In this post we shall concentrate on just writing the ProtocolDecoder

The code snippet using SNMP4J library is

public class SNMP4JCodec extends ProtocolDecoderAdapter {

 static Logger logger = LoggerFactory.getLogger(SNMP4JCodec.class);

 public void decode(IoSession ioSession, IoBuffer ioBuffer,
                    ProtocolDecoderOutput protocolDecoderOutput) throws Exception {
     ByteBuffer pduBuffer = ioBuffer.buf();
     // Decode the bytes using SNMP4J API's
     PDU pdu = new PDU();
     try {
         BERInputStream berStream = new BERInputStream(pduBuffer);
         BER.MutableByte mutableByte = new BER.MutableByte();
         int length = BER.decodeHeader(berStream, mutableByte);
         int startPos = (int)berStream.getPosition();

         if (mutableByte.getValue() != BER.SEQUENCE) {
           String txt = "SNMPv2c PDU must start with a SEQUENCE";
           throw new IOException(txt);
         }
         Integer32 version = new Integer32();
         version.decodeBER(berStream);

         // decode community string
         OctetString securityName = new OctetString();
         securityName.decodeBER(berStream);

         // decode the remaining PDU
         pdu.decodeBER(berStream);
         logger.debug("PDU - "+pdu);
     } catch (Exception ex) {
         ex.printStackTrace();
     }
     protocolDecoderOutput.write(pdu);
 }
}

The code converts ByteBuffer into an SNMP PDU using SNMP4J  library. Similarly, we can use Adventnet or joesnmp to convert the byte stream into SNMP PDU's

NOTE: Custome Error handling need to be implemented, as per the strategy