Hadoop java.lang.ClassNotFoundException

Today I ran into some weird Hadoop error. It could not find my mapper class. It turns that I had defined HADOOP_CLASSPATH as only my current directory (where my classes were) and it lacked the generic Mapper class (org.apache.hadoop.mapreduce.Mapper), but instead of Hadoop reporting this later class was missing, it did so with my own class, which was clearly accessible.



So this blog entry is for those who run into this problem too, because there is no help from Stackoverflow regarding this issue.

This is the message you get:

java.lang.RuntimeException: java.lang.ClassNotFoundException: ProcessMapper
        at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:857)
        at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:199)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:718)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:364)
        at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:415)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1190)