All I was trying to upgrade my application to use the latest 4.0.2 release but I seem to be getting errors when my application is calling import org.drools.compiler.DroolsParserException; import org.drools.compiler.PackageBuilder; import org.drools.rule.Package; import org.drools.rule.Rule; PackageBuilder builder = new PackageBuilder();new PackageBuilder(); here is the error thrown: java.lang.IncompatibleClassChangeError: Implementing class at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:620) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124) at org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:1817) at org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:872) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1325) at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1204) at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319) at org.drools.rule.builder.dialect.mvel.MVELDialect.<init>(MVELDialect.java:78) at org.drools.rule.builder.dialect.mvel.MVELDialectConfiguration.getDialect(MVELDialectConfiguration.java:26) at org.drools.compiler.PackageBuilderConfiguration.buildDialectRegistry(PackageBuilderConfiguration.java:166) at org.drools.compiler.PackageBuilder.<init>(PackageBuilder.java:142) at org.drools.compiler.PackageBuilder.<init>(PackageBuilder.java:86) at com.thomcomp.generate.action.RulesReader.readPackage(RulesReader.java:25) at com.thomcomp.generate.action.RulesReader.getRules(RulesReader.java:55) at com.thomcomp.generate.action.GetRulesDef.execute(GetRulesDef.java:47) at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419) at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224) at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194) at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414) at javax.servlet.http.HttpServlet.service(HttpServlet.java:690) at javax.servlet.http.HttpServlet.service(HttpServlet.java:803) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581) at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447) at java.lang.Thread.run(Thread.java:595) Did I miss something I can't seem to figure out where to start looking. some advise would be appreciated. running under tomcat 6 with jdk1.5.0_09