Difference between applicationdir/lib and domaindir/lib/ext

Question

Hello,
In my application I use scriptura (A document generation product). The jar containing this products code uses the JAXP API.
If I include the jar in my ear file there is no problem.
If I don't put the jar in my ear file but put it in the lib/ext directory of my domain (i.e. c:\sun\applicationserver\domains\domain1\lib\ext) then the application throws a NoClassDefFoundError (org/apache/xml/serialize/XMLSerializer)
However If I put the jar in the lib directory of my application directory (i.e. c:\sun\applicationserver\lib) then everything works fine again.
I have read some of the documentation and if I understand it correctly third party jars should be in the lib/ext directory. Therefore I would like to see it work with scriptura jar in the lib/ext directory.
I have also looked up some information on classloaders but the way I understand the documentation both directory will be loaded by the common classloader (which is situated between the bootclasspath and the application classloader)?
Can you explain me the difference between putting a jar in the lib of the application directory or putting it in the lib/ext directory of the domain.
Thanks in advance,
Kind Regards,
Niels

Answer 1

domain_dir/lib/ext is an extension directory used by the Java Optional Package Mechanism. Any libraries placed in this directory are loaded just after the core JDK classes and before any application server or application libraries. Hence, the libraries placed in this directory should not refer to any classes in application server or application libraries.
In this particular case, scriptura depends on the JAXP API which is not part of the core JDK Classes. Hence, a NoClassDefFoundError is thrown.
However, when scriptura is placed in the appserver\lib directory, the required JAXP classes are also loaded by the same classloader and scriptura finds the required classes.
Bottom line, if a class is loaded by a classloader, then all the classes referred to by this class (directly or indirectly) must either be loaded by this classloader or its parent classloaders.