Note: I am not an expert at JAVA, Class loaders, or Hadoop. I am just a hacker. My solution might be entirely wrong.
AbstractMapWritable.readFields throws a ClassNotFoundException when reading custom writables. Debugging the job using remote debugging in IntelliJ revealed that the class loader being used in Class.forName() is different than that used by the Thread's current context (Thread.currentThread().getContextClassLoader()). The class path for the system class loader does not include the libraries of the job jar. However, the class path for the context class loader does. The proposed patch changes the class loading mechanism in readFields to use the Thread's context class loader instead of the system's default class loader.