code: public Set<String> getOutputDirectories() {
  		Set<String> outputDirectories = Sets.newLinkedHashSet();
  		if (isUseOutputPerSourceFolder()) {
  			for (SourceMapping mapping : getSourceMappings()) {
  				outputDirectories.add(getOutputDirectory(mapping.getSourceFolder()));
  			}
  		} else {
  			outputDirectories.add(getOutputDirectory());
  		}
  		return outputDirectories;
  	}
docs: The output directories returned are project relative, e.g. "build/gen" or "src-gen"
  @since 2.6