To use prefer-application-packages in weblogic.xml, it looks like this:
<weblogic-web-app xmlns="http://xmlns.oracle.com/weblogic/weblogic-web-app" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd http://xmlns.oracle.com/weblogic/weblogic-web-app http://xmlns.oracle.com/weblogic/weblogic-web-app/1.2/weblogic-web-app.xsd">
<jsp-descriptor>
<keepgenerated>true</keepgenerated>
<debug>true</debug>
</jsp-descriptor>
<context-root>/quicktest</context-root>
<container-descriptor>
<prefer-application-packages>
<package-name>org.codehaus.jackson.*</package-name>
<package-name>org.codehaus.jettison.*</package-name>
<package-name>org.objectweb.asm.*</package-name>
</prefer-application-packages>
</container-descriptor>
</weblogic-web-app>