Parse a DatagramPacket after converting it to a by

问题:

I am trying to parse a DatagramPacket that I will receive at a socket. I know the format of the packet I will receive, which is a DHCPREQUEST packet, but I don't think that really matters. For simplicity's sake, let's just consider the first six fields:

First field is the "opcode", which is 1 byte.
Second field is the "hardware type" which is 1 byte.
Third, "hardware address length", 1 byte.
Fourth, "hops", 1 byte.
Fifth, "transaction identifier xid", 4 bytes.
Sixth, "seconds", 2 bytes.

After I receive the packet, my approach is to convert it to a byte array.

DatagramPacket request = new DatagramPacket(new byte[1024], 1024);
socket.receive(request);
byte[] buf = request.getData();

At this point, the packet is stored in the byte array buf as a series of bytes. Since I know what the structure of this byte sequence is, how can I parse it? The one-byte fields are simple enough, but how about the multiple-bit fields? For example, how can I extract bytes 4 to 7, and store them in a variable named xid?

I could manually put each byte into an array:

byte[] xid = new byte[4];
xid[0] = buf[4];
xid[1] = buf[5];
xid[2] = buf[6];
xid[3] = buf[7];

But that's just tedious, and impractical for fields that are hundreds of bytes in length. The String class can parse substrings given an offset and length; is there a similar method for byte arrays in Java?

Or am I somehow making things difficult for myself?

回答1:

The cleanest way to do something like this is probably to use the utility method Arrays.copyOfRange.

回答2:

Wrap the byte array in a ByteArrayOutputStream; wrap a DataInputStream around that; then use the methods of DataInputStream.

回答3:

What you do is write yourself some helper methods to extract 2 byte, 4 byte, etc values from the packet, reading the bytes and assembling them into Java short, int or whatever values.

For example

    public short getShort(byte[] buffer, int offset) {
        return (short) ((buffer[offset] << 8) | buffer[offset + 1]);
    }

Then you use these helper methods as often as you need to. (If you want to be fancy, you could have the methods update an attribute that holds the current position, so that you don't have to pass an offset argument.)

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Alternatively, if you were not worried by the overheads, you could wrap the byte array in ByteArrayInputStream and a DataInputStream, and use the latter's API to read bytes, shorts, ints, and so on. IIRC, DataInputStream assumes that numbers are represented in the stream in "network byte order" ... which is almost certainly what the DHCP spec mandates.

回答4:

I'm a bit late to this, but there's a ByteBuffer class:

ByteBuffer b = ByteBuffer.wrap(request.getData());

byte opcode = b.get();
byte hwtype = b.get();
byte hw_addr_len = b.get();
byte hops = b.get();
int xid = b.getInt();
short seconds = b.getShort();

Or, if you only need a single field:

ByteBuffer b = ByteBuffer.wrap(request.getData());
int xid = b.getInt(4);