Servlet Filter与HandlerInterceptor的对比

对于Servlet Filter，官方文档中说的很好，并且给出了常见的应用场景。

A filter is an object that performs filtering tasks on either the request to a resource (a servlet or static content), or on the response from a resource, or both.
Filters perform filtering in the doFilter method. Every Filter has access to a FilterConfig object from which it can obtain its initialization parameters, and a reference to the ServletContext which it can use, for example, to load resources needed for filtering tasks.

Filters are configured in the deployment descriptor of a web application.

Examples that have been identified for this design are:

Authentication Filters
Logging and Auditing Filters
Image conversion Filters
Data compression Filters
Encryption Filters
Tokenizing Filters
Filters that trigger resource access events
XSL/T filters
Mime-type chain Filter

接下来用简单的例子进行理解。

示例

实现Filter接口。

public class SimpleServletFilter implements Filter{
    private static Logger logger = Logger.getLogger(SimpleServletFilter.class);

    public void init(FilterConfig filterConfig) throws ServletException {
        logger.info("SimpleServletFilter init....");
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        String id = request.getParameter("id");
        if(id != null && !id.isEmpty() && !id.equals("123")){
            chain.doFilter(request,response);
        }

        // Reply directly here
        HttpServletResponse httpResponse = (HttpServletResponse)response;
        httpResponse.getWriter().print("Another Page...enjoy");
    }

    public void destroy() {
        logger.info("SimpleServletFilter destroy....");
    }
}

配置部署文件, filter所有的请求（？有问题，后面会说）。

<filter>
        <filter-name>simpleFilter</filter-name>
        <filter-class>com.vonzhou.learning.filter.SimpleServletFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>simpleFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

通过运行日志输出，结合前文HandlerInterceptor的对比,可以很清楚的看出filter和interceptor大概的生命周期。filter比interceptor更早出生，更晚死去。

2016-07-23 14:07:18,581  INFO SimpleServletFilter:16 - SimpleServletFilter init....
[2016-07-23 02:07:18,965] Artifact spring-interceptor:war exploded: Artifact is deployed successfully
[2016-07-23 02:07:18,965] Artifact spring-interceptor:war exploded: Deploy took 5,416 milliseconds
2016-07-23 14:07:24,133  INFO LogInterceptor:32 - Will call public java.lang.String com.vonzhou.learning.controller.UserController.serveUser(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse,java.util.Locale,org.springframework.ui.ModelMap)
2016-07-23 14:07:25,164  INFO LogInterceptor:38 - before return view  page, i can get model = Some Data
2016-07-23 14:07:25,848  INFO LogInterceptor:48 - Call cost time 1715 mills!
2016-07-23 14:07:28,389  INFO LogInterceptor:32 - Will call public java.lang.String com.vonzhou.learning.controller.UserController.serveUser(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse,java.util.Locale,org.springframework.ui.ModelMap)
2016-07-23 14:07:29,393  INFO LogInterceptor:38 - before return view  page, i can get model = Some Data
2016-07-23 14:07:29,395  INFO LogInterceptor:48 - Call cost time 1006 mills!
/usr/local/apache-tomcat-8.0.33/bin/catalina.sh stop
2016-07-23 14:08:12,607  INFO SimpleServletFilter:31 - SimpleServletFilter destroy....
Disconnected from server

完整代码示例

知识点总结

filter是比interceptor强大
filter操纵request，response的能力更大，可以直接response回应客户端，但是在interceptor的preHandle方法中操纵response不起作用
filter的粒度是更靠近前端的request级别，而interceptor处理的粒度测试靠近后端的Controller（或称为handler）的被映射的请求处理方法
filter可以双向的，根据request或者response来响应，但是interceptor只能是拦截进入这一端
过程中发现，如果对静态资源配置了<mvc:resources mapping="/resources/**" location="/resources/"/>，那么如果访问的静态资源文件存在的话，filter并不会起作用（web.xml中的filter配置如下）,即使把url-pattern指定为<url-pattern>/resources/*</url-pattern>（其实/就匹配了所有的请求）。*说明了mvc:resources配置的静态文件并不会经过Servlet filter，虽然说filter很强大。

<filter>
    <filter-name>simpleFilter</filter-name>
    <filter-class>com.vonzhou.learning.filter.SimpleServletFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>simpleFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

前面我说在interceptor的preHandle方法中操纵response不起作用，其实不够全面, 比如在preHandle方法中设置断点，当response回复消息后，此时shutdown服务器，这时在客户端会收到这里的消息。
另一种情况就更加奇怪，如果在interceptor中的 preHandle 方法中 response write消息之后是 return false，此时返回的消息同时包含了filter中被拦截成功后应该返回的消息（见下面），所以这时就崩溃了，一片混乱，所以只能在使用 handlerinterceptor 的时候不要使用response回复消息了。

➜  ~ curl http://localhost:8888/user/service\?id\=11
 --- From Interceptor, you are rejected.
+++ From Filter, Another Page...enjoy
➜  ~

参考

(1)The Essentials of Filters
(2) interface HandlerInterceptor