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From "Jason Dere (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-11878) ClassNotFoundException can possibly occur if multiple jars are registered one at a time in Hive
Date Wed, 28 Oct 2015 18:24:27 GMT

    [ https://issues.apache.org/jira/browse/HIVE-11878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14978962#comment-14978962
] 

Jason Dere commented on HIVE-11878:
-----------------------------------

Apologies for not getting back to you on this before now. As to your comments:

{quote}
I have some doubts about using the thread context classloader as the parent. This does not
seem to provide clean isolation between jars/resources between different sessions. Case in
point: a thread context classloader could be a previous session's classloader .This can happen
when the same thread was used to work on a previous session, and is now being used to work
on the newer current session. The thread context classloaer could contain a different implementation
of the same class also present in the session classloader. Do you see this a a problem?
{quote}
I think you are right, that the new session's classloader would be polluted with the loaded
JARs from the thread's previous SessionState. What would be a better parent class loader to
use here - the system class loader? Will it have any JARs added from the AUX_JARS options?
Checking some of the various AUX_JARS related options, not sure if I missed any others:
 - HIVE_AUX_JARS_PATH environment variable: This gets added to the CLASSPATH, so the System
class loader will have these
 - hive.aux.jars: As far as I can tell this is used when shipping JARs to the Map/Reduce tasks,
so this does not seem to get used by the SessionState. Someone correct me if I am wrong.
 - hive.reloadable.aux.jars.path: These jars are added at the start of a HiveServer2 session,
so it may not matter that the System class loader does not have these jars.

{quote}
Another potential problem I'm thinking about – which is present in the proposed approach
(see RB) is – in HiveServer2 any worker thread can serve any request by mapping it to a
persistent session. Couldn't this lead to a situation where for a specific session the session
specific classloader (conf.getClassLoader()) and the thread context classloader end up being
different? Say we have two worker thread t1 and t2 .The very first query is handled by t1
where a fresh session s1 is created along with a fresh classloader c1, which is set as the
session specific classloader and the thread context classloader. The next query for the same
session is handled by t2. I guess since it is the same session s1, we do not create a fresh
classloader. The session specific classloader is c1, but since it is a different thread and
no classloader has been set on it, the thread will have the system classloader as its context
classloader. Couldn't this cause potential CNF exceptions? If I understood correctly this
problem also exists in the current implementation, doesn't it?
{quote}
I'm not sure if I understand the problem here - I believe that if a new thread handles an
existing session, the thread calls Session.setCurrentSessionState() which should set the thread's
context class loader to the SessionState's class loader. Or did I not understand the issue?

> ClassNotFoundException can possibly  occur if multiple jars are registered one at a time
in Hive
> ------------------------------------------------------------------------------------------------
>
>                 Key: HIVE-11878
>                 URL: https://issues.apache.org/jira/browse/HIVE-11878
>             Project: Hive
>          Issue Type: Bug
>          Components: Hive
>    Affects Versions: 1.2.1
>            Reporter: Ratandeep Ratti
>            Assignee: Ratandeep Ratti
>              Labels: URLClassLoader
>         Attachments: HIVE-11878.patch, HIVE-11878_approach3.patch, HIVE-11878_approach3_per_session_clasloader.patch,
HIVE-11878_qtest.patch
>
>
> When we register a jar on the Hive console. Hive creates a fresh URL classloader which
includes the path of the current jar to be registered and all the jar paths of the parent
classloader. The parent classlaoder is the current ThreadContextClassLoader. Once the URLClassloader
is created Hive sets that as the current ThreadContextClassloader.
> So if we register multiple jars in Hive, there will be multiple URLClassLoaders created,
each classloader including the jars from its parent and the one extra jar to be registered.
The last URLClassLoader created will end up as the current ThreadContextClassLoader. (See
details: org.apache.hadoop.hive.ql.exec.Utilities#addToClassPath)
> Now here's an example in which the above strategy can lead to a CNF exception.
> We register 2 jars *j1* and *j2* in Hive console. *j1* contains the UDF class *c1* and
internally relies on class *c2* in jar *j2*. We register *j1* first, the URLClassLoader *u1*
is created and also set as the ThreadContextClassLoader. We register *j2* next, the new URLClassLoader
created will be *u2* with *u1* as parent and *u2* becomes the new ThreadContextClassLoader.
Note *u2* includes paths to both jars *j1* and *j2* whereas *u1* only has paths to *j1* (For
details see: org.apache.hadoop.hive.ql.exec.Utilities#addToClassPath).
> Now when we register class *c1* under a temporary function in Hive, we load the class
using {code} class.forName("c1", true, Thread.currentThread().getContextClassLoader()) {code}
. The currentThreadContext class-loader is *u2*, and it has the path to the class *c1*, but
note that Class-loaders work by delegating to parent class-loader first. In this case class
*c1* will be found and *defined* by class-loader *u1*.
> Now *c1* from jar *j1* has *u1* as its class-loader. If a method (say initialize) is
called in *c1*, which references the class *c2*, *c2* will not be found since the class-loader
used to search for *c2* will be *u1* (Since the caller's class-loader is used to load a class)
> I've added a qtest to explain the problem. Please see the attached patch



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