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Extending Tomcat WebappLoader to Share Library jars

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Extending Tomcat WebappLoader to Share Library jars

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Using Tomcat + Spring + Hibernate can quickly result in a 20M war. It took me 10mins to deploy such a war so to fix a typo in the HTML code. Wouldn't it be nice if we can somehow externalise the lib jars and different applications can share the same jars.

Reading Tomcat docs, I find out there is a shared lib folder for all the applications, provided each app is using exactly the same version. So this would not be very helpful either.

Finally I came across the Loader Component.

With a bit help from tomcat's src, I managed to extend org.apache.catalina.loader.WebappLoader that is able to load addtional lib by reading the external lib config settings from application's context.xml.

The idea is to intercept Tomcat's normal web app loading process and check if there are any tags in the context.xml. If so treat each tag as a folder that contains a set of jars, then make those jars available to the application. Then proceed with normal Tomcat web app loading.

There are 2 things to change

1. build the TomcatWebappLoader.jar (can be found in the attached zip) and put it to ${catalina.home}/lib, restart tomcat (this only needs to be done once). So the new WebappLoader is available to Tomcat.

2. change your context.xml

from
  <Context path="/testapp" />
to
  <Context path="/testapp">
    <Loader className='com.spiralcomms.tomcat.WebappLoader' />
    <lib>spring_2.5.6</lib>
    <lib>hibernate_3.3.0</lib>
  </Context>

and copy the required jars to ${catalina.home}/repo/spring_2.5.6 and ${catalina.home}/repo/hibernate_3.3.0 folders

DONE.

now the war size goes from 20M down to 300K, and the resulting war only takes a few secs to upload and deploy using Tomcat manager

I have included both the project src as well the compiled jar in the attached zip

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