2012-05-07 12:59:33:480[WARN]: Use GenericOptionsParser for parsing the arguments. Applications should implement Tool for the same.
2012-05-07 12:59:33:519[WARN]: No job jar file set.  User classes may not be found. See JobConf(Class) or JobConf#setJar(String).
2012-05-07 12:59:33:551[INFO]: Total input paths to process : 1
2012-05-07 12:59:33:927[INFO]: Running job: job_201205071251_0001
2012-05-07 12:59:34:930[INFO]:  map 0% reduce 0%
2012-05-07 12:59:45:966[INFO]: Task Id : attempt_201205071251_0001_m_000000_0, Status : FAILED
java.lang.RuntimeException: java.lang.ClassNotFoundException: com.test.WordCount$TokenizerMapper
at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:809)
at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:157)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:569)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:305)
at org.apache.hadoop.mapred.Child.main(Child.java:170)
Caused by: java.lang.ClassNotFoundException: com.test.WordCount$TokenizerMapper
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:247)
at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:762)
at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:807)
... 4 more



表面上看是空指针异常，实际上则不是，求解释