private void preprocess(File appDir, File dir, Zone zone) throws IOException {
        validateServicesFile();
        IOUtils.copyDirectory(appDir, dir, - 1,
                              (__, name) -> ! List.of(preprocessed, SERVICES, HOSTS, CONFIG_DEFINITIONS_DIR).contains(name));
        preprocessXML(applicationFile(dir, SERVICES), getServicesFile(), zone);
        preprocessXML(applicationFile(dir, HOSTS), getHostsFile(), zone);
    }
Reads a ranking expression from file to a string and returns it.

@param name the name of the file to return,
            relative to the search definition directory in the application package
@return the content of a ranking expression file
@throws IllegalArgumentException if the file was not found or could not be read
config-application-package/src/main/java/com/yahoo/config/model/application/provider/FilesApplicationPackage.java