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

["Ratandeep Ratti (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-11878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14824525#comment-14824525 ] 

Ratandeep Ratti commented on HIVE-11878:
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There are a few approaches in my mind on how to solve this problem.

1. Do not use the current classloader as the parent when creating a new URLClassLoader in {{Utilities.addToClassPath}} method.  In this case, every URLClassLoader constructed will have, as its parent, the SystemClassLoader.  Also as per the {{addToClassPath}} method, every URLClassLoader  will  have all the jar paths from the parent (including the current jar to be registered).

Now looking at the above scenario again class-loader *u2* will have as its parent the system class-loader and will have the jar paths for jars *j1* and *j2* both. Now, when *c1* is instantiated using the classloader *u2*, *u2* will load and define the class *c1* as the parent classloader will not have the jar. Now class *c2* required by *c1* will also be found in *u2* and will be correctly loaded and defined by *u2*.

Note that {{Utilities.removeFromClassPath}} also creates new URLClassloaders without passing the current classloader as their parent.

2. Have a new classloader which extends the URLClassloader and uses the servlet spec to load classes. That is instead of delegating to parent first, it first tries to find the class in its own classpath.

3. Have a new classloader which extends the URLClassloader and which changes the scope of the {{addURL}} method from protected to public. Using this custom classloader we will not have to create fresh-classloaders for every jar being registered. We do have to think about how  delete/remove jars will be implemented.


> ClassNotFoundException can possibly  occur if multiple jars are registered 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_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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