<form action="output.jsp" name="form" method="post">
    <table>
        <tr>
            <td>name</td>
            <td><input type="text" name="name" /></td>
        </tr>
        <tr>
            <td>age</td>
            <td><input type="text" name="age"/></td>
        </tr>
        <tr>
            <td><input type="submit" name="ok" value="ok"/></td>
        </tr>
    </table>
</form>

2.web.xml

<filter>
    <description></description>
    <display-name>encodefilter</display-name>
    <filter-name>encodefilter</filter-name>
    <filter-class>servletbean.encodefilter</filter-class>
    <init-param>
           <param-name>encoding</param-name>
           <param-value>GB2312</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>encodefilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>
<filter>
    <description></description>
    <display-name>myfilter</display-name>
    <filter-name>myfilter</filter-name>
    <filter-class>servletbean.myfilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>myfilter</filter-name>
    <url-pattern>/output.jsp</url-pattern>
</filter-mapping>

3.encodefilter.java

package servletbean;
public class encodefilter implements Filter {
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        if(request.getCharacterEncoding()==null) {
            System.out.println(encoding);
            request.setCharacterEncoding(encoding);
        }
        chain.doFilter(request, response);//到下一個鏈
    }
    public void init(FilterConfig fConfig) throws ServletException {
        this.config=fConfig;
        encoding=fConfig.getInitParameter("encoding");//獲得目標編碼格式
    }
}

3.myfilter.java

package servletbean;
import javax.swing.JOptionPane;
public class myfilter implements Filter {
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        response.setContentType("text/html");
        response.setCharacterEncoding("GB2312");
        PrintWriter out=response.getWriter();
        String name="";
        String age="";
        int age1;
        name=request.getParameter("name");
        age=request.getParameter("age");
        RequestDispatcher dispatch=request.getRequestDispatcher("erroroutput.jsp");
        if(name==null||name==""||name==" "||age==null) {
            JOptionPane.showMessageDialog(null,"用戶名和年齡輸入錯誤！");
            dispatch.forward(request, response);
            return;
        }
        else{
            try {
                age1=Integer.parseInt(age);
            }catch(Exception e){
                //JOptionPane.showMessageDialog(null,"年齡必須為數(shù)字！");
                dispatch.forward(request,response);
                return;//如果是錯誤頁面就到erroroutput.jsp中
            }
        }
        //這里表示是正確的，也就是說，他回去找下一個鏈，但是它下面已經(jīng)沒有了，所以就會去跳轉(zhuǎn)頁面了，此跳轉(zhuǎn)的頁面就是action="output.jsp"了
        chain.doFilter(request, response);
        }
}

Filter中chain.doFilter(req, res)的理解

在寫代碼中，每次看到Filter(過濾器)中 chain.doFilter(req, res)；都不懂為什么要加這一句，他的作用是什么；

代碼下面也有：

@Override
 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
  ContentCachingRequestWrapper  req=new ContentCachingRequestWrapper((HttpServletRequest) request);
  ContentCachingResponseWrapper res=new ContentCachingResponseWrapper((HttpServletResponse) response);
  long startTime = System.currentTimeMillis();
  String serverUrl = req.getServletPath();
  //不懂這句什么意思  
  chain.doFilter(req, res);  
  long endTime = System.currentTimeMillis();
  try {
   int status = res.getStatus();
   String charset=res.getCharacterEncoding();
   //響應(yīng)體
   String responsePayload = getPayLoad(res.getContentAsByteArray(),res.getCharacterEncoding());
   res.copyBodyToResponse();
   if(writeLogService==null) {
    writeLogService = (WriteLogService) ServiceLocator.getInstance().getCtx().getBean("writeLogServiceImpl");
   } 
 writeLogService.writeLog(startTime,endTime,serverUrl,postparams,status,responsePayload,charset);
  } catch (Exception e) {
   e.printStackTrace();
  }
 }