Investigate performance improvement in java MessagePack deserializer

[wtgodbe]

In the below block, we effectively deserialize maps/arrays twice - first to find out how long they are, second to feed that number of bytes into ObjectMapper.readValue. We should investigate if its possible to pass the bytes in from their starting offset, and to find out after the fact how many bytes were read so we can advance the underlying buffer. Perhaps with a custom Reader that keeps track of how many bytes it has read?

aspnetcore/src/SignalR/clients/java/signalr/src/main/java/com/microsoft/signalr/MessagePackHubProtocol.java

Lines 568 to 610 in 8522ba8

	case ARRAY:
	readBytesStart = unpacker.getTotalReadBytes();
	length = unpacker.unpackArrayHeader();
	for (int i = 0; i < length; i++) {
	readValue(unpacker, Object.class, payload, false);
	}
	if (outermostCall) {
	// Check how many bytes we've read, grab that from the payload, and deserialize with objectMapper
	byte[] payloadBytes = payload.array();
	// If itemType was null, we were just in this method to advance the buffer. return null.
	if (itemType == null) {
	return null;
	}
	return objectMapper.readValue(payloadBytes, payload.position() + (int) readBytesStart, (int) (unpacker.getTotalReadBytes() - readBytesStart),
	typeFactory.constructType(itemType));
	} else {
	// This is an inner call to readValue - we just need to read the right number of bytes
	// We can return null, and the outermost call will know how many bytes to give to objectMapper.
	return null;
	}
	case MAP:
	readBytesStart = unpacker.getTotalReadBytes();
	length = unpacker.unpackMapHeader();
	for (int i = 0; i < length; i++) {
	readValue(unpacker, Object.class, payload, false);
	readValue(unpacker, Object.class, payload, false);
	}
	if (outermostCall) {
	// Check how many bytes we've read, grab that from the payload, and deserialize with objectMapper
	byte[] payloadBytes = payload.array();
	byte[] mapBytes = Arrays.copyOfRange(payloadBytes, payload.position() + (int) readBytesStart,
	payload.position() + (int) unpacker.getTotalReadBytes());
	// If itemType was null, we were just in this method to advance the buffer. return null.
	if (itemType == null) {
	return null;
	}
	return objectMapper.readValue(payloadBytes, payload.position() + (int) readBytesStart, (int) (unpacker.getTotalReadBytes() - readBytesStart),
	typeFactory.constructType(itemType));
	} else {
	// This is an inner call to readValue - we just need to read the right number of bytes
	// We can return null, and the outermost call will know how many bytes to give to objectMapper.
	return null;
	}

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