我在Hadoop 2.2.0安装的本地模式的测试环境中定期运行hadoop作业。该作业大部分时间都成功，但偶尔会因上述错误消息而失败。调用堆栈如下：
Exception in thread "main" java.lang.IllegalArgumentException: Can not create a Path from an empty string
at org.apache.hadoop.fs.Path.checkPathArg(Path.java:127)
at org.apache.hadoop.fs.Path.<init>(Path.java:135)
at org.apache.hadoop.fs.Path.<init>(Path.java:94)
at org.apache.hadoop.mapred.LocalJobRunner$Job.<init>(LocalJobRunner.java:150)
at org.apache.hadoop.mapred.LocalJobRunner.submitJob(LocalJobRunner.java:636)
at org.apache.hadoop.mapreduce.JobSubmitter.submitJobInternal(JobSubmitter.java:430)
at org.apache.hadoop.mapreduce.Job$10.run(Job.java:1268)
at org.apache.hadoop.mapreduce.Job$10.run(Job.java:1265)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1491)
at org.apache.hadoop.mapreduce.Job.submit(Job.java:1265)
at org.apache.hadoop.mapreduce.Job.waitForCompletion(Job.java:1286)
at com.mathworks.hadoop.MWMapReduceDriver.run(MWMapReduceDriver.java:1443)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:70)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:84)
at com.mathworks.hadoop.MWMapReduceDriver.main(MWMapReduceDriver.java:1454)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.hadoop.util.RunJar.main(RunJar.java:212)
使用Hadoop 1.2.1，同样的工作不会失败。我查看了LocalJobRunner.java的源代码，发现第150行如下：
this.localJobDir = localFs.makeQualified(new Path(new Path(conf.getLocalPath(jobDir), user), jobid.toString()));
我在异常catch块中打印出conf.getLocalPath（jobDir），user和jobid.toString（），并在作业失败时发现用户为空。根据源代码，用户通过调用UserGroupInformation.getCurrentUser().getShortUserName()获得。有谁知道这个调用在什么条件下可以返回一个空字符串？