JavaWeb-11-監聽器

監聽器的接口有很多,比如,HttpSessionListener…

以統計線上人數為例(監聽session的接口)

• 實作監聽接口

  

• 重寫接口中的方法

  //統計線上人數
  public class OnlineCount implements HttpSessionListener {
      public void sessionCreated(HttpSessionEvent se) {
          ServletContext ctx = se.getSession().getServletContext();
          System.out.println(se.getSession().getId());
          Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
          if (onlineCount==null){
              onlineCount=new Integer(1);
          }else {
              int count = onlineCount.intValue();
              onlineCount = new Integer(count+1);
          }
          ctx.setAttribute("OnlineCount",onlineCount);
  
      }
  
      public void sessionDestroyed(HttpSessionEvent se) {
          ServletContext ctx = se.getSession().getServletContext();
          Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
          if (onlineCount==null){
              onlineCount=new Integer(0);
          }else {
              int count = onlineCount.intValue();
              onlineCount = new Integer(count-1);
          }
          ctx.setAttribute("OnlineCount",onlineCount);
      }
  }
             

• 在web.xml注冊監聽器

  <!--    注冊監聽器-->
      <listener>
          <listener-class>com.cmy.listener.OnlineCount</listener-class>
      </listener>
             

監聽器的應用

GUI

import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;


public class TestPanel {
    public static void main(String[] args) {
        Frame frame = new Frame("hello frame");//建構一個窗體
        Panel panel = new Panel(null);//面闆
        frame.setLayout(null);//設定窗體布局

        frame.setBounds(500,400,500,500);
        frame.setBackground(Color.blue);
        panel.setBounds(100,100,300,300);
        panel.setBackground(Color.ORANGE);

        frame.add(panel);

        frame.setVisible(true);
        frame.addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                System.out.println("closing");
                System.exit(0);
            }

            @Override
            public void windowActivated(WindowEvent e) {
                System.out.println("active");
            }
        });
        System.out.println("hello");
    }
}