WARN mapred.JobClient: No job jar file set.  User classes may not be found. See JobConf(Class) or JobConf#setJar(String).
10/08/16 15:25:49 INFO input.FileInputFormat: Total input paths to process : 4
10/08/16 15:25:50 INFO mapred.JobClient: Running job: job_201008161439_0004
10/08/16 15:25:51 INFO mapred.JobClient:  map 0% reduce 0%
10/08/16 15:26:00 INFO mapred.JobClient: Task Id : attempt_201008161439_0004_m_000000_0, Status : FAILED
java.lang.RuntimeException: java.lang.ClassNotFoundException: org.apache.hadoop.examples.WordCount2$WordCountMapper

 at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:808)

 at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:157)

 at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:532)

 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: org.apache.hadoop.examples.WordCount2$WordCountMapper

 at java.net.URLClassLoader$1.run(URLClassLoader.java:202)

 at java.security.AccessController.doPrivileged(Native Method)

 at java.net.URLClassLoader.findClass(URLClassLoader.java:190)

找不到job任务运行的资源，如map类、reduce类等。

新增

public class InitMain{

    Job job = new Job();

    job.setJarByClass(InitMain.class); //只要是Initmain类所在包下的任一个类名都可以，默认为当前 job所在类名。

}