On Fri, May 16, 2014 at 2:22 PM, Chamil Jeewantha <[email protected]> wrote:
> 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. > - ++ > > In my POV there can be two types of problems. 1. In contrast to vanilla Tomcat in AS Tomcat is deployed as OSGi service hence we have to modify some TomEE integration points to work under a OSGi run time. When we integrate Tomcat with OSGi we ran into similar problems that's why you can see Carbon Jar scanner ,Carbon Classloaders etc. We are more or less aware of these generic issues and if you can elaborate more on case by case we can help. 2. In AS we use Carbon deployers to deploy web applications instead of default Tomcat deployers, it seems TomEE also override default Tomcat deployers and take the deployment control. In this case we need to study TomEE deployment logic and improve Carbon deployers accordingly. > *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 > IMHO we need to start by improving web application deployers as it very clear that we can't use TomEE deployers. > > 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 Thanks ! > > > [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 > > -- Sagara Gunathunga Senior Technical Lead; WSO2, Inc.; http://wso2.com V.P Apache Web Services; http://ws.apache.org/ Linkedin; http://www.linkedin.com/in/ssagara Blog ; http://ssagara.blogspot.com
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