import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.RequestDispatcher;
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;
/**
* 过滤器使用：用户没用登录，不可以在浏览器输入地址访问页面
* @author Administrator
*
*/
public class OnlineFilter extends HttpServlet implements Filter {
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
RequestDispatcher dispatcher = request.getRequestDispatcher("Login.jsp");
HttpServletRequest req =(HttpServletRequest)request;
HttpServletResponse res =(HttpServletResponse)response;
HttpSession session =req.getSession(true);
//从session 里面获取用户名的信息
String user =(String)session.getAttribute("user");
//判断如果没有取到用户信息，就跳转到登陆页面，提示用户进行登陆
if(user == null || "".equals(user)){
//跳转到登陆的页面，进行用户登录
dispatcher.forward(request,response);
System.out.println("用户没有登录，请登陆！");
}else{
System.out.println("用户已经登陆成功，允许继续操作！");
}
chain.doFilter(request, response);
}
public void init(FilterConfig arg0) throws ServletException {
}
/**
* Destruction of the servlet. <br>
*/
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
}
public void init() throws ServletException {
}
}
Web.xml配置
<!-- 过滤器的 过滤用户登陆的session对象 -->
<filter>
<filter-name>sessionFilter</filter-name>
<filter-class>biz.sdna.cbrc.util.OnlineFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>sessionFilter</filter-name>
<url-pattern>/jsp/*</url-pattern>
</filter-mapping>