Unless the EU Directive is not in conformance with EU law as interpreted by the ECJ, a member state must implement it as EU law takes precedence even over the constitution of member states.  That is what the pooling of Sovereignty is all about.  Germany is no longer absolutely Sovereign in its own right.  It is Sovereign only insofar as it acts within the EU Treaties it has signed up to, and these are subject to ECJ interpretation, not a German Court.  The most a German Court could do is require a modification of the German Constitution in conformance with EU law.

notes from no w here