對于Java Web中的Filter和Interceptor的理解
寫在前面：因為部門項目中有用戶登錄驗證方面的需求，故而學習了一下相關的驗證技術，本文僅是作者個人學習的心得，由于水平有限，如有錯誤之處還請指出、見諒。
1. 背景
在設計web應用的時候，用戶登錄/注冊是必不可少的功能，對用戶登錄信息進行驗證的方法也是多種多樣，大致可以認為如下模式：前端驗證+后臺驗證。根據筆者的經驗，一般會在前端進行一些例如是否輸入數據、輸入的數據的格式是否正確等一系列的驗證，在后臺會查詢數據庫進行驗證。
一般在后臺進行驗證的時候，都會選擇使用Servlet的Filter作為攔截器，本文主要介紹Servlet的Filter，然后再拿它跟Spring MVC的HnadlerInterceptor進行對比。
2. Filter
2.1 什么是Filter
Servlet作為Java Web的基礎，它的一個比較核心也被廣泛應用的功能就是Filter，又叫攔截器。顧名思義，攔截器就是起到攔截作用的。一般情況下，用戶從客戶端發出請求到服務器后，整個的流程是：
HttpRequest ----> Filter ----> Servlet ----> Controller/Action/... ----> Filter ----> HttpResponse
根據上面的流程可以看出，Filter的作用就是在用戶請求到達Servlet之前，進行攔截。在攔截到用戶的請求后，我們可以實現一些自定義的業務邏輯，例如之前說到的對用戶登錄信息進行驗證。Filter還可以在服務器響應到達客戶端之前對響應的數據進行修改，本文主要介紹第一個功能。
2.2 Filter的工作原理
Filter跟Servlet一樣都是由服務器負責創建和銷毀的，在web應用程序啟動時，服務器會根據應用程序的web.xml文件中的配置信息調用 public void init(FilterConfig filterConfig) throws ServletException 方法來初始化Filter，在web應用程序被移除或者是服務器關閉時，會調用 public void destroy() 來銷毀Filter。在一個應用程序中一個Filter只會被創建和銷毀一次，在進行完初始化之后，Filter中聲明了 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException 方法，用來實現一些需要在攔截完成之后的業務邏輯。
注意到上面的 doFilter() 方法的參數中，有 chain 這個參數，它是傳遞過來的攔截鏈對象，里面包含了用戶定義的一系列的攔截器，這些攔截器根據其在web.xml中定義的順序依次被執行。當用戶的信息驗證通過或者當前攔截器不起作用時，我們可以執行 chain.doFilter() 方法來跳過當前攔截器來執行攔截器鏈中的下一個攔截器。
2.3 自己實現Filter
自己實現Filter時，需要繼承接口 javax.servlet.Filter 并且實現相關的方法。
2.3.1所用到的工具：
IDE: IntelliJ IDEA
構建工具：gradle
本地服務器：Tomcat
2.3.2 具體代碼
build.gradle
group 'xiangang.wei'
version '1.0-SNAPSHOT'
apply plugin: 'java'
apply plugin: 'war'
sourceCompatibility = 1.8
repositories {
jcenter()
mavenCentral()
}
dependencies {
testCompile group: 'junit', name: 'junit', version: '4.11'
// servlet-api
compile group: 'javax.servlet', name: 'servlet-api', version: '2.5'
}
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">
<filter>
<filter-name>loginValidation</filter-name>
<filter-class>filter.LoginValidation</filter-class>
<init-param>
<param-name>redirectPath</param-name>
<param-value>/index.jsp</param-value>
</init-param>
<init-param>
<param-name>disableloginValidation</param-name>
<param-value>N</param-value>
</init-param>
<init-param>
<param-name>logonString</param-name>
<param-value>/index.jsp</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>loginValidation</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
</web-app>
web.xml中<init-param>標簽被用來配置Filter的初始化時使用的參數，其中<param-name>標簽表示參數的名字，可以是自己定義的任何名字，<param-value>標簽表示對應的初始化參數的值。上面的初始化參數中， redirectPath 定義了當驗證不成功時頁面重定向的的路徑， logonString 定義了攔截器攔截的指定URL。 <filter-mapping> 標簽定義了攔截器的攔截模式，在 <url-pattern> 標簽定義了攔截模式，上面的 /* 表示攔截所有。它和之前定義的指定攔截的URL標簽結合起來使用。
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>用戶登陸界面</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>用戶名：<input type="text" name="name" /></td>
</tr>
<tr>
<td>密 碼：<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對應的實現類：
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;
}
}
登錄成功之后的頁面：
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>登錄成功！</title>
</head>
<body>
歡迎！
</body>
</html>
配置好Tomcat之后，開啟應用程序：
登錄失敗時不會跳轉，仍然會停留在原頁面。
3. Interceptor
之前提到的Filter是Servlet層面的攔截器，在許多的Java Web框架中，都實現了自己的攔截器Interceptor。例如Struts2中的Interceptor、Spring MVC中的HandlerInterceptor等。相比于Filter,框架中的Interceptor的產生作用的時間和位置不一樣，下面描述了應用了Spring MVC中的HandlerInterceptor的web請求流程：
HttpRequest ----> DispactherServlet ----> HandlerInterceptor ---->Controller----> HandlerInterceptor ----> HttpResponse
兩者的主要區別在于Filter起作用的時機是在請求到達Servlet之前，二HandlerInterceptor其作用的時機是在DispactherServlet接收到用戶請求完成請求到相應的Handler映射之后。雖然都先于在具體的業務邏輯執行，但是還是存在一些差異。Filter面對的是所有的請求，而HandlerInterceptor是面對具體的Controller。Filter總是先于HandlerInterceptor發揮作用，在Filter中甚至可以中斷請求，從而使它無法到達相應的Servlet。而且兩者的配置也不一樣，Filter是在web.xml中進行配置，HandlerInterceptor是在具體的applicationContext.xml中進行配置。
3.1 HandlerInterceptor的工作原理
分析源碼，發現HandlerInterceptor接口中聲明了如下幾個方法：
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;
}
這三個方法分別會在具體的HandlerController方法執行之前，執行成功之后，和執行完成之后被執行。
3.2 自己實現HandlerInterceptor
Spring MVC框架采用了適配器的開發模式，使用一個抽象的類 HandlerInterceptorAdapter 實現 HandlerInterceptor 接口，這樣當我們需要自己實現HandlerInterceptor時，我們可以繼承 HandlerInterceptorAdapter 這樣我們就不用全部實現這三個方法，而可以選擇性的實現自己需要的方法。
3.2.1所用到的工具：
IDE: IntelliJ IDEA
構建工具：gradle
本地服務器：Tomcat
3.2.2具體的代碼：
build.gradle
group 'xiangang.wei'
version '1.0-SNAPSHOT'
apply plugin: 'java'
apply plugin: 'war'source
Compatibility = 1.8
repositories {
jcenter()
mavenCentral()
}
dependencies {
testCompile group: 'junit', name: 'junit', version: '4.11'
// servlet-api
compile group: 'javax.servlet', name: 'servlet-api', version: '2.5'
//spring相關
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相關
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解析相關
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">
<!--框架默認幫我們配置好了ApplicationContext的實現類(org.springframework.web.context.support.XmlWebApplicationContext),不需要自己手動配置-->
<!--配置ApplicationContext需要加載的配置文件-->
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:databaseAccess.xml,classpath:service.xml</param-value>
</context-param>
<!--ApplicationContext的加載和關閉-->
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<!--配置字符過濾器,防止出現中文亂碼-->
<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>
<!--配置Spring MVC的前置控制器-->
<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">
<!--開啟注解-->
<mvc:annotation-driven/>
<!--添加需要掃描的包-->
<context:component-scan base-package="ims" use-default-filters="false">
<context:include-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
</context:component-scan>
<!--添加HandlerInterceptor-->
<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>
<!--添加試圖解析器-->
<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有兩種方式，一種是如上面所示，這張方式配置的HandlerInterceptor可以指定具體的攔截路徑，另外一種方式是直接在 <mvc:interceptors> 中使用<bean>標簽進行配置： <bean class="ims.handlerInterceptor.LoginInterceptor"/> 按照這種方式配置的HandlerInterceptor會對所有的路徑進行攔截。
具體的Interceptor實現類：
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;
}
}
具體的運行結果這里也就不再貼圖了，實現的效果跟之前的Filter是一致的。
來自：http://www.jianshu.com/p/39c0cfe25997