private boolean isSomeGeneratedFileStale(File outputDirectory, List<String> defFileNames) {
        long oldestGeneratedModifiedTime = walk(outputDirectory.toPath())
                    .filter(Files::isRegularFile)
                    .map(Path::toFile)
                    .peek(f -> getLog().debug("Checking generated file " + f))
                    .mapToLong(File::lastModified)
                    .min()
                    .orElse(Long.MAX_VALUE);

        long lastModifiedSource = defFileNames.stream()
                .peek(sourceFile -> getLog().debug("Checking source file " + sourceFile))
                .map(File::new)
                .mapToLong(File::lastModified)
                .max()
                .orElse(0L);

        getLog().debug("lastModifiedSource: " + lastModifiedSource + ", oldestTGeneratedModified: " + oldestGeneratedModifiedTime);
        return lastModifiedSource > oldestGeneratedModifiedTime;
    }
Returns List of all def-files in all defFilesDirectories, including path.
@return The list of def-files.
config-class-plugin/src/main/java/com/yahoo/vespa/ConfigGenMojo.java