code: private void checkOutDirWritable(File outputDirectory) throws MojoExecutionException {
  		if (!outputDirectory.exists()) {
  			checkIfOutputCanBeCreated();
  			checkIfOutputDirIsWritable();
  			if (verbose) {
  				log.info("Output dir check OK");
  			}
  		}
  		else if (!outputDirectory.canWrite()) {
  			throw new MojoExecutionException(
  					"The output dir exists but was not writable. "
  							+ "Try running maven with the 'clean' goal.");
  		}
  	}
docs: Check if the output directory exist and is writable. If not, try to
  create an output dir and see if that is writable.
  
  @param outputDirectory The dir where the result will be placed
  
  @throws MojoExecutionException When the output directory is not writable