出于某种原因，每次我运行这个程序（在eclipse和emr上）都会收到消息13/07/18 13:22:23 warn mapred.jobclient:no job jar file set。可能找不到用户类。请参阅jobconf（类）或jobconf#setjar（字符串）。几个打印语句之后，我发现我的jarfile顽固地保持为空。我做错什么了？

public static void main(String[] args) throws Exception {
            Job job = new Job();
            job.setJarByClass(Driver.Map.class);

             FileInputFormat.addInputPath(job, new Path(args[0]));
             FileOutputFormat.setOutputPath(job, new Path(args[1]));

            MultithreadedMapper.setMapperClass(job, Driver.Map.class);
            MultithreadedMapper.setNumberOfThreads(job, 64);

            job.setMapperClass(MultithreadedMapper.class);

            System.out.println(job.getJar());
            System.out.println(MultithreadedMapper.getNumberOfThreads(job) + "  "
                    + MultithreadedMapper.getMapperClass(job));

            job.setOutputKeyClass(Text.class);
            job.setOutputValueClass(Text.class);
            job.setNumReduceTasks(0);
            Config.LoggerProvider = LoggerProvider.DISABLED;

            job.waitForCompletion(true);

        }