The knowledge on TomEE architecture on Tomcat is important to understand
how TomEE can be integrated to WSO2 AS.
*What is OpenEJB & TomEE?*
The OpenEJB and TomEE codes exist on the same code base. The OpenEJB code
provides the ejb and other j2ee support. The TomEE code is the integration
part that integrates openejb into Tomcat. The code base generates several
distributions Such as standalone TomEE, Standalone OpenEJB, OpenEJB-Lite,
TomEE war etc... A detailed comparison can be found at [1]
*TomEE on Tomcat*
TomEE comes as a war file which can be deployed on Tomcat.
To deploy TomEE on Tomcat:
1. Download TomEE war [2]
2. put it in Tomcat's webapp directory.
3. Start Tomcat
Now the Tomcat is J2EE WP enabled.
*How does it happen internally?*
Lets look at the Important contents of TomEE war:
TomEE.war
| - libs #contains all the lib files
| - WEB-INF
| - lib #contains tomee-loader*.jar
| - web.xml
*TomEE initialization*
There are three approaches of initializing TomEE on Tomcat.
*Approach 1 - Loader Servlet*
- The web.xml file registers the LoaderServlet which is in the
tomee-loader*.jar file.
<servlet>
<servlet-name>LoaderServlet</servlet-name>
<servlet-class>org.apache.tomee.loader.LoaderServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
- The load-on-startup = 1 tells, please load this servlet on load.
- The servlet then creates another WebappClassLoader (create mini webapp)
- Loads tomee-loader.jar and openejb-loader.jar into new
WebappClassLoader
- Trigger the intialization process of TomEE within WebappClassLoader
- The tomee-loader then do lots of background work and loads OpenEJB
jars into Tomcat common classloader.
- Initialization OpenEJB within Tomcat common class loader
*Approach 2 - OpenEJBListener*
- The tomee-loader*.jar is copied into $CATALINA_HOME/lib directory
- Register org.apache.tomee.loader.OpenEJBListener as a lifecycle
listner in server.xml file
- The lifecycle listener looks for the tomee webapp directory
- It creates a new WebappClassLoader (Create a mini webapp)
- Loads tomee-loader and openejb-loader jars to that webapp
- trigger the tomee initialization process
- The OpenEJB jar files are loaded into Tomcat common class loader by
this mini Webapp
- Initialize OpenEJB within common class loader
*Approach 3 - TomEE libs in $CATALINA_HOME/libs direcotory*
- The TomEE/OpenEJB libraries are copied into $CATALINA_HOME/libs
directory
- Register org.apache.tomee.catalina.ServerListener in Tomcat server.xml
- remove tomcat annotations-api.jar because it does not contain full set
of j2ee annotations.
- The TomEE/OpenEJB libs are already in catalina classloader
- The ServerListener will trigger the TomEE initialization process
within Tomcat common classloader
In all the above approaches, They have loaded TomEE libs (and dependencies)
and initialized in Tomcat common Classloader
TomEE Then registers a lifecycle listener which will be on alert for WebApp
loading.
TomEE will create a child container per webapp when its getting deployed.
This child container is the j2ee environment for the webapp.
*TomEE AS Integration*
WSO2 AS uses Tomcat as the servlet container. TomEE has already put a big
effort to integrate OpenEJB to Tomcat. So reuse the TomEE code on AS is the
way to go.
However, non of the above TomEE approaches directly work on AS due to its
architectural differences over vanilla Tomcat. The major problems found up
to now are:
- TomEE assumes classpath/URL class loading. AS has OSGI based own class
loading mechanism
- The TomEE requires full set of j2ee annotation support. The WSO2 AS is
not bundled with the full set.
- TomEE & WSO2 Webapp deployment incompatibilities.
- TomEE uses its own Jar scanner, AS has its own Jar scanner.
- ++
*Architecture*
Bundle all TomEE & OpenEJB jars as one single plugin (Orbit bundle) with
required tweaks on code.
Register Server lifecycle listener to trigger TomEE initialization when
server is getting started.
Modify the WSO2 AS WebappClassLoader, CarbonTomcatJarScanner etc... to deal
with OpenEJB
TomEE version we are working on : TomEE 1.6.0.1
*Current Progress*
With some of the code tweaks directly done on OpenEJB code the OpenEJB jars
have put into repository/components/dropins
Registered "ServerListener" lifecycle listener on catalina-server.xml
The openejb containers up are running on WSO2 AS.
The Webapp deployment listener registered by OpenEJB on WSO2 AS, creates
OpenEJB child container upon Webapp deployment
Gives some errors due to javax.annotation dependancy
Gives duplicate webapp error by CarbonTomcat.addWebApp method.
*TODO:*
Create one single OpenEJB bundle That can be deployed as a plugin
Fix JarScanner
Fix javax.annotation dependency problem
Solve the OpenEJB - CarbonTomcat.addWebApp problem
Fix the class loading problems
Samples :- Should be able to re-use TomEE samples.
[1] http://tomee.apache.org/comparison.html
[2] https://tomee.apache.org/downloads.html
--
K.D. Chamil Jeewantha
Associate Technical Lead
WSO2, Inc.; http://wso2.com
Mobile: +94716813892
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