Since JRClassLoader does not use the classpath specified in JRProperties.COMPILER_CLASSPATH, reports cannot be compiled unless all the classes are in the runtime classpath of the compiler -- instead of the build time classpath. So, for example, if as a part of building a project data transfer beans are built and used in reports as variables, the variable verification step of the report compilation will fail -- unless the beans were in the classpath of the JRAntCompileTask itself at definition time.
The attached patch remedies this issue by modifying JRClassLoader to use a class loader which considers the compiler classpath. It's not perfect, as it relies on a single static classloader and does not monitor the COMPILER_CLASSPATH property for changes. On the other hand, it is not safe to simply construct a new classloader each time the loadClassForName(...) method is called because then each time the method is called (even with the same name) a different class object is loaded (and calls like isAssignableFrom fail).
I have not thoroughly tested the patch, but it does seem to compile and populate all of my reports (when applied to 1.2.28)
More discussion of this issue here: