Quick — without checking the documentation, what does the Export-Package instruction do in the Maven Bundle Plugin?
Did you say “sets what packages are exported by the bundle”? If so, you’re only half right.
Recently, I was trying to diagnose some issues with a SOAP Web Services implementation in an existing code base. Theoretically, everything was in place, however I was getting a
ClassCastException when attempting to retrieve the OSGi Service for accessing the SOAP Web Service. After staring at the logs and pulling out clumps of hair, I decided to find out where each class was coming from.
I executed the code below to get the source of the service interface class as well as the interface class implemented by the service implementation.
Shockingly, even though the interface for the ServiceImpl and the Service interface are the same class, the two class instances were loaded from separate bundles! It turns out what was happening is that the existing project set the export directive to:
In addition to setting the packages exported by the bundle, the Export-Package instruction will actually copy all of the classes matching the specified package from the bundle classpath into the current bundle. This is some powerful functionality, but can be easily misused. Since the Service was referenced from another bundle which used the above mentioned Export-Package instruction, all of the service code was duplicated from the service into the consuming bundle. As there was two copies of the classes, the ClassLoader didn’t believe that the Service interface and the interface the Service Implementation implemented were the same class. The fix for the problem was simply to remove the Export-Package instruction from the consuming bundle’s POM file.
Which brings me back to my original point, the Export-Package instruction can be very dangerous and can cause unexpected side-effects. Like any other tool, it can be useful, but you need to be careful and fully understand the effects when using the Export-Package instruction.
Quite likely, this project’s POM was copied from another project without the author really understanding what the POM is doing. It’s important, especially when creating a new project, to ensure you’re not just blindly copying, but instead truly comprehending every configuration in the project’s POM.