Creating device driver packets from Java

Question

Now that I have some spare time on my hands, I decided to create a Java program to connect my XBee (i.e. zigbee) chips to my new SmartThings hub. I found a nice tutorial on doing this by creating the packets by hand (https://nzfalco.jimdofree.com/electronic-projects/xbee-to-smartthings/). My next task is to create a set of Java routines to create, send, receive, and access the required packets (i.e. a sequence of bytes).

Having done similar in C for other projects, my first thought was to simple create a class with the packet structure and send it. Something like this:

class DeviceAnnounce {
    public byte frameId;
    public byte addr64[];
    public byte addr16[];
    public byte capability;
};

Problem is there does not appear to be a way to cast this "structure" to an array of bytes to send to the device.

Next I thought, we have a serialize capability built into the Java runtime. So I added Serializable to the class and used the writeObject() method to convert the instance into a byte stream. Problem here is that writeObject() converts not only your bytes, but includes the definition of the object in the encoding. Works great for reading and writing object to disk, but it's not creating the packet I need to send to the xbee device.

I finally coded it the hard way, explicitly adding a method to my class that creates the byte array.

class DeviceAnnounce {
public DeviceAnnounce(byte frameId, byte[] addr64, byte[] addr16, byte capability) {
    super();
    this.frameId = frameId;
    this.addr64 = addr64;
    this.addr16 = addr16;
    this.capability = capability;
}

public byte frameId;
public byte addr64[];
public byte addr16[];
public byte capability;

byte[] getBytes() throws IOException {
    byte[] data=new byte[12];
    data[0]=frameId;
    data[1]=addr64[7];
    data[2]=addr64[6];
    data[3]=addr64[5];
    data[4]=addr64[4];
    data[5]=addr64[3];
    data[6]=addr64[2];
    data[7]=addr64[1];
    data[8]=addr64[0];
    data[9]=addr16[1];
    data[10]=addr16[0];
    data[11]=capability;
    return data;
}

@Override
public String toString() {
    return "DeviceAnnounce [frameId=" + frameId + ", addr64=" + HexUtils.prettyHexString(addr64) + ", addr16="
            + HexUtils.prettyHexString(addr16) + ", capability=" + capability + "]";
}

}

It works, but I keep thinking there must be a better way. Now the 64 dollar (or maybe bit) question. Is there a way to convert a POJO into a simple byte stream/array?

Answer 1

To build a block of bytes for transmitting, I recommend using the built-in ByteBuffer, which e.g. has helpers for 16-, 32-, and 64-bit integers in big- or little-endian.

You should then store the values as you use them, e.g.

public byte frameId;
public long addr64;
public short addr16;
public byte capability;

byte[] getBytes() throws IOException {
    ByteBuffer buf = ByteBuffer.allocate(12)
            .order(ByteOrder.BIG_ENDIAN/*Network Byte Order*/);
    buf.put(frameId);
    buf.putLong(addr64);
    buf.putShort(addr16);
    buf.put(capability);
    return buf.array(); // or return the ByteBuffer itself
}