NoClassDefFoundError com.sun.activation.registries.LogSupport

I’m trying to run a single Junit test via Eclipse and getting this error. But I don ‘t get the error if I run the whole suite of junit tests using ant. The jars activation.jar and mail.jar are definitely in my Project classpath.

The answer: go to Project > Properties > Java Build Path > Order and Export. Move activation.jar up the list.