Java dependencies are difficult for an external dependency-based construction tool to accomodate. Determining Java class dependencies is more complicated than the simple pattern-matching of C or C++ #include files. From the point of view of an external build tool, the Java compiler behaves "unpredictably" because it may create or update multiple output class files and directories as a result of its internal class dependencies.

An obvious SCons implementation would be to have the Scanner object parse output from Java -depend -verbose to calculate dependencies, but this has the distinct disadvantage of requiring two separate compiler invocations, thereby slowing down builds.