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https://issues.apache.org/jira/browse/HIVE-11878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14940183#comment-14940183
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Ratandeep Ratti commented on HIVE-11878:
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Hi [~jdere]
I got some time to look into this today. I incorporated your suggestion
where I create a fresh classloader when a new session is created. I use, as
parent, the thread context classloader for the freshly created session
classloader (See RB: https://reviews.apache.org/r/38663/) . 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?
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?
> 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_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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