[jira] [Commented] (HIVE-11878) ClassNotFoundException can possibly occur if multiple jars are registered one at a time in Hive

["Jason Dere (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-11878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15082049#comment-15082049 ] 

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

Committed to branch-1

> 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
>             Fix For: 1.3.0, 2.0.0
>
>         Attachments: HIVE-11878 ClassLoader Issues when Registering Jars.pptx, HIVE-11878.2.patch, HIVE-11878.3.patch, HIVE-11878.4.patch, HIVE-11878.4.patch.branch-1, HIVE-11878.patch, HIVE-11878_approach3.patch, HIVE-11878_approach3_per_session_clasloader.patch, HIVE-11878_approach3_with_review_comments.patch, HIVE-11878_approach3_with_review_comments1.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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