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Multiple nested JARs are not supported; just copying the original bundle
OSGi allows to nest JARs inside of each other. A standard use case for this is source bundles which contain a manifest and the original source archive.
But binary plug-ins use this as well. One example is org.w3c.sac which contains flute.jar
Maven doesn't support nested JARs, so MT4E has to do something about it.
Usually, the bundle only contains a single nested JAR and no other code. In this case, MT4E will extract the nested JAR and use that.
But sometimes, the bundle contains several JARs or, worse, it contains code plus a nested JAR. At the time of writing, there was no generic solution to the problem.
Check if you really need the code in the nested JAR. If not, then you can ignore this warning.
If you need the nested JAR, you can unpack it yourself. Write a small POM for it and add a dependency in the original POM.