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From "John Trimble (JIRA)" <j...@apache.org>
Subject [jira] Created: (JXPATH-127) Change dynamic class loading to consult context class loader.
Date Tue, 23 Dec 2008 23:10:44 GMT
Change dynamic class loading to consult context class loader.
-------------------------------------------------------------

                 Key: JXPATH-127
                 URL: https://issues.apache.org/jira/browse/JXPATH-127
             Project: Commons JXPath
          Issue Type: Improvement
            Reporter: John Trimble
             Fix For: post-1.3


For dynamically loading classes, JXPath currently uses Class.forName(...). This means all
classes loaded by JXPath must be visible to whatever class loader loaded JXPath. This is restrictive
in large frameworks and web applications where multiple class loaders are in use. In particular,
in an EJB3 application deployed on Weblogic, the shared jars between different EJB modules
are loaded with a separate class loader than the modules themselves. Consequently, if the
jxpath.jar is among the general libraries for the application, then a specific EJB module
will be unable to reference static methods of classes within its own module in an xpath expression.
For example, consider the following lines of code in an EJB module class:

JXPathContext context = JXPathContext.newContext(new Object());
Object value = context.getValue("SomeClass.getSomething()");

This will fail, with a ClassNotFoundException, if SomeClass is a class in the module, but
jxpath is a general library for the application.

Attached (or will be once I figure out how) is a patch for JXPath that gets around this issue.
The patch changes the class loading strategy to use the context class loader, when it appears
appropriate, to dynamically load classes. The strategy is based on suggestions and example
code from:

http://www.javaworld.com/javaworld/javaqa/2003-06/01-qa-0606-load.html 

Assuming all class loaders involved delegate to their parent class loader (as recommended
in the API), currently working code, that uses JXPath, should only be affected by this change
if the current class loader (the class loader that loaded JXPath) is not in an ancestor/descendant
relationship with the context class loader.



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