Java NIO BufferUnderflowException when using DatagramChannels and Serialization

Question

I am trying to write a program to send and receive a serialized object over a Datagram channel using Java nio, but I am receiving a BufferUnderFlow exception when trying to read my object from the ObjectInputStream.

Currently I have the following code:

Sender:

public void sendMessage(InetSocketAddress destination) {
    writeBuffer = ByteBuffer.allocate(64000);
    writeBuffer.clear();
    /*
     * Put new MyObject into a ByteArrayOutputStream and put that
     * into the writeBuffer to be sent.
     */
    MyObject myObject = new MyObject();
    byteArrayOS = new ByteArrayOutputStream(writeBuffer.capacity());
    objectOS = new ObjectOutputStream(byteArrayOS);
    objectOS.writeObject(myObject);
    objectOS.flush();

    writeBuffer.put(byteArrayOS.toByteArray());
    writeBuffer.flip();
    channel.send(writeBuffer, new InetSocketAddress(ipAddress, portNum));
    // Channel is bound to correct IP / Port
}

Receiver:

public void read() {
  try { //blah blah
    channel = DatagramChannel.open();
    channel.socket().bind(new InetSocketAddress(readPort));
    channel.configureBlocking(true);

    ByteArrayInputStream byteArrayIS = null;
    ObjectInputStream objectIS = null;
    MyObject object;
    while (true) {
        inputBuffer.clear();
    client.getChannel().receive(inputBuffer);
    byte[] data = new byte[inputBuffer.capacity()];
    inputBuffer.get(data);
        /*
         * Troubleshooting: inputBuffer is returning with something
         */
        byteArrayIS = new ByteArrayInputStream(data);
        objectIS = new ObjectInputStream(byteArrayIS);
        myObject = objectIS.readObject(); // Throwing BufferUnderflowException here
        // Process object
        // ...
    }
  } catch (BlahBlah e) {}
}

This is the exception I am getting:

Exception in thread "main" java.nio.BufferUnderflowException
        at java.nio.HeapByteBuffer.get(HeapByteBuffer.java:145)
        at java.nio.ByteBuffer.get(ByteBuffer.java:694)
        at package.Class.main(Class.java:493)

What is causing this BufferUnderflowException to be thrown? I can't figure it out. Both ByteBuffers are allocated the same amount of space, and the writeBuffer is not overflowing.

Answer 1

You have to flip() the buffer to get data out of it. NB you should be using limit(), not capacity(), in that line.