Holder<DataStruct> structHolder = new Holder<DataStruct>();
ByteArrayDataSource source = new ByteArrayDataSource("foobar".getBytes(), "application/octet-stream");
DataHandler handler = new DataHandler(source);
DataStruct struct = new DataStruct();
struct.setDataRef(handler);
structHolder.value = struct;
port.echoDataRef(structHolder);
handler = structHolder.value.getDataRef();