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过滤器、监听器、拦截器的区别
内容来源：本篇为学习整理，转载自网络技术博客（版权归原作者所有）：
看在前面
一、过滤器
1.1 过滤器介绍
Servlet中的过滤器Filter是实现了javax.servlet.Filter接口的服务器端程序。主要的用途是过滤字符编码、做一些业务逻辑判断。其工作原理是，只要你在web.xml文件配置好要拦截的客户端请求，它都会帮你拦截到请求。此时你就可以对请求或相应（Request、Response）统一设置编码，简化操作。同时还可以做逻辑判断，例如用户是否已经登录、有没有权限访问该页面。
它是随着你的web应用启动而启动，只初始化一次。以后就可以拦截相关的请求。只有当你的web应用停止或重新部署的时候才能销毁。
在java.servlet.Filter接口中定义了3个方法：
java
package javax.servlet;
import java.io.IOException;
/**
* 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.
*
* <p>Filters perform filtering in the <code>doFilter</code> 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.
*
* <p>Filters are configured in the deployment descriptor of a web
* application.
*
* <p>Examples that have been identified for this design are:
* <ol>
* <li>Authentication Filters
* <li>Logging and Auditing Filters
* <li>Image conversion Filters
* <li>Data compression Filters
* <li>Encryption Filters
* <li>Tokenizing Filters
* <li>Filters that trigger resource access events
* <li>XSL/T filters
* <li>Mime-type chain Filter
* </ol>
*
* @since Servlet 2.3
*/
public interface Filter {
/**
* 用于完成过滤器的初始化
*
* Called by the web container to indicate to a filter that it is
* being placed into service.
*
* <p>The servlet container calls the init
* method exactly once after instantiating the filter. The init
* method must complete successfully before the filter is asked to do any
* filtering work.
*
* <p>The web container cannot place the filter into service if the init
* method either
* <ol>
* <li>Throws a ServletException
* <li>Does not return within a time period defined by the web container
* </ol>
*/
public void init(FilterConfig filterConfig) throws ServletException;
/**
* 实现过滤功能，该方法对每个请求增加额外的处理
*
* The <code>doFilter</code> method of the Filter is called by the
* container each time a request/response pair is passed through the
* chain due to a client request for a resource at the end of the chain.
* The FilterChain passed in to this method allows the Filter to pass
* on the request and response to the next entity in the chain.
*
* <p>A typical implementation of this method would follow the following
* pattern:
* <ol>
* <li>Examine the request
* <li>Optionally wrap the request object with a custom implementation to
* filter content or headers for input filtering
* <li>Optionally wrap the response object with a custom implementation to
* filter content or headers for output filtering
* <li>
* <ul>
* <li><strong>Either</strong> invoke the next entity in the chain
* using the FilterChain object
* (<code>chain.doFilter()</code>),
* <li><strong>or</strong> not pass on the request/response pair to
* the next entity in the filter chain to
* block the request processing
* </ul>
* <li>Directly set headers on the response after invocation of the
* next entity in the filter chain.
* </ol>
*/
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain)
throws IOException, ServletException;
/**
* 用于过滤器销毁前，完成某些资源的回收
*
* Called by the web container to indicate to a filter that it is being
* taken out of service.
*
* <p>This method is only called once all threads within the filter's
* doFilter method have exited or after a timeout period has passed.
* After the web container calls this method, it will not call the
* doFilter method again on this instance of the filter.
*
* <p>This method gives the filter an opportunity to clean up any
* resources that are being held (for example, memory, file handles,
* threads) and make sure that any persistent state is synchronized
* with the filter's current state in memory.
*/
public void destroy();
}1.2 自定义过滤器：
java
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class FilterUtil implements Filter {
private FilterConfig filterConfig;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
this.filterConfig = filterConfig;
System.out.println("过滤器Filter初始化");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException {
if (!(request instanceof HttpServletRequest) || !(response instanceof HttpServletResponse)) {
throw new ServletException("FilterUtil just supports HTTP requests");
}
HttpServletRequest httpServletRequest = (HttpServletRequest) request;
HttpServletResponse httpServletResponse = (HttpServletResponse) response;
httpServletRequest.setCharacterEncoding(this.filterConfig.getInitParameter("encoding"));
httpServletResponse.setCharacterEncoding(this.filterConfig.getInitParameter("encoding"));
filterChain.doFilter(httpServletRequest, httpServletResponse);
}
@Override
public void destroy() {
System.out.println("过滤器Filter销毁");
}
}web.xml配置：
java
<filter>
<filter-name>encodingFilter</filter-name>
<!-- <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class> -->
<filter-class>com.cn.util.FilterUtil</filter-class>
<async-supported>true</async-supported>
<init-param>
<param-name>encoding</param-name>
<param-value>UTF-8</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>encodingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>1.3 filter的实际应用
过滤器只需要继承javax.servlet.filter即可。一般来说我们只要添加tomcat运行时环境就能够包含javax.servlet的jar包。但是eclipse在tomcat8中没有找到，tomcat8中确实没有，只有通过maven来添加了：
java
<!-- https://mvnrepository.com/artifact/javax.servlet/servlet-api -->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>2.5</version>
</dependency>
<!-- https://mvnrepository.com/artifact/javax.servlet/jsp-api -->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>jsp-api</artifactId>
<version>2.0</version>
</dependency>1.3.1 请求编码转换
java
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class urlEncodeFilter implements Filter{
Logger logger = LoggerFactory.getLogger(urlEncodeFilter.class);
Map<String,Object> paramMap = new HashMap<String,Object>();
@Override
public void destroy() {
}
@Override
public void doFilter(ServletRequest arg0, ServletResponse arg1,
FilterChain arg2) throws IOException, ServletException {
System.out.println("urlEncodeFilter doFilter..."+paramMap.get("urlEncode").toString());
arg0.setCharacterEncoding(paramMap.get("urlEncode").toString());
arg2.doFilter(arg0, arg1);
}
@Override
public void init(FilterConfig arg0) throws ServletException {
String urlEncode = arg0.getInitParameter("urlEncode");
paramMap.put("urlEncode",urlEncode);
}
}web.xml配置：
java
<filter>
<filter-name>urlEncodeFilter</filter-name>
<filter-class>com.mycompany.mvc.filter.urlEncodeFilter</filter-class>
<init-param>
<param-name>urlEncode</param-name>
<param-value>UTF-8</param-value>
</init-param>filter
java
<filter-mapping>
<filter-name>urlEncodeFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>1.3.2 日志记录，比如记录所有对网站发起请求的地址
java
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class logFilter implements Filter{
Logger logger = LoggerFactory.getLogger(logFilter.class);
@Override
public void destroy() {
}
@Override
public void doFilter(ServletRequest arg0, ServletResponse arg1,
FilterChain arg2) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest)arg0;
System.out.println("logFilter doFilter servletPath:"+request.getRemoteHost());
arg2.doFilter(arg0, arg1);
}
@Override
public void init(FilterConfig arg0) throws ServletException {
}
}web.xml:
java
<filter>
<filter-name>logFilter</filter-name>
<filter-class>com.mycompany.mvc.filter.logFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>logFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>1.3.4 对未登录用户的判断
java
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import org.apache.commons.lang.StringUtils;
import com.mycompany.mvc.utils.Constant;
public class loginFilter implements Filter{
private String dispatchUrl = "";
private String excludeUrl = "";
@Override
public void destroy() {
}
@Override
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest)arg0;
String servletPath = request.getServletPath();
HttpSession session = request.getSession();
String sessionKey = (String) session.getAttribute(Constant.SESSIONKEY);
/*就是登陆界面不进行过滤*/
if(servletPath.equals(dispatchUrl) || servletPath.equals(excludeUrl)){
arg2.doFilter(arg0, arg1);
}else{
if(!StringUtils.isEmpty(sessionKey)){
arg2.doFilter(arg0, arg1);
}else{
request.getRequestDispatcher(dispatchUrl).forward(arg0, arg1);
}
}
}
@Override
public void init(FilterConfig arg0) throws ServletException {
dispatchUrl = arg0.getInitParameter("dispatchUrl");
excludeUrl = arg0.getInitParameter("excludeUrl");
}
}web.xml:
java
<filter>
<filter-name>loginFilter</filter-name>
<filter-class>com.mycompany.mvc.filter.loginFilter</filter-class>
<init-param>
<!-- 不进行过滤的url，因为它就是跳转到登陆界面, -->
<param-name>excludeUrl</param-name>
<param-value>/main</param-value>
</init-param>
<init-param>
<!-- 未登录用户跳转的url -->
<param-name>dispatchUrl</param-name>
<param-value>/system/login</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>loginFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>之所以上面的/main能够直接跳转到index这个登录界面，是因为SpringMvc中配置了这个（上篇文章有讲述到）：
java
<mvc:view-controller path="${adminPath}" view-name="index"/>它的意思就是不经过controller层，直接把index放入ModelAndView，然后由渲染层渲染。 讲到这里，再结合上面说到的拦截器，我们发现，这个时候拦截器还是能够拦截2次的。就是视图渲染前和渲染后。但是进入controller层之前肯定拦截不到了，因为请求根本就没有进入controller。
systemAction:
java
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.servlet.ModelAndView;
@Controller
@RequestMapping("/system")
public class systemAction {
@RequestMapping("/login")
public ModelAndView login(HttpServletRequest request){
ModelAndView mv = new ModelAndView();
HttpSession session = request.getSession();
/*假设用户输入的用户名密码正确，则放入sessionKey中，对应的value可以
* 是User对象，这里以字符串"test"代替*/
session.setAttribute("sessionKey","test");
mv.setViewName("login");
return mv;
}
@RequestMapping("/view")
public ModelAndView view(HttpServletRequest request){
ModelAndView mv = new ModelAndView();
mv.setViewName("view");
return mv;
}
}Constant.java:
java
public class Constant {
public static final String SESSIONKEY = "sessionKey";
}二、监听器
2.1 监听器介绍
Servlet的监听器Listener，它是实现了javax.servlet.ServletContextListener接口的服务器端程序，它也是随web应用的启动而启动，只初始化一次，随web应用的停止而销毁。主要作用是：做一些初始化的内容添加工作、设置一些基本的内容、比如一些参数或者是一些固定的对象等等。
在javax.servlet.ServletContextListener接口中定义了2种方法：
java
package javax.servlet;
import java.util.EventListener;
/**
* Interface for receiving notification events about ServletContext
* lifecycle changes.
*
* In order to receive these notification events, the implementation
* class must be either declared in the deployment descriptor of the web
* application, annotated with {@link javax.servlet.annotation.WebListener},
* or registered via one of the addListener methods defined on
* {@link ServletContext}.
*
* Implementations of this interface are invoked at their
* {@link #contextInitialized} method in the order in which they have been
* declared, and at their {@link #contextDestroyed} method in reverse
* order.
*
* @see ServletContextEvent
*
* @since Servlet 2.3
*/
public interface ServletContextListener extends EventListener {
/**
* Receives notification that the web application initialization
* process is starting.
*
* All ServletContextListeners are notified of context
* initialization before any filters or servlets in the web
* application are initialized.
*
* @param sce the ServletContextEvent containing the ServletContext
* that is being initialized
*/
public void contextInitialized(ServletContextEvent sce);
/**
* Receives notification that the ServletContext is about to be
* shut down.
*
* All servlets and filters will have been destroyed before any
* ServletContextListeners are notified of context
* destruction.
*
* @param sce the ServletContextEvent containing the ServletContext
* that is being destroyed
*/
public void contextDestroyed(ServletContextEvent sce);
}2.2 自定义监听器：
java
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class ServletContextListenerUtil implements ServletContextListener {
/**
* 项目启动时，先启动监听器，再启动过滤器。
*/
@Override
public void contextInitialized(ServletContextEvent sce) {
System.out.println("监听器ServletContextListenerUtil初始化");
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
System.out.println("监听器ServletContextListenerUtil销毁");
}
}2.3 listener实际应用
listener具体分为八种，能够监听包括request域，session域，application域的产生，销毁和属性的变化；
具体使用，可以看之前转载一篇文章（再次感谢"孤傲苍狼"，他的主页http://www.cnblogs.com/xdp-gacl/）：<a href="https://blog.csdn.net/Jintao_Ma/article/details/51464124">http://blog.csdn.net/Jintao_Ma......</a>
在配置完然后我们在web.xml中诸如下面的配置即可：
java
<listener> <listener-class>
com.mycompany.mvc.listener.myServletContextListener
</listener-class>
</listener>
<listener>
<listener-class>
com.mycompany.mvc.listener.myServletContextAttributeListener
</listener-class>
</listener>2.4.1 获取当前在线人数
java
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class myHttpSessionListener implements HttpSessionListener{
public static int peopleOnLine = 0;
@Override
public void sessionCreated(HttpSessionEvent arg0) {
System.out.println("myHttpSessionListener.sessionCreated():"+arg0);
peopleOnLine++;
arg0.getSession().setAttribute("peopleOnLine",peopleOnLine);
}
@Override
public void sessionDestroyed(HttpSessionEvent arg0) {
System.out.println("myHttpSessionListener.sessionDestroyed():"+arg0);
peopleOnLine--;
arg0.getSession().setAttribute("peopleOnLine",peopleOnLine);
}
}在页面中就可以获取：
当前在线人数：当前在线人数：
<%=session.getAttribute("peopleOnLine")%>
其实也可以获得历史所有在线人数，只需要把历史所有在线人数保存在文件中，然后每次项目启动读取这个文件，当前人数增加时，把历史所有人数也相应增加，项目关闭时，再保存起来。
<h4>在系统初始化时，获取项目绝对路径</h4>
如下，获得绝对路径后保存到系统变量System中：
java
@Override
public void contextInitialized(ServletContextEvent servletContext) {
System.out.println("myServletContextListener.contextInitialized()");
System.setProperty("realPath", servletContext.getServletContext().getRealPath("/"));
System.out.println("myServletContextListener.contextInitialized()");
}三、拦截器
3.1 拦截器介绍
拦截器是在面向切面编程中应用的，就是在你的service或者一个方法前调用一个方法，或者在方法后调用一个方法比如动态代理就是拦截器的简单实现，在你调用方法前打印出字符串（或者做其它业务逻辑的操作），也可以在你调用方法后打印出字符串，甚至在你抛出异常的时候做业务逻辑的操作。拦截器不是在web.xml配置的，比如struts在struts.xml配置，在springMVC在spring与springMVC整合的配置文件中配置。
在springmvc中，定义拦截器要实现HandlerInterceptor接口，并实现该接口中提供的三个方法。其中HandlerInterceptor接口源码如下：
java
public interface HandlerInterceptor {
/**
* Intercept the execution of a handler. Called after HandlerMapping determined
* an appropriate handler object, but before HandlerAdapter invokes the handler.
* DispatcherServlet processes a handler in an execution chain, consisting
* of any number of interceptors, with the handler itself at the end.
* With this method, each interceptor can decide to abort the execution chain,
* typically sending a HTTP error or writing a custom response.
* **Note:** special considerations apply for asynchronous
* request processing. For more details see
* {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
* @param request current HTTP request
* @param response current HTTP response
* @param handler chosen handler to execute, for type and/or instance evaluation
* @return {@code true} if the execution chain should proceed with the
* next interceptor or the handler itself. Else, DispatcherServlet assumes
* that this interceptor has already dealt with the response itself.
* @throws Exception in case of errors
*/
boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception;
/**
* Intercept the execution of a handler. Called after HandlerAdapter actually
* invoked the handler, but before the DispatcherServlet renders the view.
* Can expose additional model objects to the view via the given ModelAndView.
* DispatcherServlet processes a handler in an execution chain, consisting
* of any number of interceptors, with the handler itself at the end.
* With this method, each interceptor can post-process an execution,
* getting applied in inverse order of the execution chain.
* **Note:** special considerations apply for asynchronous
* request processing. For more details see
* {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
* @param request current HTTP request
* @param response current HTTP response
* @param handler handler (or {@link HandlerMethod}) that started asynchronous
* execution, for type and/or instance examination
* @param modelAndView the {@code ModelAndView} that the handler returned
* (can also be {@code null})
* @throws Exception in case of errors
*/
void postHandle(
HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
throws Exception;
/**
* Callback after completion of request processing, that is, after rendering
* the view. Will be called on any outcome of handler execution, thus allows
* for proper resource cleanup.
* Note: Will only be called if this interceptor's {@code preHandle}
* method has successfully completed and returned {@code true}!
* As with the {@code postHandle} method, the method will be invoked on each
* interceptor in the chain in reverse order, so the first interceptor will be
* the last to be invoked.
* **Note:** special considerations apply for asynchronous
* request processing. For more details see
* {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
* @param request current HTTP request
* @param response current HTTP response
* @param handler handler (or {@link HandlerMethod}) that started asynchronous
* execution, for type and/or instance examination
* @param ex exception thrown on handler execution, if any
* @throws Exception in case of errors
*/
void afterCompletion(
HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
throws Exception;
}3.2 自定义拦截器如下：
java
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;
public class InterceptorUtil implements HandlerInterceptor{
//进入Handler方法之前执行
//可以用于身份认证、身份授权。如果认证没有通过表示用户没有登陆，需要此方法拦截不再往下执行，否则就放行
@Override
public boolean preHandle(HttpServletRequest request,
HttpServletResponse response, Object handler) throws Exception {
// TODO Auto-generated method stub
System.out.println("InterceptorUtil...........preHandle");
String user= (String) request.getSession().getAttribute("user");
if(user != null){
return true;
}
request.getRequestDispatcher("/WEB-INF/jsp/index.jsp").forward(request, response);
//true表示放行，false表示不放行
return false;
}
@Override
public void postHandle(HttpServletRequest request,
HttpServletResponse response, Object handler,
ModelAndView modelAndView) throws Exception {
// TODO Auto-generated method stub
}
@Override
public void afterCompletion(HttpServletRequest request,
HttpServletResponse response, Object handler, Exception ex)
throws Exception {
// TODO Auto-generated method stub
System.out.println("InterceptorUtil...........afterCompletion");
}
}preHandle方法：进入Handler方法之前执行。可以用于身份认证、身份授权。比如如果认证没有通过表示用户没有登陆，需要此方法拦截不再往下执行（return false），否则就放行（return true）。
postHandle方法：进入Handler方法之后，返回ModelAndView之前执行。可以看到该方法中有个modelAndView的形参。应用场景：从modelAndView出发：将公用的模型数据（比如菜单导航之类的）在这里传到视图，也可以在这里同一指定视图。
afterCompletion方法：执行Handler完成之后执行。应用场景：统一异常处理，统一日志处理等。
在springmvc中，拦截器是针对具体的HandlerMapping进行配置的，也就是说如果在某个HandlerMapping中配置拦截，经过该 HandlerMapping映射成功的handler最终使用该拦截器。
spring-mvc.xml配置文件中：
java
<!-- 拦截器配置 -->
<mvc:interceptors>
<!--多个拦截器,顺序执行 -->
<!-- 登陆认证拦截器 -->
<mvc:interceptor>
<!-- /** 表示拦截所有url包括子url路径，/*只拦截根下的url -->
<mvc:mapping path="/**"/>
<bean class="com.cn.util.InterceptorUtil"></bean>
</mvc:interceptor>
<!-- 其他拦截器 -->
</mvc:interceptors>3.3 过滤器和拦截器的区别
- 拦截器是基于Java的反射机制的，而过滤器是基于函数回调
- 过滤器依赖与servlet容器，而拦截器不依赖与servlet容器
- 拦截器只能对action请求起作用，而过滤器则可以对几乎所有的请求起作用
- 拦截器可以访问action上下文、值栈里的对象，而过滤器不能
- 在action的生命周期中，拦截器可以多次被调用，而过滤器只能在容器初始化时被调用一次
3.4 过滤器执行顺序
过滤前 - 拦截前 - Action处理 - 拦截后 -过滤后。个人认为过滤是一个横向的过程，首先把客户端提交的内容进行过滤(例如未登录用户不能访问内部页面的处理)；过滤通过后，拦截器将检查用户提交数据的验证，做一些前期的数据处理，接着把处理后的数据发给对应的Action；Action处理完成返回后，拦截器还可以做其他过程，再向上返回到过滤器的后续操作。
过滤器（Filter）：当你有一堆东西的时候，你只希望选择符合你要求的某一些东西。定义这些要求的工具，就是过滤器。
拦截器（Interceptor）：在一个流程正在进行的时候，你希望干预它的进展，甚至终止它进行，这是拦截器做的事情。
监听器（Listener）：当一个事件发生的时候，你希望获得这个事件发生的详细信息，而并不想干预这个事件本身的进程，这就要用到监听器。
servlet：就是对request和response进行处理的容器，它在filter之后执行，servlet其中的一部分就是controller层（标记为servlet_2），还包括渲染视图层(标记为servlet_3)和进入controller之前系统的一些处理部分(servlet_1)，另外我们把servlet开始的时刻标记为servlet_0，servlet结束的时刻标记为servlet_4。
拦截器(interceptor)：就是对请求和返回进行拦截，它作用在servlet的内部，具体来说有三个地方：
servlet_1和servlet_2之间，即请求还没有到controller层
servlet_2和servlet_3之间，即请求走出controller层次，还没有到渲染时图层
servlet_3和servlet_4之间，即结束视图渲染，但是还没有到servlet的结束
它们之间的关系，可以用一张图来表示：
拦截器这个要详细讲述一下了，上一篇文章说到，Spring的配置文件应该扫描service层及以下，SpringMvc的配置文件应该扫描controller层； 我们在service层如果想做日志的话，可以使用spring aop特性，在spring.xml中配置aspect即可，那么如果想在controller层做日志，相应地，在SpringMvc.xml中应该怎么配置呢？
这个时候就需要拦截器，它其实也是一种aop的实现(aop本身是一种思想)，而且这种实现本质上和aspect是一样的，只是做了更多的事情，我们当然可以在SpringMvc.xml中也配置aspect，不过现在有一个更好的实现，为什么不用呢。
关于拦截器细节，可以参考这篇文章：<a href="https://www.iteye.com/blog/elim-1750680">http://elim.iteye.com/blog/175...</a>
3.5 拦截器的实际应用
3.5.1 可以全局做日志
java
import java.lang.reflect.Method;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.web.method.HandlerMethod;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;
/**@Description
* logInterceptor公共拦截器，做日志记录
*/
public class logInterceptor implements HandlerInterceptor{
@Override
public void afterCompletion(HttpServletRequest arg0, HttpServletResponse arg1, Object arg2, Exception arg3)
throws Exception {
/*做一些清理工作*/
}
@Override
public void postHandle(HttpServletRequest arg0, HttpServletResponse arg1, Object arg2, ModelAndView arg3) throws Exception {
System.out.println("logInterceptor.postHandle()---view Name:"+arg3.getViewName());
}
@Override
public boolean preHandle(HttpServletRequest arg0, HttpServletResponse arg1, Object arg2) throws Exception {
/*取得调用的controller方法等*/
if(arg2 instanceof HandlerMethod){
HandlerMethod hMethod = (HandlerMethod)arg2;
Method method = hMethod.getMethod();
System.out.println("logInterceptor.preHandle()--method Name:"+method.getName());
}
return true;
}
}3.5.2 记录部分调用的时间
java
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;
/**@Description
* 登陆时间拦截器，记录用户登录时间
*
*/
public class timeInterceptor implements HandlerInterceptor{
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
return true;
}
@Override
public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
ModelAndView modelAndView) throws Exception {
System.out.println("timeInterceptor.postHandle()--time:"+System.currentTimeMillis());
}
@Override
public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
}
}上述两个拦截器功能的配置如下，SpringMvc.xml:
java
<!-- 拦截器配置 -->
<mvc:interceptors>
<bean class="com.mycompany.mvc.interceptor.logInterceptor"></bean>
<mvc:interceptor>
<mvc:mapping path="/system/view"/>
<bean class="com.mycompany.mvc.interceptor.timeInterceptor"></bean>
</mvc:interceptor>
</mvc:interceptors>四、过滤器、监听器、拦截器使用原则
对整个流程清楚之后，然后就是各自的使用，在使用之前应该有一个使用规则，为什么这个说，因为有些功能比如判断用户是否登录，既可以用过滤器，也可以用拦截器，用哪一个才是合理的呢？那么如果有一个原则，使用起来就会更加合理。实际上这个原则是有的：
把整个项目的流程比作一条河，那么监听器的作用就是能够听到河流里的所有声音，过滤器就是能够过滤出其中的鱼，而拦截器则是拦截其中的部分鱼，并且作标记。所以当需要监听到项目中的一些信息，并且不需要对流程做更改时，用监听器；当需要过滤掉其中的部分信息，只留一部分时，就用过滤器；当需要对其流程进行更改，做相关的记录时用拦截器。