字符编码的过滤器Filter（即输入的汉字，能在页面上正常显示，不会出现乱码）

自定义抽象的 HttpFilter类, 实现自 Filter 接口

package com.lanqiao.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;
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
     
    @Override
    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
     */
    @Override
    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. 
    
    public abstract void doFilter(HttpServletRequest request, HttpServletResponse response, 
            FilterChain filterChain) throws IOException, ServletException;

    //空的 destroy 方法。 
    @Override
    public void destroy() {}

}

建立一个Filter类，其继承于父类HttpFilter

package com.atguigu.javaweb.encoding;

import java.io.IOException;

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

import com.atguigu.javaweb.HttpFilter;

public class EncodingFilter extends HttpFilter{
　　//配置参数
    private String encoding;
    
    @Override
　　//获取web.jsp文件里配置和映射里的encoding属性
    protected void init() {
        encoding = getFilterConfig().getServletContext().getInitParameter("encoding");
    }
    
    @Override
    public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain filterChain) throws IOException, ServletException {
        //System.out.println(encoding); 
        request.setCharacterEncoding(encoding);
        filterChain.doFilter(request, response);
    }

}

WEB-INF下的web.xml文件的配置：

<?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" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
 
  <filter>
      <filter-name>encodingFilter</filter-name>
      <filter-class>com.lanqiao.javatest.encoding.EncodingFilter</filter-class>
  </filter>
  
  <filter-mapping>
      <filter-name>encodingFilter</filter-name>
      <url-pattern>/encoding/*</url-pattern>
  </filter-mapping>

　　<context-param>
　　　　<param-name>encoding</param-name>
　　　　<param-value>UTF-8</param-value>
　　</context-param>


</web-app>

在WebContent下的encoding文件里的login.jsp和index.jsp页面：

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!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>
    
    <form action="login.jsp" method="post">
        username:<input type="text" name="username"/><br><br>
        <input type="submit" name="tt" />
    
    </form>
</body>
</html>

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!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>
    hello:${param.username }
</body>
</html>

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原文地址：https://www.cnblogs.com/lxnlxn/p/5834025.html