mvn:jetty unable to load WebAppContext - maven

Question

I have a weird situation, when trying to run jetty from eclipse. I updated my old project from jetty 7 to jetty 9.3.7.v20160115. But, now when starting the jetty:run with m2eclipse I got the following exception:
java.lang.IllegalArgumentException: Object of class 'org.eclipse.jetty.maven.plugin.JettyWebAppContext' is not of type 'org.eclipse.jetty.webapp.WebAppContext'. Object Class and type Class are from different loaders. in file:/xx/WebApp/src/main/webapp/WEB-INF/jetty-env.xml
at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.configure(XmlConfiguration.java:295)
at org.eclipse.jetty.xml.XmlConfiguration.configure(XmlConfiguration.java:245)
at org.eclipse.jetty.plus.webapp.EnvConfiguration.configure(EnvConfiguration.java:116)
From the message I can see that the two classes are not the same. But from the documentation I need to use the class org.eclipse.jetty.webapp.WebAppContext and org.eclipse.jetty.maven.plugin.JettyWebAppContext is a subclass of WebAppContext. So it is not clear for me if the condition in line 292
oClass.isInstance(obj)
is correctly used there.
I have defined in jetty-env.xml file the following entries:
<Configure id='wac' class="org.eclipse.jetty.webapp.WebAppContext">
<New id="resInspector" class="org.eclipse.jetty.plus.jndi.Resource">
<Arg><Ref refid="wac"/></Arg>
<Arg>jdbc/xxx</Arg>
<Arg>
<New class="org.apache.commons.dbcp.BasicDataSource">
<Set name="driverClassName">org.postgresql.Driver</Set>
<Set name="url">jdbc:postgresql://xx:5432/xx</Set>
<Set name="username">xx</Set>
<Set name="password">xx</Set>
<Set name="maxActive">100</Set>
<Set name="maxIdle">30</Set>
<Set name="maxWait">-1</Set>
<Set name="defaultAutoCommit">false</Set>
</New>
</Arg>
</New>
</Configure>
Next I added in web.xml those entries
<resource-ref>
<description> JNDI resource</description>
<res-ref-name>jdbc/xxx</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
thanks for any help.
Markus

Answer 1

The error message says :
Object Class and type Class are from different loaders
So the object class obj.getClass() and the type Class oClass may be equal or the same, but loaded from different classloaders.
Possible reason could be that you have this class twice in your classpath. Once through dependencies of the jetty plugin and once in your pom.
Use mvn dependecy:tree to check that and exclude the dependency of your pom.
You also may try to change the class in
<Configure id='wac' class="org.eclipse.jetty.webapp.WebAppContext">
to
<Configure id='wac' class="org.eclipse.jetty.maven.plugin.JettyWebAppContext">

Answer 2

indeed it was related to some duplicated jar files. In my maven pom file I had unnecessary files (jetty-plus,jetty-annotations,jetty-jndi,jetty-xml).
Removing those jar file from my maven file solved the problem.
thanks for your suggestion