FIX "Reflection not allowed" with Jersey by adding this option to appengine :
    <system-properties>
        <property name="com.sun.xml.bind.v2.bytecode.ClassTailor.noOptimize" value="true" />
    </system-properties>
    source : http://code.google.com/p/googleappengine/issues/detail?id=1267 (thks christia...@gmail.com ! )