1、简单介绍一下监听器
所谓监听器就是监听事件，然后针对相应的事件进行处理
监听器举例：ServletcontextListener、HttpSessionListener、HttpSessionAttributeListener
2、设计一个统计在线人数的实例
import java.util.LinkedList;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class OnlineListener implements HttpSessionAttributeListener,
ServletContextListener, HttpSessionListener {
private ServletContext application=null;
@Override
public void sessionCreated(HttpSessionEvent arg0) {
// TODO Auto-generated method stub
}
@Override
public void sessionDestroyed(HttpSessionEvent arg0) {
// TODO Auto-generated method stub
HttpSession hs=arg0.getSession();
String user=(String) hs.getAttribute("username");
LinkedList<String> online = (LinkedList<String>) application.getAttribute("online");
online.remove(user);
application.setAttribute("online", online);
}
@Override
public void contextDestroyed(ServletContextEvent arg0) {
// TODO Auto-generated method stub
}
@Override
public void contextInitialized(ServletContextEvent arg0) {
// TODO Auto-generated method stub
application=arg0.getServletContext();
application.setAttribute("online", new LinkedList<String>());
}
@Override
public void attributeAdded(HttpSessionBindingEvent arg0) {
// TODO Auto-generated method stub
HttpSession hs=arg0.getSession();
LinkedList<String> online = (LinkedList<String>) application.getAttribute("online");
String name=arg0.getName();
if(name.equals("username")){
online.add((String)arg0.getValue());
}
application.setAttribute("online", online);
}
@Override
public void attributeRemoved(HttpSessionBindingEvent arg0) {
// TODO Auto-generated method stub
}
@Override
public void attributeReplaced(HttpSessionBindingEvent arg0) {
// TODO Auto-generated method stub
}
}
import java.io.IOException;
import java.io.PrintWriter;
import java.util.LinkedList;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class LoginServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doGet(req, resp);
doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doPost(req, resp);
PrintWriter out=resp.getWriter();
String user=(String) req.getParameter("username");
if(user!=null && !user.equals("")){
req.getSession().setAttribute("username", user);
}
LinkedList<String> online= (LinkedList<String>) getServletContext().getAttribute("online");
resp.setContentType("text/html;charset=utf-8");
out.println("<html>");
out.println("<head><title>Login</title></head>");
out.println("<body>");
out.println("<b><font size=\"+4\">");
out.println("<font color=\"#FF0000\">The curent user's list is:<br/>");
for(int i=0;i<online.size();i++){
out.println(i+1+"."+online.get(i)+"<br/>");
}
out.println("<hr/>");
out.println("<a href=\""+resp.encodeURL("logout")+"\">Logout</a>");
out.println("</b></font>");
out.println("</body>");
out.println("</html>");
out.flush();
out.close();
}
}
import java.io.IOException;
import java.io.PrintWriter;
import java.util.LinkedList;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class LogoutServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doGet(req, resp);
doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doPost(req, resp);
req.getSession().invalidate();
LinkedList<String> online=(LinkedList<String>) getServletContext().getAttribute("online");
resp.setContentType("text/html;charset=utf-8");
PrintWriter out=resp.getWriter();
out.println("<html>");
out.println("<head><title>Log out</title></head>");
out.println("<body>");
out.println("<b><font size=\"+4\">");
out.println("<font color=\"#FF0000\">The curent user's list is:<br/>");
for(int i=0;i<online.size();i++){
out.println(1+i+"."+online.get(i)+"<br/>");
}
out.println("<hr/><a href=\"index.html\">home page</a>");
out.println("</font></body>");
out.println("</html>");
}
}
<listener>
<listener-class>OnlineListener</listener-class>
</listener>
<servlet>
<servlet-name>login</servlet-name>
<servlet-class>LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>login</servlet-name>
<url-pattern>/login</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>logout</servlet-name>
<servlet-class>LogoutServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>logout</servlet-name>
<url-pattern>/logout</url-pattern>
</servlet-mapping>
<html> <head><title>index</title></head> <body> <form action="login" method="post"> username:<input type="text" name="username"/> <input type="submit" value="login"/><br/><br/> </form> </body> </html>
时间： 2024-10-27 12:27:16