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From Mark Benzel <>
Subject RE: Classloader, JNI and "already loaded in another classloader"
Date Fri, 01 Jun 2001 18:02:14 GMT
Yes, it makes sense.  However, I'm not sure if it solves my problem.  In
the scenario you describe, I would want to have 2 Java Beans, each accessing
a separate instance of the C++/DLL.  That's because the data contained in
C++/DLL used by webapps/aaa is different than the data contained in the
used by webapps/bbb.  An equivalent case would be running the same
Java application twice with each instance of the Java application accessing
different set of data.



-----Original Message-----
From: Pae Choi []
Sent: Friday, June 01, 2001 10:30 AM
Subject: Re: Classloader, JNI and "already loaded in another classloader"

Just to give a thought for your situation, how would you think if we
place a plain java bean(i.e., common wrapper class) that interact with the
c++/dll module and provide the access from the Web components, i.e.,
servlet/jsp --- the jb's user --- to the java bean(jb). In that case,
multiple jb's users are accessing a single dll through the jb.

Does it make sense to you?


>I have a webapp running under Tomcat 3.2.1 that needs to make JNI calls in
>order to access data and methods in legacy C++ code.  A servlet is loaded
>startup of the webapp that, as part if its init method, causes a data set
>specific to that webapp instance to be loaded into the C++ data structures.
>This Java code for this servlet contains the following:
>    static
>    {
>        try {
>            System.loadLibrary("JCoreImpl");
>            System.out.println("JCoreImpl loaded");
>            m_bLibraryLoaded = true;
>        } catch (UnsatisfiedLinkError e) {
>            m_bLibraryLoaded = false;
>            System.out.println("JCoreImpl NOT loaded " + e);
>        }
>    }
>Things work fine if there is only one webapp (let's call it "webapps/aaa").
>If I have a second webapp ("webapps/bbb") that is identical to webapps/aaa
>except for the data set used in the C++ data structures, then webapps/aaa
>starts up just fine, but when webapps/bbb is started I get an error stating
>JCoreImpl NOT loaded java.lang.UnsatisfiedLinkError: Native Library
>E:\WebStation\binDebug\JCoreImpl.dll already loaded in another classloader
>I need to have a separate instance of the native library for each of my
>webapps as each instance needs to contain data that is unique to that
>particular webapp.  I have searched through the mail archives and read
>emails by Craig McLanahan explaining the classloader hierarchy.  But I have
>not been able to find anything specific to loading a unique instance of a
>native library for each webapp.
>Any ideas?  Thanks,

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