GWT, JPA, Derby and Datanucleus Exception

Hi,

I have serious trouble setting up my GWT Project.
I have to use Derby and JPA.

I installed the eclipselink plugin and tried the following:

Set up a new GWT Project (with GWT 1.7.1). Didn't touch anything there
yet.
Then converted it into a JPA Project. (Which added persistence.xml and
some Libraries)
Created a annotated class. And changed the standard
GreetingServiceImpl.java which is installed with the new web project
wizard. In it I get a EntityManagerFactory and create a Entity out of
it.

That's all i want to do for now.

My persistence.xml looks like that:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/
persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
<persistence-unit name="TestProject">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>myproject.TestClass</class>
<properties>
<property name="eclipselink.jdbc.driver"
value="org.apache.derby.jdbc.ClientDriver"/>
<property name="eclipselink.jdbc.url" value="jdbc:derby://localhost:
1527/sample;create=true"/>
<property name="eclipselink.jdbc.user" value="app"/>
<property name="eclipselink.jdbc.password" value="app"/>
<property name="eclipselink.ddl-generation" value="create-tables"/>
</properties>
</persistence-unit>
</persistence>

But when i try to run it, i get this Exception:

Caused by: org.datanucleus.exceptions.NucleusUserException: No
available StoreManager found for the datastore URL key "". Please make
sure you have all relevant plugins in the CLASSPATH (e.g datanucleus-
rdbms?, datanucleus-db4o?), and consider setting the persistence
property "datanucleus.storeManagerType" to the type of store you are
using e.g rdbms, db4o

I've been looking over the web for a solution the last 12 hours, but
just don't seem to find the right thing.

Can somebody help me?

Regards,
Nik

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