java.lang.NullPointerException at java.lang.ProcessBuilder.start(Unknown Source) at org.apache.had

java.lang.NullPointerException
    at java.lang.ProcessBuilder.start(Unknown Source)
    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:715)
    at org.apache.hadoop.util.Shell.execCommand(Shell.java:808)
    at org.apache.hadoop.util.Shell.execCommand(Shell.java:791)
    at org.apache.hadoop.fs.RawLocalFileSystem.setPermission(RawLocalFileSystem.java:659)
    at org.apache.hadoop.fs.FilterFileSystem.setPermission(FilterFileSystem.java:490)
    at org.apache.hadoop.fs.ChecksumFileSystem.create(ChecksumFileSystem.java:462)
    at org.apache.hadoop.fs.ChecksumFileSystem.create(ChecksumFileSystem.java:428)
    at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:908)
    at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:889)
    at org.apache.hadoop.fs.FileSystem.create(FileSystem.java:786)
    at org.apache.hadoop.fs.FileUtil.copy(FileUtil.java:365)
    at org.apache.hadoop.fs.FileUtil.copy(FileUtil.java:338)
    at org.apache.hadoop.fs.FileUtil.copy(FileUtil.java:289)
    at org.apache.hadoop.fs.FileSystem.copyToLocalFile(FileSystem.java:1970)
    at org.apache.hadoop.fs.FileSystem.copyToLocalFile(FileSystem.java:1939)
    at org.apache.hadoop.fs.FileSystem.copyToLocalFile(FileSystem.java:1915)
    at com.bie.lesson01.HdfsClientTest.downloadFile(HdfsClientTest.java:64)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)