filter

filter 基本上和servlet一样。

package com.atguigu.javaweb;

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;

public class HelloFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("init..");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        System.out.println("1. Before HelloFilter's chain.doFilter ..."); //1
        
        //放行
        chain.doFilter(request, response);
        
        System.out.println("2. After HelloFilter's chain.doFilter ..."); //2
    }

    @Override
    public void destroy() {
        System.out.println("destroy...");
    }

}

在web.xml中进行配置：

<filter>
    <filter-name>helloFilter</filter-name>
    <filter-class>com.atguigu.javaweb.HelloFilter</filter-class>
    <init-param>
      <param-name>name</param-name>
      <param-value>root</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>helloFilter</filter-name>
    <url-pattern>/test.jsp</url-pattern>
  </filter-mapping>

filter的一些基本应用：

　　1）：浏览器页面缓存

　　

package cache;


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;

/**
 * 自定义的 HttpFilter, 实现自 Filter 接口
 *
 */
public abstract class HttpFilter implements Filter {

    /**
     * 用于保存 FilterConfig 对象. 
     */
    private FilterConfig filterConfig;
    
    /**
     * 不建议子类直接覆盖. 若直接覆盖, 将可能会导致 filterConfig 成员变量初始化失败
     */
    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
        init();
    }

    /**
     * 供子类继承的初始化方法. 可以通过 getFilterConfig() 获取 FilterConfig 对象. 
     */
    protected void init() {}

    /**
     * 直接返回 init(ServletConfig) 的 FilterConfig 对象
     */
    public FilterConfig getFilterConfig() {
        return filterConfig;
    }
    
    /**
     * 原生的 doFilter 方法, 在方法内部把 ServletRequest 和 ServletResponse 
     * 转为了 HttpServletRequest 和 HttpServletResponse, 并调用了 
     * doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
     * 
     * 若编写 Filter 的过滤方法不建议直接继承该方法. 而建议继承
     * doFilter(HttpServletRequest request, HttpServletResponse response, 
     *        FilterChain filterChain) 方法
     */
    
    public void doFilter(ServletRequest req, ServletResponse resp,
            FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        
        doFilter(request, response, chain);
    }
    
    /**
     * 抽象方法, 为 Http 请求定制. 必须实现的方法. 
     * @param request
     * @param response
     * @param filterChain
     * @throws IOException
     * @throws ServletException
     */
    public abstract void doFilter(HttpServletRequest request, HttpServletResponse response, 
            FilterChain filterChain) throws IOException, ServletException;

    /**
     * 空的 destroy 方法。 
     */

    public void destroy() {}

}

package cache;

import java.io.IOException;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class NoCacheFilter extends HttpFilter{

    @Override
    public void doFilter(HttpServletRequest request,
            HttpServletResponse response, FilterChain filterChain)
            throws IOException, ServletException {
        response.setDateHeader("Expires", -1);
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Pragma", "no-cache");
        filterChain.doFilter(request, response);
            
    }
   }

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>    
    <a href="b.html">TO BBB PAGE</a>
    <br><br>
    <img alt="" src="Hydrangeas.jpg">
</body>
</html>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>    
    <a href="a.html">TO AAA PAGE</a>
    <br><br>
</body>
</html>

配置web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
    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_2_5.xsd">
 <filter>
    <display-name>NoCacheFilter</display-name>
    <filter-name>NoCacheFilter</filter-name>
    <filter-class>cache.NoCacheFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>NoCacheFilter</filter-name>
    <url-pattern>/cache/*</url-pattern>
  </filter-mapping>
</web-app>

2）：编码过滤器

　　要不每次遇到中文编码的时候都会加上

　　<%
      request.setCharacterEncoding("UTF-8");
   %>

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>My JSP 'encodingA.jsp' starting page</title>
  </head>
  <body>
      <form action="encodingB.jsp" method="post">
      name:<input type="text" name="name"/>
      <input type="submit" value="Submit"/>
      </form>
  </body>
</html>

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>My JSP 'encodingA.jsp' starting page</title>
  </head>
  <body>
      Hello: ${param.name }
  </body>
</html>

package com.encoding;

import java.io.IOException;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import cache.HttpFilter;

public class EncodingFilter extends HttpFilter{
    private String encoding;
    @Override
    protected void init() {
        encoding=getFilterConfig().getServletContext().getInitParameter("encoding");
    }
    @Override
    public void doFilter(HttpServletRequest request,
            HttpServletResponse response, FilterChain filterChain)
            throws IOException, ServletException {
        request.setCharacterEncoding(encoding);
        filterChain.doFilter(request, response);
    }

}

配置web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
    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_2_5.xsd">
  <context-param>
      <param-name>encoding</param-name>
      <param-value>UTF-8</param-value>
  </context-param>
  <filter>
      <display-name>EncodingFilter</display-name>
      <filter-name>EncodingFilter</filter-name>
      <filter-class>com.encoding.EncodingFilter</filter-class>
  </filter>
  <filter-mapping>
      <filter-name>EncodingFilter</filter-name>
      <url-pattern>/encoding/*</url-pattern>
  </filter-mapping>
  
</web-app>