Dec 1, 2010

Tracking Session Lifecycle in Grails

Web applications often need to keep track when new user sessions are created and when user sessions are terminated - either by timing out or by active invalidation.
The Java Servlet Specification defines Event Listeners for this purpose. Have a look at this article for an introduction on how to utilize these servlet technologies.

I want to explain here how to integrate HttpSessionListeners smoothly with Grails.

We start by registering a listener in the deployment descriptor - the file web.xml.
By default this file is not visible when developing with Grails since it is generated dynamically. You can get a static version by executing the following command in the application's root directory:

grails install-templates

Instead of executing this command and creating a static web.xml file we are going to choose the more modular approach of extending the web.xml generation process.

The first thing to do is to create the file _Events.groovy in the scripts directory of the application root. Note that this directory was created when executing grails create-app. The contents of _Events.groovy should look something like this:

import groovy.xml.StreamingMarkupBuilder
eventWebXmlEnd = {String tmpfile -> 
  def root = new XmlSlurper().parse(webXmlFile)
  root.appendNode {
    'listener' { 
      'listener-class' (
  webXmlFile.text = new StreamingMarkupBuilder().bind {
      "": "")

This defines a closure that is called by the Grails build mechanism right after creation of web.xml. In the closure Groovy's XmlSlurper is used to parse the web.xml into a tree data structure. Xml elements for listener and listener-class are added and the tree data structure is written back using Groovy's StreamingMarkupBuilder.

The class org.test.MyHttpSessionListener is registered as event listener.
The file  src/groovy/org/test/MyHttpSessionListener.groovy contains this class. Obviously the org/test part of the path depends on the package name you want to choose. The file's contents should look like this:

package org.test

import org.codehaus.groovy.grails.commons.ApplicationHolder
import javax.servlet.http.HttpSession
import javax.servlet.http.HttpSessionEvent
import javax.servlet.http.HttpSessionListener
class MyHttpSessionListener implements HttpSessionListener {
  HttpSessionService httpSessionService
  // called by servlet container upon session creation
  void sessionCreated(HttpSessionEvent event) {
    HttpSession session = event.getSession()

  // called by servlet container upon session destruction
  void sessionDestroyed(HttpSessionEvent event) {
    HttpSession session = event.getSession()

  private synchronized HttpSessionService
  getHttpSessionService() {
    if (httpSessionService == null) {
      httpSessionService =
        (HttpSessionService) ApplicationHolder
    return httpSessionService

The class implements the sessionCreated and sessionDestroyed methods as defined in the HttpSessionListener interface. These methods are called by the servlet container upon session creation and destruction. However to integrate our listener with the rest of the Grails artefacts it would be nicer if conventional Grails Service methods were called on these events.

To achieve the desired Service calls we cannot utilize Spring injections in our SessionListener. We therefore manually fetch a bean called httpSessionService from the application's MainContext. This bean is a conventional Grails Service that we have to implement in a file called grails-app/services/org/test/HttpSessionService.groovy.  The file could have the following contents:

package org.test

import javax.servlet.http.HttpSession

class HttpSessionService {

  // method called upon session creation
  def sessionCreated(HttpSession session) {"Session created: "

  //method called upon session destruction
  def sessionDestroyed(HttpSession session) {"Session destroyed: "

Other services can be injected into the HttpSessionService and relevant business logic can be called from the sessionCreated and sessionDestroyed Service methods.