首先實現Filter接口(該實例是用於判斷session中是否存在用戶)

package com.xinzhi.filter;

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;

public class LoginFilter implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse res,
            FilterChain chain) 
 
throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        // 1. 獲取請求資源，截取
        String uri = request.getRequestURI(); // /emp_sys/login.jsp
        // 截取 【login.jsp或login】
        String requestPath = uri.substring(uri.lastIndexOf("/") + 1, uri
                .length());

        
 
// 2. 判斷： 先放行一些資源：/login.jsp、/login
        if ("LoginServlet".equals(requestPath)
                || "login.jsp".equals(requestPath)) {
            // 放行
            chain.doFilter(request, response);
        } else {
            // 3. 對其他資源進行攔截
            // 3.1 先獲取Session、獲取session中的登陸用戶(loginInfo)
            HttpSession session = request.getSession(false);
            // 判斷
            if (session != null) {
                Object obj = session.getAttribute("adminname");
                // 3.2如果獲取的內容不為空，說明已經登陸，放行
                if (obj != null) {
                    // 放行
                    uri = requestPath;
                    // chain.doFilter(request, response);
                } else {
                    // 3.3如果獲取的內容為空，說明沒有登陸； 跳轉到登陸
                    uri = "jsp/login.jsp";
                }

            } else {
                // 肯定沒有登陸
                uri = "jsp/login.jsp";
            }
            request.getRequestDispatcher(uri).forward(request, response);
        }
    }

    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub

    }

}

然後要在web.xml配置文件中指定相關信息

<filter>
        <filter-name>filter</filter-name>
        <filter-class>com.xinzhi.filter.LoginFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>filter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

j2ee過濾器實現的主要代碼