

冷不防 2022-06-05 08:11 222阅读 0赞

作者：meepo  
链接：https://www.zhihu.com/question/24304289/answer/150310001  
来源：知乎  
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

今天又被学员问到这个问题了，索性专门做个小知识点来讲解反射是怎么起作用的，发现还是可以在不学习Spring等框架的基础上，举一些例子来辅助理解反射如何起作用。

鉴于手机知乎上看代码格式会乱掉，所以代码都截图

1. 首先准备两个很简单的业务类

\&lt;img src="https://pic2.zhimg.com/50/v2-aacd60547555dd41116a3288ca549785\_hd.png" data-rawwidth="495" data-rawheight="206" class="origin\_image zh-lightbox-thumb" width="495" data-original="https://pic2.zhimg.com/v2-aacd60547555dd41116a3288ca549785\_r.png"\&gt; ![v2-aacd60547555dd41116a3288ca549785_hd.jpg][]  
\&lt;img src="https://pic1.zhimg.com/50/v2-4410be8aedf7a91db4350ca4e9b89e6c\_hd.png" data-rawwidth="496" data-rawheight="202" class="origin\_image zh-lightbox-thumb" width="496" data-original="https://pic1.zhimg.com/v2-4410be8aedf7a91db4350ca4e9b89e6c\_r.png"\&gt; ![v2-4410be8aedf7a91db4350ca4e9b89e6c_hd.jpg][]

2. 非反射方式切换不同的业务方法调用

\&lt;img src="https://pic2.zhimg.com/50/v2-6d07614e59386775c738db0e573ca9a5\_hd.png" data-rawwidth="538" data-rawheight="207" class="origin\_image zh-lightbox-thumb" width="538" data-original="https://pic2.zhimg.com/v2-6d07614e59386775c738db0e573ca9a5\_r.png"\&gt; ![v2-6d07614e59386775c738db0e573ca9a5_hd.jpg][]

当需要从第一个业务方法切换到第二个业务方法的时候，使用非反射方式，必须修改代码，并且重新编译运行，才可以达到效果

3. 反射方式

使用反射方式，首先准备一个配置文件，就叫做spring.txt吧, 放在src目录下。 里面存放的是类的名称，和要调用的方法名。  
在测试类Test中，首先取出类名称和方法名，然后通过反射去调用这个方法。

当需要从调用第一个业务方法，切换到调用第二个业务方法的时候，不需要修改一行代码，也不需要重新编译，只需要修改配置文件spring.txt，再运行即可。

\&lt;img src="https://pic2.zhimg.com/50/v2-77ffdf1131e3cf1cfea3c48aac79f951\_hd.png" data-rawwidth="346" data-rawheight="144" class="content\_image" width="346"\&gt; ![v2-77ffdf1131e3cf1cfea3c48aac79f951_hd.jpg][]  
\&lt;img src="https://pic3.zhimg.com/50/v2-80cd1a6134c5e5c051807ddb949e9282\_hd.png" data-rawwidth="884" data-rawheight="678" class="origin\_image zh-lightbox-thumb" width="884" data-original="https://pic3.zhimg.com/v2-80cd1a6134c5e5c051807ddb949e9282\_r.png"\&gt; ![v2-80cd1a6134c5e5c051807ddb949e9282_hd.jpg][]

使用这个例子，可以较好得理解反射的一个应用场景。

这也是[Spring框架][Spring]的最基本的原理，只是它做的更丰富，安全，健壮。

这个例子如果有兴趣自己照着做一遍，我放在这里：

[http://how2j.cn/k/reflection/reflection-usage/1111.html][http_how2j.cn_k_reflection_reflection-usage_1111.html]

## 二、反射的主要用途 ##

很多人都认为反射在实际的Java开发应用中并不广泛，其实不然。  
 当我们在使用IDE(如Eclipse，IDEA)时，当我们输入一个对象或类并想调用它的属性或方法时，一按点号，编译器就会自动列出它的属性或方法，这里就会用到反射。  
 反射最重要的用途就是开发各种通用框架。  
 很多框架（比如Spring）都是配置化的（比如通过XML文件配置JavaBean,Action之类的），为了保证框架的通用性，它们可能需要根据配置文件加载不同的对象或类，调用不同的方法，这个时候就必须用到反射——运行时动态加载需要加载的对象。

举一个例子，在运用Struts 2框架的开发中我们一般会在struts.xml里去配置Action，比如：

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   <td style="margin:0px; padding:0px; border:none; outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; border:none; outline:0px; font-weight:inherit; font-style:inherit; font-family:Menlo,Consolas,'Source Code Pro',Inconsolata,Monaco,monospace,'Microsoft YaHei','PingFang SC','Hiragino Sans GB','WenQuanYi Micro Hei'; font-size:1em; vertical-align:baseline; overflow:auto; color:rgb(77,77,76); line-height:1.3rem; background:none">
        
         
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      <span style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; color:rgb(200,40,41)">&lt;<span style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline">action</span> <span style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline">name</span>=<span style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; color:rgb(113,140,0)">"login"</span></span>
        
         
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  </tr> 
 </tbody> 
</table>

配置文件与Action建立了一种映射关系，当View层发出请求时，请求会被StrutsPrepareAndExecuteFilter拦截，然后StrutsPrepareAndExecuteFilter会去动态地创建Action实例。  
——比如我们请求login.action，那么StrutsPrepareAndExecuteFilter就会去解析struts.xml文件，检索action中name为login的Action，并根据class属性创建SimpleLoginAction实例，并用invoke方法来调用execute方法，这个过程离不开反射。  
对与框架开发人员来说，反射虽小但作用非常大，它是各种容器实现的核心。而对于一般的开发者来说，不深入框架开发则用反射用的就会少一点，不过了解一下框架的底层机制有助于丰富自己的编程思想，也是很有益的。

## 四、反射的一些注意事项（待补充） ##

由于反射会额外消耗一定的系统资源，因此如果不需要动态地创建一个对象，那么就不需要用反射。  
另外，反射调用方法时可以忽略权限检查，因此可能会破坏封装性而导致安全问题

[v2-aacd60547555dd41116a3288ca549785_hd.jpg]: /images/20220605/831376dd62bd45079bf8eda621b0bb82.png
[v2-4410be8aedf7a91db4350ca4e9b89e6c_hd.jpg]: /images/20220605/feb28f72fa004d6fb392bde6506846d8.png
[v2-6d07614e59386775c738db0e573ca9a5_hd.jpg]: /images/20220605/32985872cbd7414aabd6fd6e4953eaa2.png
[v2-77ffdf1131e3cf1cfea3c48aac79f951_hd.jpg]: /images/20220605/e0802d8f65234ac5a13169bbd5238bfa.png
[v2-80cd1a6134c5e5c051807ddb949e9282_hd.jpg]: /images/20220605/cbff0090c3774acfa10222391dec7862.png
[Spring]: https://link.zhihu.com/?target=http%3A//how2j.cn/k/spring/spring-ioc-di/87.html
[http_how2j.cn_k_reflection_reflection-usage_1111.html]: https://link.zhihu.com/?target=http%3A//how2j.cn/k/reflection/reflection-usage/1111.html

java反射简单例子

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