1、Servlet过滤器的作用描述
（1）在HttpServletRequest到达Servlet 之前，拦截客户的HttpServletRequest。
根据需要检查HttpServletRequest，也可以修改HttpServletRequest头和数据。
（2）在HttpServletResponse 到达客户端之前，拦截HttpServletResponse。
根据需要检查HttpServletResponse，可以修改HttpServletResponse头和数据。
2、应用Servlet过滤器进行身份验证
假设网站根目录下的login1.htm、longin1.jsp用于用户登录，而chap08目录下的文件需要用户登录后才能访问。
（1）编写Servlet过滤器
@WebFilter("/FilterStation")
public class FilterStation extends HttpServlet implements Filter {
PRivate FilterConfig filterConfig;
public FilterStation() {
super();
}
public void destroy() {
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
Httpsessionsession=((HttpServletRequest)request).getSession();
response.setCharacterEncoding("gb2312");
if(session.getAttribute("me")==null){
PrintWriter out=response.getWriter();
out.print("<script>alert('请登录！');location.href='../login1.htm'</script>");
}
else{
// pass the request along the filter chain
chain.doFilter(request, response);
}
}
public void init(FilterConfig fConfig) throws ServletException {
// TODO Auto-generated method stub
this.filterConfig=fConfig;
}
}
（2）配置web.xml
<filter>
<filter-name>filterstation</filter-name>
<filter-class>zhou.FilterStation</filter-class>
</filter>
<filter-mapping>
<filter-name>filterstation</filter-name>
<url-pattern>/chap08/*</url-pattern>
</filter-mapping>
（3）login1.htm代码
<html>
<head>
<title>用户登录</title>
</head>
<body>
<form method="POST" action="login1.jsp">
<p>用户名：<input type="text" name="user" size="18"></p>
<p>密码：<input type="text" name="pass" size="20"></p>
<p><input type="submit" value="提交" name="ok">
<input type="reset" value="重置" name="cancel"></p>
</form>
</body>
</html>
（4）login1.jsp代码
<%@ page contentType="text/html;charset=GB2312" %>
<html>
<head><title>Session 应用演示</title></head>
<%
if (request.getParameter("user")!=null && request.getParameter("pass")!=null)
{
String strName=request.getParameter("user");
String strPass=request.getParameter("pass");
if (strName.equals("admin") && strPass.equals("admin"))
{
session.setAttribute("login","OK");
session.setAttribute("me",strName);
response.sendRedirect("chap08/welcome.jsp");
}
else
{
out.print("<script>alert('用户名或密码错误');location.href='login1.htm'</script>");
}
}
%>
</html>