MapReduce Program Operation error Java.lang.ClassNotFoundException solution

When the custom mapper was created, the compilation was correct, but an error occurred while uploading to the cluster execution:

11/16/05 22:53:16INFO mapred. Jobclient:task Id:attempt_201111301626_0015_m_000000_0, status:failedjava.lang.RuntimeException: Java.lang.ClassNotFoundException:actiondemo. Myjob$mapclass at Org.apache.Hadoop.conf.Configuration.getClass (Configuration.java:866) at Org.apache.hadoop.mapreduce.JobContext.getMapperClass (Jobcontext.java:199) at Org.apache.hadoop.mapred.MapTask.runNewMapper (Maptask.java:719) at Org.apache.hadoop.mapred.MapTask.run (Maptask.java:370) at org.apache.hadoop.mapred.child$4.run (child.java:255) at java.security.AccessController.doPrivileged (Native Method) at Javax.security.auth.Subject.doAs (subj Ect.java:396) at Org.apache.hadoop.security.UserGroupInformation.doAs (Usergroupinformation.java:1059) at Org.apache.hadoop.mapred.Child.main (Child.java:249) caused By:java.lang.ClassNotFoundException:actiondemo. Myjob$mapclass at java.net.urlclassloader$1.run (urlclassloader.java:202At java.security.AccessController.doPrivileged (Native Method) at Java.net.URLClassLoader.findClass (URL Classloader.java:190) at Java.lang.ClassLoader.loadClass (Classloader.java:60S) at Sun.misc.launcher$appclassloader.loadclass (Launcher.java:301) at Java.lang.ClassLoader.loadClass (Classloader.java:247At JAVA.LANG.CLASS.FORNAME0 (Native Method) at Java.lang.Class.forName (Class.java:247) at Org.apache.hadoop.conf.Configuration.getClassByName (Configuration.java:819) at Org.apache.hadoop.conf.Configuration.getClass (Configuration.java:864)        ... 8 more

The program development process is developed under Windows Eclipse, and then the jar package is poured out and the jar package is placed on the cluster to run.

The version of the JDK for eclipse cannot be higher than the JDK version configured in Hadoop, or it will be an error. (What type of error, not to be remembered)

　　The cause of the problem and the solution are as follows:

Since you are using a Hadoop version of more than 0.20, you need to set the Setjarbyclass method when invoking a custom mapper in the jar, set the method as follows:

　　Job.setjarbyclass (Myjob.class);

In fact, in the output log also has the prompt message:
11/16/05 22:53:03 WARN mapred.  Jobclient:no job jar file set. User classes May is not found. See jobconf (Class) or Jobconf#setjar (String).
Job.setjarbyclass (Myjob.class);

MapReduce Program Operation error Java.lang.ClassNotFoundException solution