* @return Returns the combined {@link FileTree} for test and main resources from Java projects. Returns null if no files exist.
     */
    @Nullable
    public static FileTree getJavaTestAndMainSourceResources(Project project, Action<? super PatternFilterable> filter) {
        final Optional<FileTree> testFileTree = getJavaTestSourceSet(project).map(SourceSet::getResources).map(FileTree::getAsFileTree);

        Provider<ValidateJsonAgainstSchemaTask> validateRestSpecTask = project.getTasks()
            .register("validateRestSpec", ValidateJsonAgainstSchemaTask.class, task -> {
                task.setInputFiles(Util.getJavaTestAndMainSourceResources(project, filter -> {
                    filter.include(DOUBLE_STAR + "/rest-api-spec/api/" + DOUBLE_STAR + "/*.json");
                    filter.exclude(DOUBLE_STAR + "/_common.json");
                }));