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manifest classpath + taglib jars + Tomcat = FAIL


I found an interesting side-effect of using manifest classpaths in our application jar files as it applies to taglib jar dependencies and embedded Tomcat 6.

·         Manifest classpaths do not appear to be expanded in the URLClassLoaders that load the jars with the manifests, so only the jar file referenced directly on the classpath are included in its URLs. The taglib search mechanism in Tomcat 6 (haven't checked Tomcat 7 to see if there's a difference) only walks the classpath looking for URLClassLoaders, looking at the underlying JAR URLs. Thus, if the taglib jar is referenced only as a dependency in the manifest of the main jar file and is not specifically on the classpath, it will not be spotted and use of that taglib by the JSP engine will throw an exception.

This problem won’t actually show up in tests as IDE environments and builds generally include all the jar files in the classpath, so this one will bite you at runtime.

The workaround for this is to include taglib jar files specifically on the classpath, which is something that using the manifest classpath is supposed to help avoid.

Le sigh.
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