java - Control the classpath ordering of jars in WEB-INF/lib on Tomcat 5?

Question

I have a legacy web app running in Tomcat 5.0.

This web app has two jars in WEB-INF/lib, let's say Foo-2.0.jar and Bar-2.0.jar. Bar-2.0.jar actually includes a Foo-1.0.jar inside of it. Bar is also a dead project, meaning no upgrading, no source, but still important to the application.

The latest release of this application requires Foo-2.0.jar for some other stuff. Having both Foo-1.0.jar and Foo-2.0.jar in the classpath creates a conflict, specifically a ClassDefNotFound type of error, where a class that was later added in 2.0 cannot be found in 1.0, etc.

In Eclipse, the simple solution is to right click on your Project, click Properties > Java Built Path > Order and Export and to move Foo-2.0.jar above Bar-2.0.jar so it's resolved first.

How does one accomplish this type of classpath ordering for jars in WEB-INF/lib in Tomcat?

Answer 1

Tomcat 5's classloading precedence for webapps is roughly as follows: first the bootstrap/system (JRE/lib, then Tomcat's internal classes), then the webapp libraries (first WEB-INF/classes, then WEB-INF/lib), then the common libraries (first Tomcat/common, then Tomcat/lib) and finally the webapp-shared libraries (Tomcat/shared).

So to get Foo-2.0.jar loaded before Bar-2.0.jar, best what you can do is to move Bar-2.0.jar from WEB-INF/lib to Tomcat/common or Tomcat/shared.

The JAR's aren't loaded in alphabetic order of their name. At least, there's no spec which says that. Renaming them to change the alphabetical filename order makes no sense.