监听器详解与实例演练

   创建监听器时，只要创建一个实现监听器接口的java类即可

监听器接口有ServletContextListener,HttpSessionListener,HttpSessionBindingListener,ServletRequestListener等接口，下面那进行详细解读

在Servlet3.0中，有两种方法创建

@webListener

public class ListenerClass implements ListenerInterface（）{

}

第二种方法时在web.xml部署描述符中使用一个listener元素

</listener>

<listener-class>listener class name</listener-class>

</listener>

一,Servlet Context监听器

ServletContext有两个监听器：ServletContextListener和ServletContextAttributeListener

1ServletContextListener会对ServletContext的初始化和解构做出响应，下面通过一个实例来演示，

一旦ServletContext被初始化，它会将属性map保存在ServletContext属性中

import java.util.HashMap;
import java.util.Map;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class AppListener implements ServletContextListener {

	@Override
	public void contextDestroyed(ServletContextEvent sce) {
		// TODO Auto-generated method stub

	}

	@Override
	public void contextInitialized(ServletContextEvent sce) {
		// TODO Auto-generated method stub
		ServletContext servletContext=sce.getServletContext();
		
		Map<String, String> countries=new HashMap<String, String>();
		countries.put("ca", "Canada");
		countries.put("cn", "China");
		servletContext.setAttribute("countries", countries);

	}

}

2，ServletContextAttributeListener

   每当ServletContext中添加，删除，替换了某个属性，ServletContextAttributeListener分别

会运行  void attributeAdded(ServletContextAttributeEvent event)

             void attributeRemoved(ServletContextAttributeEvent event)

             void attributeReplaced(ServletContextAttributeEvent event)方法

ServletContextAttributeEvent类派生与ServletContetAttribute，并添加了下面两个方法

java.lang.String getName()

java.lang.Object getValue()

二，Session监听器

1，HttpSessionListener

当有httpSession被创建或者销毁，Servlet容器就会调用所有已注册的HttpSessionListener

下面通过一个十分实用的例子来演示，用了一个AtomicInteger作为计数器，并保存为ServletContext属性

每当创建HttpSession时，这个计数器就会递增，每当销毁HttpSession时，计数器就会递减，可以用于显示在线人数

import java.util.concurrent.atomic.AtomicInteger;
import javax.print.attribute.standard.Severity;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
@WebListener
public class SessionListener implements HttpSessionListener,
		ServletContextListener {
	int userCount;

	@Override
	public void contextDestroyed(ServletContextEvent arg0) {
		// TODO Auto-generated method stub

	}

	@Override
	public void contextInitialized(ServletContextEvent arg0) {
		// TODO Auto-generated method stub
		ServletContext servletContext=arg0.getServletContext();
		servletContext.setAttribute("userCounter", new AtomicInteger());

	}

	@Override
	public void sessionCreated(HttpSessionEvent arg0) {
		// TODO Auto-generated method stub
		HttpSession session=arg0.getSession();
		ServletContext servletContext=session.getServletContext();
		AtomicInteger userCounter= (AtomicInteger) servletContext.
				getAttribute("userCounter");
		int userCount=userCounter.incrementAndGet();
		System.out.println("userCount incremented to :"+userCount);

	}

	@Override
	public void sessionDestroyed(HttpSessionEvent arg0) {
		// TODO Auto-generated method stub
		HttpSession session=arg0.getSession();
		ServletContext servletContext=session.getServletContext();
		AtomicInteger userCounter= (AtomicInteger) servletContext.
				getAttribute("userCounter");
		int userCount=userCounter.decrementAndGet();
		System.out.println("userCount incremented to :"+userCount);

	}

}

2，HttpSessionBindingListener

当HttpSessionBindingListener绑定到HttpSession，或者解除绑定时，都会收到通知

3，HttpSessionAttributeListener 

当HttpSession中有添加，删除，替换属性的时候它才会被调用，如ServletContextAttributeListener中一样

4，HttpSessionActivationListener

在分布式中，多个Servlet容器会被配置成可伸缩的，为了节省内存，Servlet容器可以对session属性进行迁移或者序列化。

一般来说，当内存比较时，相对较少访问的对象可以序列化到备用设备中，这个接口定义了两个方法

 

void sessionDidActive(HttpSessionEvent event)

void sessionWillPassivate(HttpSessionEvent event)

三，ServletRequest监听器

ServletRequestListener对ServletRequest的创建和销毁做出响应

下面通过一个实例，来计算完成一个HTTP请求要花费的时间。通过比较运行requestInitiallized 和requestDestroyed

方法中间所隔的时间差，得知完成一个HTTP所花的时间

import javax.servlet.ServletRequest;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpServletRequest;
@WebListener
public class PerfStatListener implements ServletRequestListener {

	@Override
	public void requestDestroyed(ServletRequestEvent arg0) {
		// TODO Auto-generated method stub
		ServletRequest servletRequest=arg0.getServletRequest();
		long start=(Long) servletRequest.getAttribute("start");

//nanoTime方法返回一个表示任意时间的long，在同一台JVM得到两个返回值可以用于所间隔的时间差
		long end=System.nanoTime();
		HttpServletRequest httpServletRequest=(HttpServletRequest)
				arg0.getServletRequest();
		String url=httpServletRequest.getRequestURI();
		System.out.println("time taken to execute"+url+":"+
		(end-start)/1000+" microseconds");

	}

	@Override
	public void requestInitialized(ServletRequestEvent arg0) {
		// TODO Auto-generated method stub
		ServletRequest servletRequest=arg0.getServletRequest();
		servletRequest.setAttribute("start", System.nanoTime());

	}

}