filter过滤

web.xml
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>ShowImgServlet</servlet-name>
    <servlet-class>rcom.rf.common.ShowImgServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ShowImgServlet</servlet-name>
    <url-pattern>/servlet/ShowImgServlet</url-pattern>
  </servlet-mapping>
  <filter>
    <filter-name>checksessionfilter</filter-name>
    <filter-class>rcom.rf.common.CheckSessionFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>checksessionfilter</filter-name>
    <url-pattern>/admin/*</url-pattern>
  </filter-mapping>

CheckSessionFilter.java
  package rcom.rf.common;

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.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class CheckSessionFilter extends HttpServlet implements Filter {

 private FilterConfig filterConfig;
   //Handle the passed-in FilterConfig
   public void init(FilterConfig filterConfig)
       throws ServletException
   {
     this.filterConfig = filterConfig;
   }

   //校验session
   public void doFilter(ServletRequest request, ServletResponse response,
          FilterChain filterChain)
   {
     HttpSession session = ((HttpServletRequest)request).getSession();
     try {
   if(session.getAttribute("username")==null){
    //System.out.println("session is null!没有登陆···");/////////////////
    ((HttpServletResponse)response).sendRedirect("/rfnet/index.jsp");
   }
   filterChain.doFilter(request, response);
  } catch (IOException e) {
   System.out.println("CheckSessionFilter ERR IOException: "+e.getMessage());///////////
    filterConfig.getServletContext().log(e.getMessage());
  } catch (ServletException e) {
   System.out.println("CheckSessionFilter ERR ServletException: "+e.getMessage());///////////////////////////////
    filterConfig.getServletContext().log(e.getMessage());
  }
   }
  

   //Clean up resources
   public void destroy()
   {
   }

}
ShowImgServlet.java
  package rcom.rf.common;

import java.io.BufferedOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.sql.Blob;
import java.sql.SQLException;

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

import rcom.rf.dao.ImgNewsDAO;

public class ShowImgServlet extends HttpServlet {
 private static final String CONTENT_TYPE = "text/html; charset=GBK";
 /**
  * Constructor of the object.
  */
 public ShowImgServlet() {
  super();
 }

 /**
  * Destruction of the servlet. <br>
  */
 public void destroy() {
  super.destroy(); // Just puts "destroy" string in log
  // Put your code here
 }

 /**
  * The doGet method of the servlet. <br>
  *
  * This method is called when a form has its tag value method equals to get.
  *
  * @param request the request send by the client to the server
  * @param response the response send by the server to the client
  * @throws ServletException if an error occurred
  * @throws IOException if an error occurred
  */
 //显示图片
 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  response.setContentType(CONTENT_TYPE);
  InputStream in = null;
  String id = request.getParameter("id");
  byte[] buff = new byte[10240];
  Blob blob = null;
  ImgNewsDAO imgDAO = null;
  int length = 0 ;
  try {
   BufferedOutputStream buffout = new BufferedOutputStream(response.getOutputStream());
   if(id != null && !id.equals("")){
    imgDAO = new ImgNewsDAO();
    blob = imgDAO.getImg(new Long(id));
    in = blob.getBinaryStream();
   }
   while((length = in.read(buff))!=-1){
    buffout.write(buff,0,length);
   }
   in.close();
   buffout.close();
  } catch (NumberFormatException e) {
   e.printStackTrace();
  } catch (IOException e) {
   e.printStackTrace();
  } catch (SQLException e) {
   e.printStackTrace();
  }
 }

 /**
  * The doPost method of the servlet. <br>
  *
  * This method is called when a form has its tag value method equals to post.
  *
  * @param request the request send by the client to the server
  * @param response the response send by the server to the client
  * @throws ServletException if an error occurred
  * @throws IOException if an error occurred
  */
 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
 }

 /**
  * Initialization of the servlet. <br>
  *
  * @throws ServletException if an error occure
  */
 public void init() throws ServletException {
  // Put your code here
 }

}
http://www.javaworld.com.tw/confluence/pages/viewpage.action?pageId=2727