I found out a solution. It is very simple.
Just defining the parameters as java array (e.g. String P1). The first value (P1) is the returned value.
At last just set in JDeveloper in the "Edit Method Signature" Dialog the parametermode to OUT.
The dialog can be found by rightclicking on the stored procedure in the dbexport file. You can read this
in Section 6 Publishing Java Classes With Call Specifications -> Setting Parameter Modes in
Oracle Database Java Developer's Guide.