What is a listener in java

A component that monitors the operation of the Web server and takes preset processing measures when a specific event occurs. It is a very important content in JAVA Web development. Let's learn about the java listener with the editor.

1 What is a web listener

Web listeners are special classes in Servlets that can help developers monitor specific events in the web. event.

For example, the creation and destruction of ServletRequest; the creation, destruction and modification of variables, etc. You can also add processing before and after certain actions to achieve monitoring.

2 Common uses of listeners

Web listeners are usually used to do the following:

Count the number of people online and use HttpSessionLisener

to load initialization information: Using ServletContextListener

Count website visits

Implement access monitoring

3 Let’s look at the creation and execution process of a listener

First you need to create A listener that implements a certain interface. For example, if I want to monitor the number of people online, I can create the following listener:

public class MyListener implements HttpSessionListener{
    private int userNumber = 0;
    public void sessionCreated(HttpSessionEvent arg0) {
        userNumber++;
        arg0.getSession().setAttribute("userNumber", userNumber);
    }
    public void sessionDestroyed(HttpSessionEvent arg0) {
        userNumber--;
        arg0.getSession().setAttribute("userNumber", userNumber);
    }
}

Then configure the listener in web.xml and in web-app Add:

 
      com.test.MyListener
  

Add the number of visitors in JSP:

    
在线人数：<%=session.getAttribute("userNumber") %>

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