Java Servlet (1) —— Filter过滤请求与响应
版本: Java EE 6
参考来源:
Oracle：The Java EE 6 Tutorial: Filtering Requests and Responses
CSDN：Java中Filter、Servlet、Listener的学习
正文
在oracle javaee 6的官方文档中短短的一段话，分别从定义、内容、应用、实现这四个方面对Filter这个东西做了详细的说明
定义
A filter is an object that can transform the header and content (or both) of a request or response. Filters differ from web components in that filters usually do not themselves create a response. Instead, a filter provides functionality that can be “attached” to any kind of web resource. Consequently, a filter should not have any dependencies on a web resource for which it is acting as a filter; this way, it can be composed with more than one type of web resource.
以上定义有几层意思：
-
Filter是一个对象
（A filter is an object）
-
Filter对象的功能是可以变换请求或相应的头和内容
（can transform the header and content (or both) of a request or response）
-
Filter与web components不同，不自己创建相应
（Filters differ from web components in that filters usually do not themselves create a response）
Web Components是什么？（Wiki:Web Components）
Wiki上的定义比较抽象，但是它也给出了Web Components所表现的几个具象形式：
-
自定义元素（Custom Elements）
-
隐藏DOM（Shadow DOM）
-
HTML引入（HTML Imports）
-
HTML模板（HTML Templates）
总而言之，Web Components可以认为是一些资源（resource）的组件。
为什么我将它看成资源的组件？下面这点可以看出（Instead...web resource）
-
-
Filter可以“附在”（attached）任何web资源上
（Instead, a filter provides functionality that can be “attached” to any kind of web resource）
-
Filter不应依赖与它“依附”的web资源
（Consequently, a filter should not have any dependencies on a web resource for which it is acting as a filter）
这点是与上第4点对应。第4点为正说：应该怎样；这里为反说：不应怎样。
-
Filter可以与多个web资源组合在一起使用
（this way, it can be composed with more than one type of web resource）
正因为有4、5两特点，所以Filter具有这种能力。
何种能力呢？
功能
The main tasks that a filter can perform are as follows:
- Query the request and act accordingly.
- Block the request-and-response pair from passing any further.
- Modify the request headers and data. You do this by providing a customized version of the request.
- Modify the response headers and data. You do this by providing a customized version of the response.
- Interact with external resources.
Filter的主要功能包括：
-
查询请求然后做相应动作
(Query the request and act accordingly)
这里“查询”（Query）主要体现在filter-mapping中的url-pattern。
-
拦截请求与响应对（在向下传递时）
(Block the request-and-response pair from passing any further)
注意这里是请求与响应对，这个“对”（pair）十分重要。
-
修改请求的头与数据
(Modify the request headers and data. You do this by providing a customized version of the request)
-
修改响应的头与数据
(Modify the response headers and data. You do this by providing a customized version of the response)
-
与外部资源交互
(Interact with external resources)
以上这点比较抽象。与什么样的外部资源？如何交互？
暂且不回答这个问题，看Filter的应用场景。
应用
-
验证(Authentication)
例如SSO等验证实现都有AuthenticationFilter。
-
日志(Logging)
为了实现任何Filter的应用，都可以加入日志之类的功能。
-
图像转换(Image Conversion)
主要常见于图像格式的转换，根据不同客户端可能支持显示的格式不同，处理图片响应。
-
数据压缩(Data Compression)
对于较大的请求与响应体，可以设置数据压缩GZipFilter。
-
加密(Encryption)
对于SSL或者自行实现的安全措施，会对请求与响应进行加密。
-
标记流(Tokenizing Streams)
这个主要见于搜索应用中，比如Elastic会有TokenFilter。
-
XML变换(XML transformations)
一个典型应用可能是使用xslt转换xml的内容。
-
等
如此看来，功能中的最后一点中提到的“与外部资源的交互”就很好理解了，以上的这些验证、加密、压缩、变换等功能都需要外部资源的支持。
实现
最后实现也只是两句话，但是足以将Filter的内涵说清楚。
You can configure a web resource to be filtered by a chain of zero, one, or more filters in a specific order.
这里提到了几个关键点：
- 目标——配置web资源（web resource）
- 方式——链式（chain）
- 数量——0、1或多（zero, one, or more filters）
- 顺序——特定的顺序（in a specific order）
This chain is specified when the web application containing the component is deployed and is instantiated when a web container loads the component.
补充说明链式是如何工作的：
- 编译时（静态）——在编译部署的时候，这个链就已经定义好了。
- 运行时（动态）——在加载组件的时候，这个链被实例化。
至于详细实现方式，另开文章做具体说明。
原文
Filtering Requests and Responses
A filter is an object that can transform the header and content (or both) of a request or response. Filters differ from web components in that filters usually do not themselves create a response. Instead, a filter provides functionality that can be “attached” to any kind of web resource. Consequently, a filter should not have any dependencies on a web resource for which it is acting as a filter; this way, it can be composed with more than one type of web resource.
The main tasks that a filter can perform are as follows:
- Query the request and act accordingly.
- Block the request-and-response pair from passing any further.
- Modify the request headers and data. You do this by providing a customized version of the request.
- Modify the response headers and data. You do this by providing a customized version of the response.
- Interact with external resources.
Applications of filters include authentication, logging, image conversion, data compression, encryption, tokenizing streams, XML transformations, and so on.
You can configure a web resource to be filtered by a chain of zero, one, or more filters in a specific order. This chain is specified when the web application containing the component is deployed and is instantiated when a web container loads the component.
*扩展
问题
Filter有以上的职责，那么Interceptor的主要作用是什么呢？