Partition Key Cannot be Send Out by Producer

[Haoming Zhang]

Hi all,

I'm a beginner of Kafka, currently I'm stuck by how to send out a KeyedMessage 
by producer. I would like to design a partition
function to route the message based on the key, but the producer cannot send 
the KeyedMessage and I got this exception:
java.lang.ClassCastException: [B cannot be cast to java.lang.String

What I tried is hardcode a partition key ( I tried String and Integer, 
currently it is Integer ), then convert the partition key to Byte Array:
          val converter = new DataTypeConvertion
          val hardKey = 2
          val partkey = converter.intToByteArray(hardKey)

 Then create a KeyedMessage by the following function:

  private def toMessage(value: Val, key: Option[Key] = None, topic: 
Option[String] = None): KeyedMessage[Key, Val] = {
    val t = topic.get
    require(!t.isEmpty, "Topic must not be empty")
    key match {
      case Some(key) => new KeyedMessage(t, key, value)
      case _ => new KeyedMessage(t, value)
    }
  }

Then try to send the KeyedMessage by a Kafka producer:

  def send(key: Key, value: Val, topic: Option[String] = None) {
    val msg = toMessage(value, Option(key), topic)
    print(msg + "\n")
    print("msg.key" + msg.key + "\n")
    print("msg.message" + msg.message + "\n")
    print("msg.partKey" + msg.partKey + "\n")
    print("msg.topic" + msg.topic + "\n")
    try {
      p.send(msg) //P is an instance of producer, exception happens in this line
    } catch {
      case e: Exception =>
        e.printStackTrace()
        System.exit(1)
    }
  }

As you can see, I added many print statement in the above function, and the 
following is the output of above function:
KeyedMessage(testingInput,[B@7ad40950,[B@7ad40950,[B@7ce18764)
msg.key: [B@7ad40950
msg.message: [B@7ce18764
msg.partKey: [B@7ad40950
msg.topic: testingInput

The key of KeyedMessage is displayed as [B@7ad40950 , I think it is a memory 
address and the exception (java.lang.ClassCastException: [B cannot be cast to 
java.lang.String) happens when "send" function try to convert the Byte Array to 
String.

Am I wrong on creating a key in Byte Array type?
Some examples of how to use KeyedMessage will be great!

Regards,
Haoming