尝试在小结中解析urls文件夹时遇到了该nullpointer异常。 该如何解决？
$ ./nutch inject urls
InjectorJob: starting at 2019-06-07 10:19:55
InjectorJob: Injecting urlDir: urls
InjectorJob: Using class org.apache.gora.mongodb.store.MongoStore as the
Gora storage class.
InjectorJob: java.lang.NullPointerException
at java.lang.ProcessBuilder.start(ProcessBuilder.java:1012)
at org.apache.hadoop.util.Shell.runCommand(Shell.java:482)
at org.apache.hadoop.util.Shell.run(Shell.java:455)
at
org.apache.hadoop.util.Shell$ShellCommandExecutor.execute(Shell.java:702)
at org.apache.hadoop.util.Shell.execCommand(Shell.java:791)
at org.apache.hadoop.util.Shell.execCommand(Shell.java:774)
at org.apache.hadoop.fs.RawLocalFileSystem.setPermission(RawLocalFileSystem.java:646)
at org.apache.hadoop.fs.RawLocalFileSystem.mkdirs(RawLocalFileSystem.java:434)
at org.apache.hadoop.fs.FilterFileSystem.mkdirs(FilterFileSystem.java:281)
at org.apache.hadoop.mapreduce.JobSubmissionFiles.getStagingDir(JobSubmissionFiles.java:125)
at org.apache.hadoop.mapreduce.JobSubmitter.submitJobInternal(JobSubmitter.java:348)