代码示例

CharacterEncodingFilter.java

//导包必须要导javax.servlet下的Filter，千万不要导错！！！
import javax.servlet.*;
import java.io.IOException;

/**
 *  1,过滤器中的所有代码，在过滤特定请求的时候都会执行
 *  2,必须要让过滤器继续通行
 *      filterChain.doFilter(servletRequest, servletResponse);
 */
public class CharacterEncodingFilter implements Filter {

    //需要重写三个方法

    //初始化：web服务器启动，就已经初始化了，随时等待过滤对象的出现
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("CharacterEncodingFilter     初始化");
    }

    //和Servlet的doGet和doPost一样的方法
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

        //解决中文乱码
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
//        servletResponse.setHeader("content-type","text/html;charset=UTF-8");
        servletResponse.setContentType("text/html;charset=UTF-8");

        System.out.println("CharacterEncodingFilter     执行前....");
        //filterChain：链
        //只有一个doFilter方法，作用是让请求继续往下走，这个必须要写！！！不写的话，请求就会被过滤器卡住
        filterChain.doFilter(servletRequest, servletResponse);
        System.out.println("CharacterEncodingFilter     执行后....");

    }

    //销毁：服务器关闭的时候才会销毁
    public void destroy() {
        System.out.println("CharacterEncodingFilter     销毁");
    }
}

ShowServlet.java

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class ShowServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().println("你好，世界！");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
				http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">

  <servlet>
    <servlet-name>ShowServlet</servlet-name>
    <servlet-class>com.mahe666.servlet.ShowServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>ShowServlet</servlet-name>
    <!--这个请求经过过滤器-->
    <url-pattern>/servlet/show</url-pattern>
    <!--这个请求没有经过过滤器-->
    <url-pattern>/show</url-pattern>
  </servlet-mapping>


	<!--注册过滤器-->
  <filter>
    <filter-name>CharacterEncodingFilter</filter-name>
    <filter-class>com.mahe666.filter.CharacterEncodingFilter</filter-class>
  </filter>

  <filter-mapping>
    <filter-name>CharacterEncodingFilter</filter-name>
    <!--只要是 /servlet 下的请求，都经过过滤器-->
    <url-pattern>/servlet/*</url-pattern>
    <!--可以在一个 filter-mapping 标签下写多个 url-pattern-->
<!--    <url-pattern></url-pattern>-->
  </filter-mapping>

</web-app>