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From "Nadeem Douba (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-12406) AbstractMapWritable.readFields throws ClassNotFoundException with custom writables
Date Mon, 02 Nov 2015 23:46:27 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-12406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14986295#comment-14986295
] 

Nadeem Douba commented on HADOOP-12406:
---------------------------------------

Hi Vinod,

I was running this on top of YARN with the default configuration enabled on top of Java 1.8
from Oracle. I think the new isolation strategy used in the newer versions of the JVM for
class loaders prevents the system class loader from loading a job jar class. I didn't do anything
fancy with configuration options or anything else. The only thing I added were the remote
debugging options to allow me to attach to the map and reduce threads remotely and debug them
step by step. I can tell you that the job jar definitely isn't in the system's class loader
but in the current thread's context class loader. I verified this observation by manually
inspecting the class loader object from the system and the one that comes with the thread
context.

Cheers,

Nadeem

> AbstractMapWritable.readFields throws ClassNotFoundException with custom writables
> ----------------------------------------------------------------------------------
>
>                 Key: HADOOP-12406
>                 URL: https://issues.apache.org/jira/browse/HADOOP-12406
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: io
>    Affects Versions: 2.7.1
>         Environment: Ubuntu Linux 14.04 LTS amd64
>            Reporter: Nadeem Douba
>            Priority: Blocker
>              Labels: bug, hadoop, io, newbie, patch-available
>         Attachments: HADOOP-12406.patch
>
>
> 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.



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