Listener listener

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Monitor Listener

summary

Listener Monitor it's JavaWeb One of the three components of ,JavaWeb The three major components of are ：Servlet Program 、Listener Monitor 、Filter filter

The function of the monitor ： Monitor a change ( Generally, it is object creation / The destruction , Property changes ), Trigger the corresponding method to complete the corresponding task

JavaWeb There are eight listeners in the , At present, the most commonly used is ServletContextListener

ServletContextListener

effect ： monitor ServletContext Create or destroy ( When we Web When app starts , Will be created ServletContext） Life cycle monitoring

Application scenarios

• Load the initialized configuration file such as spring Configuration file for
• Task scheduling （ Cooperation timer Timer/TimerTask)

Method

void contextInitialized(ServletContextEvent sce)  //  establish  Servletcontext  Trigger when 
void contextDestroyed(ServletContextEvent sce) //  The destruction  Servletcontext  Trigger when 

Code example

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0">
    <listener>
        <listener-class>com.al_tair.listener.HttpServletContextListener</listener-class>
    </listener>
</web-app>

public class HttpServletContextListener implements ServletContextListener {
    
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
    
        System.out.println("ServletContext  Be created ");
    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
    
        System.out.println("ServletContext  Be destroyed ");
    }
}

ServletContextAttributeListener

effect ： monitor ServletContext Property changes

Method

void attributeAdded(ServletContextAttributeEvent event) //  Add attribute 
void attributeReplaced(ServletContextAttributeEvent event) //  Replace attributes  
void attributeRemoved(ServletContextAttributeEvent event)  //  Remove properties 

Sample code

// Servlet
@WebServlet(urlPatterns = {
    "/listener"})
public class ServletListenerDemo extends HttpServlet {
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        System.out.println("doGet Called ");
        ServletContext servletContext = req.getServletContext();
        servletContext.setAttribute("name"," Niansheng ");
        servletContext.setAttribute("name"," Zhang Luorong ");
        servletContext.removeAttribute("name");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        doGet(req, resp);
    }
}
// listener
public class HttpServletContextAttributeListener implements ServletContextAttributeListener {
    
    @Override
    public void attributeAdded(ServletContextAttributeEvent servletContextAttributeEvent) {
    
        Object name = servletContextAttributeEvent.getServletContext().getAttribute("name");
        if(name != null)
            System.out.println(" Add attribute " + name.toString());
        else
            System.out.println(" Add attribute ");
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent servletContextAttributeEvent) {
    
        System.out.println(" Remove properties ...");
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent servletContextAttributeEvent) {
    
        System.out.println(" Replace attributes ...");
    }
}

HttpSessionListener

effect ： monitor Session Create or destroy , Life cycle monitoring

Method

void sessionCreated(HttpSessionEvent se)  //  establish  session  Called when the  
void sessionDestroyed(HttpSessionEvent se)  //  The destruction  session  Called when the 

HttpSessionAttributeListener

effect ： monitor Session Changes in attributes

Method

void attributeAdded(ServletRequestAttributeEvent srae) //  Add attribute 
void attributeReplaced(ServletRequestAttributeEvent srae) //  Replace attributes 
void attributeRemoved(ServletRequestAttributeEvent srae)  //  Remove properties 

ServletRequestListener

effect ： monitor Request Create or destroy , namely Request Life cycle monitoring , Can be used to monitor , Some IP Frequency of visiting our website , logging , Access to resources

Method

void requestInitialized(ServletRequestEvent sre) //  establish  request 
void requestDestroyed(ServletRequestEvent sre) //  The destruction  request 

ServletRequestAttributeListener

effect ： monitor Request Property changes

Method

void attributeAdded(ServletRequestAttributeEvent srae) //  When adding attributes 
void attributeReplaced(ServletRequestAttributeEvent srae) //  Replace attributes 
void attributeRemoved(ServletRequestAttributeEvent srae) //  Remove properties  

Other listeners

HttpSessionBindingListener Perception monitor

HttpSessionActivationListener Perception monitor