core/play/src/main/java/play/Environment.java
public Optional<File> getExistingFile(String relativePath) {
    return OptionConverters.toJava(env.getExistingFile(relativePath));
  }
public Optional<File> getExistingFile(String relativePath) {
    return OptionConverters.toJava(env.getExistingFile(relativePath));
  }
Retrieves a file relative to the application root path. This method returns an Optional, using
empty if the file was not found.

@param relativePath relative path of the file to fetch
@return an existing file