onlinecountlistener.java

精通java核心技术》随书源代码

// ==================== Program Discription =====================
// 程序名称：示例14-9 : EncodingFilter .java
// 程序目的：学习使用监听器
// ==============================================================
import javax.servlet.http.*;
import javax.servlet.*;

public class OnLineCountListener implements HttpSessionListener,ServletContextListener,ServletContextAttributeListener
{
	private int count;
	private ServletContext context = null;
	
	public OnLineCountListener()
	{
		count=0;
		//setContext();
	}
	//创建一个session时激发
	public void sessionCreated(HttpSessionEvent se) 
	{
		count++;
		setContext(se);
		
	}
	//当一个session失效时激发
	public void sessionDestroyed(HttpSessionEvent se) 
	{
		count--;
		setContext(se);
	}
	//设置context的属性，它将激发attributeReplaced或attributeAdded方法
	public void setContext(HttpSessionEvent se)
	{
		se.getSession().getServletContext().setAttribute("onLine",new Integer(count));
	}
	 //增加一个新的属性时激发
	public void attributeAdded(ServletContextAttributeEvent event) {

	log("attributeAdded('" + event.getName() + "', '" +
	    event.getValue() + "')");

    }
    
   //删除一个新的属性时激发
    public void attributeRemoved(ServletContextAttributeEvent event) {

	log("attributeRemoved('" + event.getName() + "', '" +
	    event.getValue() + "')");

    }

	//属性被替代时激发
    public void attributeReplaced(ServletContextAttributeEvent event) {

		log("attributeReplaced('" + event.getName() + "', '" +
		    event.getValue() + "')");
    }
    ////context删除时激发
     public void contextDestroyed(ServletContextEvent event) {

		log("contextDestroyed()");
		this.context = null;

    }

    //context初始化时激发
    public void contextInitialized(ServletContextEvent event) {

		this.context = event.getServletContext();
		log("contextInitialized()");

    }
    private void log(String message) {

	    System.out.println("ContextListener: " + message);
    }
    
    
}