Environment variable expansion in persistence.xml (JPA)

Question

I am developing a Eclipse RCP plugin which uses JPA. I tried to specify the database path via a variable give to the JVM on runtime. The property is set correctly but the database is created in a folder named after the variable name (here: ${DBHOME}).

<property name="javax.persistence.jdbc.url" value="jdbc:derby:${DBHOME};create=true"/>

Is there a possibility to fix this?

Thx