一个简单的Filter列子

filter代码：

package servlet.filterpackage;

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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
/*
 * this is the first program we test the filter use.
 * maybe we can learn more information about this.
 * @author:*****
 * @date:**** 
 * */
public class ServletFilter implements Filter{
	
	private FilterConfig  config=null;

	public void destroy() {
		
		System.out.println("========login filter destroy==============");
		
	}

	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		
		HttpServletRequest req=(HttpServletRequest)request; 
		HttpServletResponse resp=(HttpServletResponse)response;
		HttpSession session=req.getSession(true);
		Object login=session.getAttribute("login");
		
		if(login==null){
			req.setAttribute("error Message","please login");
			req.getRequestDispatcher("/error").forward(req, resp);
		}else{
			chain.doFilter(req, resp);
		}
		
		
	}

	public void init(FilterConfig config) throws ServletException {
	    System.out.println("=========login server filter beginning=======");
	    this.config=config;
		
	}
	
}

在配置文件中加入如下代码：

 <filter>
    <filter-name>loginFilter</filter-name>
    <filter-class>servlet.filterpackage.ServletFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>loginFilter</filter-name>
    <url-pattern>/protected/*</url-pattern>
  </filter-mapping>

登陆成功后转向的页面

package servlet.filterpackage;

import java.io.IOException;
import java.io.PrintWriter;

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

public class SucceedLogin extends HttpServlet{

	@Override
	protected void service(HttpServletRequest req, HttpServletResponse resp)
		throws ServletException, IOException {
		
		resp.setContentType("text/html");
		PrintWriter out=resp.getWriter();
		
    	  out.println("login succeed");
    	  
	}

}

判断是否成功的登陆：

package servlet.filterpackage;

import java.io.IOException;

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

public class LoginJudge extends HttpServlet{

	@Override
	protected void service(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
   
		HttpSession session=request.getSession(true);
		String name=request.getParameter("name");
		String pwd=request.getParameter("pwd");
		System.out.println(name.equals("kkk"));
		System.out.println(pwd.equals("123")); 
		if(name.equals("kkk")&&pwd.equals("123")){
			session.setAttribute("login", "true");
			response.sendRedirect("/JSP/protected/succeed");
		}
		else{
			session.setAttribute("error", "failure");
			request.getRequestDispatcher("/error").forward(request, response);
		}
		
	}
	
	

}