<%

Object user = session.getAttribute("user");

// 如果等于 null，说明还没有登录

if (user == null) {
request.getRequestDispatcher("/login.jsp").forward(request,response);

return;
}

%>

public class AdminFilter implements Filter {

/**
* doFilter 方法，专门用于拦截请求。可以做权限检查

*/

@Override

public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain
filterChain) throws IOException, ServletException {
HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
HttpSession session = httpServletRequest.getSession();
Object user = session.getAttribute("user");

// 如果等于 null，说明还没有登录

if (user == null) {
servletRequest.getRequestDispatcher("/login.jsp").forward(servletRequest,servletResponse);

return;
} else {

// 让程序继续往下访问用户的目标资源

filterChain.doFilter(servletRequest,servletResponse);
}
}
}

<!--filter 标签用于配置一个 Filter 过滤器-->

<filter>

<!--给 filter 起一个别名-->

<filter-name>AdminFilter</filter-name>

<!--配置 filter 的全类名-->

<filter-class>com.atguigu.filter.AdminFilter</filter-class>
</filter>
<!--filter-mapping 配置 Filter 过滤器的拦截路径-->

<filter-mapping>

<!--filter-name 表示当前的拦截路径给哪个 filter 使用-->

<filter-name>AdminFilter</filter-name>

<!--url-pattern 配置拦截路径

/ 表示请求地址为：http://ip:port/工程路径/ 映射到 IDEA 的 web 目录

/admin/* 表示请求地址为：http://ip:port/工程路径/admin/*
-->

<url-pattern>/admin/*</url-pattern>
</filter-mapping>

这是登录页面。login.jsp 页面 <br>
<form action="http://localhost:8080/15_filter/loginServlet" method="get">

用户名：<input type="text" name="username"/> <br>

密 码：<input type="password" name="password"/> <br>
<input type="submit" />
</form>

public class LoginServlet extends HttpServlet {

@Override

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException,
IOException {
resp.setContentType("text/html; charset=UTF-8");
String username = req.getParameter("username");
String password = req.getParameter("password");

if ("wzg168".equals(username) && "123456".equals(password)) {
req.getSession().setAttribute("user",username);
resp.getWriter().write("登录 成功！！！");
} else {
req.getRequestDispatcher("/login.jsp").forward(req,resp);
}
}
}

@Override

public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("2.Filter 的 init(FilterConfig filterConfig)初始化");

// 1、获取 Filter 的名称 filter-name 的内容

System.out.println("filter-name 的值是：" + filterConfig.getFilterName());

// 2、获取在 web.xml 中配置的 init-param 初始化参数

System.out.println("初始化参数 username 的值是：" + filterConfig.getInitParameter("username"));
System.out.println("初始化参数 url 的值是：" + filterConfig.getInitParameter("url"));

// 3、获取 ServletContext 对象

System.out.println(filterConfig.getServletContext());
}

<!--filter 标签用于配置一个 Filter 过滤器-->

<filter>

<!--给 filter 起一个别名-->

<filter-name>AdminFilter</filter-name>

<!--配置 filter 的全类名-->
<filter-class>com.atguigu.filter.AdminFilter</filter-class>
<init-param>
<param-name>username</param-name>
<param-value>root</param-value>
</init-param>
<init-param>
<param-name>url</param-name>
<param-value>jdbc:mysql://localhost3306/test</param-value>
</init-param>
</filter>