Java Web学习总结（29）——Java Web中的Filter和Interceptor比较

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1. 背景

在设计web应用的时候，用户登录/注册是必不可少的功能，对用户登录信息进行验证的方法也是多种多样，大致可以认为如下模式：前端验证+后台验证。根据笔者的经验，一般会在前端进行一些例如是否输入数据、输入的数据的格式是否正确等一系列的验证，在后台会查询数据库进行验证。

一般在后台进行验证的时候，都会选择使用Servlet的Filter作为拦截器，本文主要介绍Servlet的Filter，然后再拿它跟Spring MVC的HnadlerInterceptor进行对比。

2. Filter

2.1 什么是Filter

Servlet作为Java Web的基础，它的一个比较核心也被广泛应用的功能就是Filter，又叫拦截器。顾名思义，拦截器就是起到拦截作用的。一般情况下，用户从客户端发出请求到服务器后，整个的流程是：

HttpRequest ----> Filter ----> Servlet ----> Controller/Action/... ----> Filter ----> HttpResponse

根据上面的流程可以看出，Filter的作用就是在用户请求到达Servlet之前，进行拦截。在拦截到用户的请求后，我们可以实现一些自定义的业务逻辑，例如之前说到的对用户登录信息进行验证。Filter还可以在服务器响应到达客户端之前对响应的数据进行修改，本文主要介绍第一个功能。

2.2 Filter的工作原理

Filter跟Servlet一样都是由服务器负责创建和销毁的，在web应用程序启动时，服务器会根据应用程序的web.xml文件中的配置信息调用 public void init(FilterConfig
filterConfig) throws ServletException 方法来初始化Filter，在web应用程序被移除或者是服务器关闭时，会调用 public void destroy() 来销毁Filter。在一个应用程序中一个Filter只会被创建和销毁一次，在进行完初始化之后，Filter中声明了 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,
ServletException 方法，用来实现一些需要在拦截完成之后的业务逻辑。

注意到上面的 doFilter() 方法的参数中，有 chain 这个参数，它是传递过来的拦截链对象，里面包含了用户定义的一系列的拦截器，这些拦截器根据其在web.xml中定义的顺序依次被执行。当用户的信息验证通过或者当前拦截器不起作用时，我们可以执行
chain.doFilter() 方法来跳过当前拦截器来执行拦截器链中的下一个拦截器。

2.3 自己实现Filter

自己实现Filter时，需要继承接口 javax.servlet.Filter 并且实现相关的方法。

2.3.1所用到的工具：

IDE: IntelliJ IDEA

构建工具：gradle

本地服务器：Tomcat

2.3.2 具体代码

build.gradle

web.xml

web.xml中<init-param>标签被用来配置Filter的初始化时使用的参数，其中<param-name>标签表示参数的名字，可以是自己定义的任何名字，<param-value>标签表示对应的初始化参数的值。上面的初始化参数中，
redirectPath 定义了当验证不成功时页面重定向的的路径， logonString 定义了拦截器拦截的指定URL。 <filter-mapping> 标签定义了拦截器的拦截模式，在 <url-pattern> 标签定义了拦截模式，上面的 /* 表示拦截所有。它和之前定义的指定拦截的URL标签结合起来使用。

index.jsp

<%--
Created by IntelliJ IDEA.
User: xiangang
Date: 2016/11/21
Time: 下午3:42
To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>用户登陆界面</title>
<style type="text/css">
div{
margin: auto;
border: gray 1px solid;
width: 70%;
}
</style>
</head>
<body>
<div>
<form action="success.jsp" method="post">
<table>
<tr>
<td>用户名：<input type="text" name="name" /></td>
</tr>
<tr>
<td>密码：<input type="password" name="password" /></td>
</tr>
<tr>
<td><input type="submit" value="提交" /></td>
<td><input type="reset" value="重置" /></td>
</tr>
</table>
</form>
</div>
</body>
</html>

Filter对应的实现类：

LoginValidation.java

package filter;
import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;/** *
Created by xiangang on 2016/11/21.
*/public class LoginValidation implements Filter{
public FilterConfig config;
public static boolean isContains(String url, String[] regx){
boolean flag = false;
for (int i = 0;i<regx.length;i++){
if (url.indexOf(regx[i])!=-1){
flag = true;
return flag;
}
}
return flag;
} @Override
public void init(FilterConfig filterConfig) throws ServletException {
this.config = filterConfig;
} @Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
HttpServletRequest httpServletRequest= (HttpServletRequest)request;
HttpServletResponse httpServletResponse = (HttpServletResponse)response;
String name = httpServletRequest.getParameter("name");
String password = httpServletRequest.getParameter("password");
String redirectPath=httpServletRequest.getContextPath()+config.getInitParameter("redirectPath");
String logonString = config.getInitParameter("logonString");
String[] logonList = logonString.split(";"); if (isContains(httpServletRequest.getRequestURI(),logonList)){
chain.doFilter(request,response);
return;
}
if ("Y".equals(config.getInitParameter("disableloginValidation"))){
chain.doFilter(request,response);
return;
}
if ("root".equals(name) && "admin".equals(password)){
chain.doFilter(request,response);
return;
}else{
httpServletResponse.sendRedirect(redirectPath);
return;
}
} @Override public void destroy() {
this.config = null;
}
}

登录成功之后的页面：

success.jsp

<%--
Created by IntelliJ IDEA.
User: xiangang
Date: 2016/11/21
Time: 下午3:56
To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>登录成功！</title>
</head>
<body>
欢迎！
</body>
</html>

配置好Tomcat之后，开启应用程序：

登录失败时不会跳转，仍然会停留在原页面。

3. Interceptor

之前提到的Filter是Servlet层面的拦截器，在许多的Java Web框架中，都实现了自己的拦截器Interceptor。例如Struts2中的Interceptor、Spring
MVC中的HandlerInterceptor等。相比于Filter,框架中的Interceptor的产生作用的时间和位置不一样，下面描述了应用了Spring MVC中的HandlerInterceptor的web请求流程：

HttpRequest ----> DispactherServlet ----> HandlerInterceptor ---->Controller----> HandlerInterceptor
----> HttpResponse

两者的主要区别在于Filter起作用的时机是在请求到达Servlet之前，二HandlerInterceptor其作用的时机是在DispactherServlet接收到用户请求完成请求到相应的Handler映射之后。虽然都先于在具体的业务逻辑执行，但是还是存在一些差异。Filter面对的是所有的请求，而HandlerInterceptor是面对具体的Controller。Filter总是先于HandlerInterceptor发挥作用，在Filter中甚至可以中断请求，从而使它无法到达相应的Servlet。而且两者的配置也不一样，Filter是在web.xml中进行配置，HandlerInterceptor是在具体的applicationContext.xml中进行配置。

3.1 HandlerInterceptor的工作原理

分析源码，发现HandlerInterceptor接口中声明了如下几个方法：

public interface HandlerInterceptor { /**
* Intercept the execution of a handler. Called after HandlerMapping determined
* an appropriate handler object, but before HandlerAdapter invokes the handler.
* <p>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.
* <p><strong>Note:</strong> 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.
* <p>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.
* <p><strong>Note:</strong> 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.
* <p>Note: Will only be called if this interceptor's {@code preHandle}
* method has successfully completed and returned {@code true}!
* <p>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.
* <p><strong>Note:</strong> 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;}

这三个方法分别会在具体的HandlerController方法执行之前，执行成功之后，和执行完成之后被执行。

3.2 自己实现HandlerInterceptor

Spring MVC框架采用了适配器的开发模式，使用一个抽象的类 HandlerInterceptorAdapter 实现 HandlerInterceptor 接口，这样当我们需要自己实现HandlerInterceptor时，我们可以继承
HandlerInterceptorAdapter 这样我们就不用全部实现这三个方法，而可以选择性的实现自己需要的方法。

3.2.1所用到的工具：

IDE: IntelliJ IDEA

构建工具：gradle

本地服务器：Tomcat

3.2.2具体的代码：

build.gradle

group 'xiangang.wei'
version '1.0-SNAPSHOT'
apply plugin: 'java'
apply plugin: 'war'sourceCompatibility = 1.8repositories {
jcenter()
mavenCentral()
}dependencies {
testCompile group: 'junit', name: 'junit', version: '4.11'
// servlet-api
compile group: 'javax.servlet', name: 'servlet-api', version: '2.5'
//spring相关
compile group: 'org.springframework', name: 'spring-webmvc', version: '4.3.3.RELEASE'
compile group: 'org.springframework', name: 'spring-orm', version: '4.3.3.RELEASE'
compile group: 'org.springframework', name: 'spring-aspects', version: '4.3.3.RELEASE'
compile group: 'org.springframework.security', name: 'spring-security-config', version: '3.2.0.RELEASE'
compile group: 'org.springframework.security', name: 'spring-security-taglibs', version: '3.2.0.RELEASE'
compile 'org.springframework.security:spring-security-web:3.2.0.RELEASE' //hibernate相关
compile 'org.hibernate:hibernate-core:4.3.6.Final'
//mysql
compile group: 'mysql', name: 'mysql-connector-java', version: '5.1.39'
//springData
compile group: 'org.springframework.data', name: 'spring-data-jpa', version: '1.10.3.RELEASE'
// https://mvnrepository.com/artifact/log4j/log4j日志
compile group: 'log4j', name: 'log4j', version: '1.2.17'
//json解析相关
compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.5.4'
compile group: 'com.fasterxml.jackson.core', name: 'jackson-core', version: '2.5.4'
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0">
 
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:databaseAccess.xml,classpath:service.xml</param-value>
</context-param> 
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

<filter>
<filter-name>CharacterEncodingFilter</filter-name>
<filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>utf-8</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>CharacterEncodingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping> 
<servlet>
<servlet-name>dispatcherServlet</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:dispatcherServlet.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>dispatcherServlet</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
</web-app>

涉及到HandlerInterceptor的配置文件 dispatcherServlet.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd
http://www.springframework.org/schema/mvc
http://www.springframework.org/schema/mvc/spring-mvc.xsd"> 
<mvc:annotation-driven/> 
<context:component-scan base-package="ims" use-default-filters="false">
<context:include-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
</context:component-scan> 
<mvc:interceptors>
<mvc:interceptor>
<mvc:mapping path="/user/**"/>
<bean class="ims.handlerInterceptor.LoginInterceptor"/>
</mvc:interceptor>
<mvc:interceptor>
<mvc:mapping path="/register/**"/>
<bean class="ims.handlerInterceptor.RegisterInterceptor"/>
</mvc:interceptor>
</mvc:interceptors> 
<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
<property name="prefix" value="/WEB-INF/views/"/>
<property name="suffix" value=".jsp"/>
</bean></beans>

配置HandlerInterceptor有两种方式，一种是如上面所示，这张方式配置的HandlerInterceptor可以指定具体的拦截路径，另外一种方式是直接在 <mvc:interceptors>
中使用<bean>标签进行配置： <bean class="ims.handlerInterceptor.LoginInterceptor"/> 按照这种方式配置的HandlerInterceptor会对所有的路径进行拦截。

具体的Interceptor实现类：

LoginInterceptor.java

package ims.handlerInterceptor;
import ims.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.stereotype.Service;
import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;/**
* Created by xiangang on 16/11/17.
*/public class LoginInterceptor extends HandlerInterceptorAdapter { @Autowired
private UserService userService; @Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
boolean flag = true;
HttpSession session = request.getSession();
if (session.getAttribute("user") == null) {
String userName = request.getParameter("userName");
String password = request.getParameter("password");
flag = userService.selectByUserName(userName, password);
if (flag){
session.setAttribute("user",userName);
}
}
if (!flag){
response.sendRedirect("index.jsp");
}
return flag;
}
}