// which silently excludes files not ending in .java
        Iterable<File> javaOnly = Iterables.filter(spec.getSourceFiles(), NormalizingJavaCompiler::hasJavaExtension);
        spec.setSourceFiles(ImmutableSet.copyOf(javaOnly));
    }

    private static boolean hasJavaExtension(@Nullable File input) {
        return hasExtension(input, ".java");
    }

    private void resolveNonStringsInCompilerArgs(JavaCompileSpec spec) {