public Object answer(InvocationOnMock invocation) throws IOException {
Object args[] = invocation.getArguments();
if (args.length == 1) {
LocatedBlocksWithMetaInfo realAnswer = realNN.open((OpenRequest) args[0]).get();
return new OpenResponse(injectFailureIfNecessary(realAnswer));
} else {
LocatedBlocksWithMetaInfo realAnswer = realNN.openAndFetchMetaInfo((String) args[0],
(Long) args[1], (Long) args[2]);
return injectFailureIfNecessary(realAnswer);
}