編寫過濾的步驟
1、編寫一個類，實現javax.servlet.Filter介面，這樣的類一般稱之為過濾器類
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("FilterDemo1攔截了");
chain.doFilter(request, response);//放行
}
2、在web.xml中進行配置，要攔截哪些資源。
<filter>
<filter-name>FilterDemo1</filter-name>
<filter-class>cn.itcast.filter.FilterDemo1</filter-class>
 </filter>
 <filter-mapping>
<filter-name>FilterDemo1</filter-name>
<url-pattern>/*</url-pattern>
 </filter-mapping>
 
過濾器的執行過程
多個過濾器的攔截順序是按照web.xml中filter-mapping元素的出現順序進行攔截的。

過濾器只會被初始化一次，應用被載入時就完成了初始化。

過濾器的高階配置
<filter-mapping>
<filter-name>FilterDemo3</filter-name>
<url-pattern>/*</url-pattern>
<dispatcher>REQUEST</dispatcher> 預設情況。如果寫了另外一種情況，預設的就沒有了
<dispatcher>FORWARD</dispatcher>
<dispatcher>INCLUDE</dispatcher>
<dispatcher>ERROR</dispatcher>

 </filter-mapping>

下面是在servlet中的應用的一個小例子：

package deng;


import java.io.*;   
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
import hu.ZcBean;    
public class ZcSevlet extends HttpServlet implements Filter{
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
System.out.println("攔截了");  
chain.doFilter(request,response);
System.out.println("放行了");
  
}  
public void init(ServletConfig config) throws ServletException {
super.init(config);
}
public void doPost(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException {
ZcBean zcbean = null;
try {  
zcbean = (ZcBean)request.getAttribute("ZcBean");
if(zcbean == null) {
zcbean = new ZcBean();//建立ZcBean的一個物件
request.setAttribute("zcbean", zcbean);
}   
}  catch(Exception e) {
zcbean = new ZcBean();
request.setAttribute("zcbean", zcbean);
  }
try { 
Class.forName("com.mysql.jdbc.Driver");//載入資料庫驅動
} catch(Exception e) { } 
String dataBase = request.getParameter("dataBase");//拿到我們要訪問的資料庫的名字跟表的名字
String tableName = request.getParameter("tableName");
String name = request.getParameter("yhm");//拿到註冊的使用者名稱跟密碼
String word = request.getParameter("mm");
if((name == null) || (word == null)) {
return ;
}         
double n = Double.parseDouble(name);//將拿到的（文字）字元型使用者名稱跟密碼轉成double型的，便於錄入資料庫中
double m = Double.parseDouble(word);
Connection con;
Statement sql; 
ResultSet rs;  
try {  
  String uri = "jdbc:mysql://127.0.0.1/" + dataBase;
  con = DriverManager.getConnection(uri,"root","961102");
  sql = con.createStatement();
  //設定使用者名稱只能是自己的學號  這樣才可以進行註冊  
  rs = sql.executeQuery("select xh from class where xh =" + n);
  rs.next();
  while(rs.next()) {
  if(rs.getString(1).equals(name)) {
  String a = "update" + "\t" +  tableName + "\t" + "set yhm = " + n +"\t" + "where xh =" + n;
  String b = "update" + "\t" +  tableName + "\t" + "set mm = " + m + "\t" + "where xh =" + n ;
  sql.executeUpdate(a);
  sql.executeUpdate(b); 
  con.close();
  //註冊成功，將註冊資訊傳給javabean  然後在另一個jsp頁面中進行顯示
  zcbean.setBackNews("恭喜你，註冊成功！！！");
  zcbean.setUsername(name);  
  zcbean.setPassword(word);    
  RequestDispatcher dispatcher = request.getRequestDispatcher("zcrecord.jsp");
  dispatcher.forward(request, response);  
   }
  } 
  zcbean.setBackNews("註冊失敗！");
  zcbean.setUsername("使用者名稱有誤！");  
  zcbean.setPassword(word);          
  RequestDispatcher dispatcher = request.getRequestDispatcher("zcrecord.jsp");
  dispatcher.forward(request, response);  
} catch(SQLException e) {
System.out.println(e);
}
}
public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
doPost(request,response);
}
public void init(FilterConfig arg0) throws ServletException {
// TODO Auto-generated method stub

}
}