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Java filter中的chain.doFilter使用详解

2024-04-02 19:55

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filter中的chain.doFilter使用

chain.doFilter作用

1.一般filter都是一个链,web.xml 里面配置了几个就有几个。一个一个的连在一起

request -> filter1 -> filter2 ->filter3 -> …. -> request resource.

2.chain.doFilter将请求转发给过滤器链下一个filter , 如果没有filter那就是你请求的资源

举例说明

1.input.jsp 是用来提交输入的 : 当提交后，过滤器检测姓名和年龄 .

2.如果正常的话会提交给output.jsp 如果不正常提交给erroroutput.jsp.

3.在此 , 同时也有一个过滤器，防止乱码问题存在 , 该过滤器检测一个页面是否设置了字符编码，如果没有则进行设置。

1.input.jsp


<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);//到下一个链
    }
    public void init(FilterConfig fConfig) throws ServletException {
        this.config=fConfig;
        encoding=fConfig.getInitParameter("encoding");//获得目标编码格式
    }
}

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,"用户名和年龄输入错误！");
            dispatch.forward(request, response);
            return;
        }
        else{
            try {
                age1=Integer.parseInt(age);
            }catch(Exception e){
                //JOptionPane.showMessageDialog(null,"年龄必须为数字！");
                dispatch.forward(request,response);
                return;//如果是错误页面就到erroroutput.jsp中
            }
        }
        //这里表示是正确的，也就是说，他回去找下一个链，但是它下面已经没有了，所以就会去跳转页面了，此跳转的页面就是action="output.jsp"了
        chain.doFilter(request, response);
        }
}

Filter中chain.doFilter(req, res)的理解

在写代码中，每次看到Filter(过滤器)中 chain.doFilter(req, res)；都不懂为什么要加这一句，他的作用是什么；

代码下面也有：


@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();
  //不懂这句什么意思  
  chain.doFilter(req, res);  
  long endTime = System.currentTimeMillis();
  try {
   int status = res.getStatus();
   String charset=res.getCharacterEncoding();
   //响应体
   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();
  }
 }