code: private void writeObject(final ObjectOutputStream out) throws IOException {
  
  		out.writeObject(reducer.getClass());
  		out.writeObject(combiner.getClass());
  		jobConf.write(out);
  	}
docs: Custom serialization methods.
  @see <a href="http://docs.oracle.com/javase/7/docs/api/java/io/Serializable.html">http://docs.oracle.com/javase/7/docs/api/java/io/Serializable.html</a>