hadoop-compat/src/main/java/com/twitter/elephantbird/util/HadoopCompat.java
public static JobContext newJobContext(Configuration conf, JobID jobId) {
    return (JobContext) newInstance(JOB_CONTEXT_CONSTRUCTOR, conf, jobId);
  }
public static JobContext newJobContext(Configuration conf, JobID jobId) {
    return (JobContext) newInstance(JOB_CONTEXT_CONSTRUCTOR, conf, jobId);
  }
Creates JobContext from a JobConf and jobId using the correct constructor
for based on Hadoop version. <code>jobId</code> could be null.