have read quite a lot about this error, the reason is 1) more than one jar containing this class have been include into the classpath, 2)include the jar twice or more, but in my case, this class is jdk class, and I have search no this class have been found under my application. any clue would be very helpful. exception stack: 2012-10-31 14:09:58,319 WARN [org.jboss.detailed.classloader.ClassLoaderManager] (http-0.0.0.0-8080-1:) Unexpected error during load of:javax.sql.DataSource java.lang.LinkageError: loader constraint violation: loader (instance of org/jboss/classloader/spi/base/BaseClassLoader) previously initiated loading for a different type with name "javax/sql/DataSource" at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631) at java.lang.ClassLoader.defineClass(ClassLoader.java:615) at org.jboss.classloader.spi.base.BaseClassLoader.access$200(BaseClassLoader.java:67) at org.jboss.classloader.spi.base.BaseClassLoader$2.run(BaseClassLoader.java:633) at org.jboss.classloader.spi.base.BaseClassLoader$2.run(BaseClassLoader.java:592) at java.security.AccessController.doPrivileged(Native Method) at org.jboss.classloader.spi.base.BaseClassLoader.loadClassLocally(BaseClassLoader.java:591) at org.jboss.classloader.spi.base.BaseClassLoader.loadClassLocally(BaseClassLoader.java:568) at org.jboss.classloader.spi.base.BaseDelegateLoader.loadClass(BaseDelegateLoader.java:135) at org.jboss.classloader.spi.filter.FilteredDelegateLoader.loadClass(FilteredDelegateLoader.java:131) at org.jboss.classloader.spi.base.ClassLoadingTask$ThreadTask.run(ClassLoadingTask.java:455) at org.jboss.classloader.spi.base.ClassLoaderManager.nextTask(ClassLoaderManager.java:267) at org.jboss.classloader.spi.base.ClassLoaderManager.process(ClassLoaderManager.java:166) at org.jboss.classloader.spi.base.BaseClassLoaderDomain.loadClass(BaseClassLoaderDomain.java:287) at org.jboss.classloader.spi.base.BaseClassLoaderDomain.loadClass(BaseClassLoaderDomain.java:1163) at org.jboss.classloader.spi.base.BaseClassLoader.loadClassFromDomain(BaseClassLoader.java:862) at org.jboss.classloader.spi.base.BaseClassLoader.doLoadClass(BaseClassLoader.java:502) at org.jboss.classloader.spi.base.BaseClassLoader.loadClass(BaseClassLoader.java:447) at java.lang.ClassLoader.loadClass(ClassLoader.java:247) at com.lombardrisk.webgui.dwr.ajax_search.UmbrellaNettingFundsSearch.getBranchesByAgreementId(UmbrellaNettingFundsSearch.java:199) at com.lombardrisk.webgui.dwr.ajax_search.UmbrellaNettingFundsSearch.buildConditionOfPrinAndCpty(UmbrellaNettingFundsSearch.java:177) at com.lombardrisk.webgui.dwr.ajax_search.UmbrellaNettingFundsSearch.getFunds(UmbrellaNettingFundsSearch.java:58) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.directwebremoting.impl.ExecuteAjaxFilter.doFilter(ExecuteAjaxFilter.java:34) at org.directwebremoting.impl.DefaultRemoter$1.doFilter(DefaultRemoter.java:428) at org.directwebremoting.impl.DefaultRemoter.execute(DefaultRemoter.java:431) at org.directwebremoting.impl.DefaultRemoter.execute(DefaultRemoter.java:283) at org.directwebremoting.servlet.PlainCallHandler.handle(PlainCallHandler.java:52) at org.directwebremoting.servlet.UrlProcessor.handle(UrlProcessor.java:101) at org.directwebremoting.servlet.DwrServlet.doPost(DwrServlet.java:146) at javax.servlet.http.HttpServlet.service(HttpServlet.java:637) at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at com.lombardrisk.webgui.filter.ValidRequestFilter.doFilter(ValidRequestFilter.java:41) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:235) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191) at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:183) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:525) at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:95) at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.process(SecurityContextEstablishmentValve.java:126) at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:70) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) at org.jboss.web.tomcat.service.request.ActiveRequestResponseCacheValve.internalProcess(ActiveRequestResponseCacheValve.java:74) at org.jboss.web.tomcat.service.request.ActiveRequestResponseCacheValve.invoke(ActiveRequestResponseCacheValve.java:47) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:330) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:829) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:599) at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:451) at java.lang.Thread.run(Thread.java:662)