ibjars not working in custom mapreduce code, How to debug

Mostly application developers bump into this issue. They ship their custom jars to map reduce job but when the classes in those are referred by code it throws a Class not found exception.

For -libjars to work your main class should satisfy the following two conditions.
 
1) Main Class should implement the Tool interface

 //wrong usage - Tool Interface not implemented

public class WordCount extends Configured {

//right usage

public class WordCount extends Configured implements Tool {

 
2) Main Class should get the existing configuration using getConf() method rather than creating anew configuration instance.

//wrong usage - creating anew instance of Conf 

public int run(String[] args) throws Exception {

   Configuration conf = new Configuration();

//right usage 

 public int run(String[] args) throws Exception {
    Configuration conf = getConf();