Java Web——过滤器

《Java Web开发技术应用——过滤器》

过滤器是一个程序，它先于与之相关的servlet或JSP页面运行在服务器上。过滤器可附加到一个或多个servlet或JSP页面上，并且可以检查进入这些资源的请求信息。在这之后，过滤器可以作如下的选择：

①以常规的方式调用资源（即，调用servlet或JSP页面）。

②利用修改过的请求信息调用资源。

③调用资源，但在发送响应到客户机前对其进行修改。

④阻止该资源调用，代之以转到其他的资源，返回一个特定的状态代码或生成替换输出。

 

用户请求——>过滤器——>WEB资源——>过滤器——>用户

过滤器的生命周期

1.实例化 web.xml  在web容器启动时依据web.xml实例化

2.初始化 init()

3.过滤 doFilter()

4.销毁 destroy()

配置：

可以通过Design界面快速配置

过滤器需要实现接口javax.servlet.Filter，开始以为是类……找了好久……

需要实现三个方法

 public void destroy() {} 

 public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException, ServletException {} 

 public void init(FilterConfig arg0) throws ServletException {} 

在doFilter中实现逻辑

如果多个过滤器对应一个路径

那么按照在web.xml的中定义的顺序执行过滤器

请求——>过滤器1——>过滤器2——>Servlet——>过滤器2——>过滤器1——>用户

过滤器的分类：（默认是request

ASYNC：Servlet中异步执行过滤器和业务逻辑内容。

ERROR：处理error-page

FORWARD：通过request.getRequestDispatcher("url").forward(request, response);或者<jsp:forward page="..."></jsp:forward>

INCLUDE：通过request.getRequestDispatcher("url").include(request, response);或者<jsp:include page="..."></jsp:include>

REQUEST：通过链接直接访问，或者通过response.sendRedirect("url");

在web.xml里面配置error-page

<error-page>
        <error-code>404</error-code>
        <location>/error.jsp</location>
</error-page>

在类上面通过注解配置过滤器

@WebFilter(filterName="...", value={"/....jsp"}, dispatcherTypes={DispatcherType.REQUEST, DispatcherType.ASYNC})

public class FilterName implements Filter {...}

案例：登录校验，如果没有登录，不能直接通过url访问登录后才能访问的页面

（突然发现右键有Servlet的选项，内心是崩溃的……窝每次都是新建java类……

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    
    <title>My JSP 'login.jsp' starting page</title>
    
    <meta http-equiv="pragma" content="no-cache">
    <meta http-equiv="cache-control" content="no-cache">
    <meta http-equiv="expires" content="0">    
    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
    <meta http-equiv="description" content="This is my page">
    <!--
    <link rel="stylesheet" type="text/css" href="styles.css">
    -->

  </head>
  
  <%
       request.setCharacterEncoding("utf-8");
   %>
  
  <body>
    <form action="<%=request.getContextPath() %>/servlet/LoginServlet" method="post">
        用户名：<input type="text" name="username">
        密码：<input type="password" name="password">
        <input type="submit" value="提交">
    </form>
    
  </body>
</html>
login.jsp
package com.imooc.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LoginServlet extends HttpServlet {

    /**
         * The doPost method of the servlet. <br>
         *
         * This method is called when a form has its tag value method equals to post.
         * 
         * @param request the request send by the client to the server
         * @param response the response send by the server to the client
         * @throws ServletException if an error occurred
         * @throws IOException if an error occurred
         */
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String username = request.getParameter("username");
        String password = request.getParameter("password");
        
        System.out.println(username);
        
        if("admin".equals(username) && "admin".equals(password)){
            //校验通过
            HttpSession session = request.getSession();
            session.setAttribute("username", username);
            response.sendRedirect(request.getContextPath()+"/success.jsp");
            return ;
        } else{
            //校验失败
            response.sendRedirect(request.getContextPath()+"/fail.jsp");
            return ;
        }
    }

}
LoginServlet.java
package com.imooc.filter;

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;
import javax.servlet.http.HttpSession;

public class LoginFilter implements Filter {
    
    
    private FilterConfig config;

    @Override
    public void destroy() {
        // TODO Auto-generated method stub

    }

    @Override
    public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) arg0;
        HttpServletResponse response = (HttpServletResponse) arg1;
        
        String charset = config.getInitParameter("charset");
        if (charset == null) {
            charset = "utf-8";
        }
        
        request.setCharacterEncoding("utf-8");
        
        String noLoginPaths = config.getInitParameter("noLoginPaths");
        if (noLoginPaths != null) {
            String[] strArray = noLoginPaths.split(";");
            for (String s: strArray) {
                if (s != null && !s.equals("") && request.getRequestURL().indexOf(s) != -1) {
                    arg2.doFilter(arg0, arg1);
                    return;
                }
            }
        }
        
        
        HttpSession session = request.getSession();
        if (session.getAttribute("username") != null) {
            arg2.doFilter(arg0, arg1);
        } else {
            response.sendRedirect("login.jsp");
        }
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-gegnerated method stub
        config = arg0;
    }

}
LoginFilter.java
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>LoginFilter</display-name>
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>LoginServlet</servlet-name>
    <servlet-class>com.imooc.servlet.LoginServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>LoginServlet</servlet-name>
    <url-pattern>/servlet/LoginServlet</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
    <filter>
        <filter-name>LoginFilter</filter-name>
        <filter-class>com.imooc.filter.LoginFilter</filter-class>
        <init-param>
            <param-name>noLoginPaths</param-name>
            <param-value>login.jsp;fail.jsp;LoginServlet</param-value>
        </init-param>
        <init-param>
            <param-name>charaset</param-name>
            <param-value>GBK</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>LoginFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>
web.xml
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原文地址：https://www.cnblogs.com/wenruo/p/6347721.html