What are the possible causes for ServletContextListener to not be called

I am studying a Java Server, and I came across a problem with ServletContextListener. I see that it is not called, so when I do and getAttribute it always return null, because that attribute is not set in the class implementing ServletContextListener. I have cheked the code in web.xml and I see that it is delcared there too.

What else could the problem be ?

The code its very long so I will post a small part of it.

package common;

// import stuff

public class ServerContextListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent event) {

      ServletContext context = event.getServletContext();
      context.setAttribute(Const.ATTR_LICENSES, licenses); 
      // code here
    }
}

@Override
    public void contextDestroyed(ServletContextEvent event) {

        // code here
    }

// other file
Licenses licenses = (Licenses) context.getAttribute(Const.ATTR_LICENSES); // this returns null

And this is a part from web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
    <session-config>
        <session-timeout>30</session-timeout>
    </session-config>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
    <display-name>mymob</display-name>

    <context-param>
        <param-name>jobQueueName<开发者_开发技巧;/param-name>
        <param-value>mobileJobQueue</param-value>
 </context-param>
  <listener>
        <listener-class>common.ServerContextListener</listener-class>
    </listener>

//other parameters