How to replicate struct.pack in java at a low level

Question

I have been working with a client/server where the client is written in Java (Android) and the server is written in C++. I basically need to be able to pass it a dataset and have it interpret that data correctly.

I wrote a prototype in Python, as I know what its supposed to look like there. With struct.pack, it will pack the data and you can specify that it do it in network byte order (Big Endian). This is the prototype that I wrote (and this works with the server).

  5 def _BuildPDUToTx(pduObj):
  6     txData = struct.pack('!i i', pduObj.Src.Type, pduObj.Src.Len)           #pduObj.Src.Type: 1
                                            #pduObj.Src.Len: 16
  7     txData += pduObj.Src.Value                          #pduObj.Src.Value: sourceid
  8     txData += struct.pack('!i i', pduObj.Dst.Type, pduObj.Dst.Len)          #pduObj.Dst.Type: 2
                                            #pduObj.Dst.Len: 14
  9     txData += pduObj.Dst.Value                          #pduObj.Dst.Value: destid
 10     txData += struct.pack('!i i i i', pduObj.Metadata.Type, pduObj.Metadata.Len,\   #pduObj.Metadata.Type: 3
 11                             pduObj.Metadata.Status, pduObj.Metadata.Remaining)  #pduObj.Metadata.Len: 16
                                            #pduObj.Metadata.Status: 0
                                            #pduObj.Metadata.Remaining: 0
 12     txData += struct.pack('!i i', pduObj.Msg.Type, pduObj.Msg.Len)          #pduObj.Msg.Type: 6
                                            #pduObj.Msg.Len: 27
 13     txData += pduObj.Msg.Value                          #pduObj.Msg.Value: This is the message
 14     return(txData)

I ran it with some example code (the example values are the comments on the side), and I get the following if I setup a raw netcat listener on the server side (just to see what the data looks like).

0000000 0000 0100 0000 1000 [6f73 7275 6563 6469] *brackets have "sourceid"
0000010 0000 0200 0000 0e00 [6564 7473 6469] 0000 *brackets have "destid"
0000020 0300 0000 1000 0000 0000 0000 0000 0000
0000030 0600 0000 1b00 [6854 7369 6920 2073 6874 *brackets have "This is the message"
0000040 2065 656d 7373 6761 0065]         

I have tried doing the same (and used the advice of Java equivalent of Python's struct.pack? and Using Java's ByteBuffer to replicate Python's struct.pack).

I used the following code:

client = new Socket(server, port);
ObjectOutputStream dos = new ObjectOutputStream(new BufferedOutputStream(client.getOutputStream()));
ByteBuffer buffer = ByteBuffer.allocate(1000);
dos.writeInt(Src.Type);         // int value 1
dos.writeInt(Src.Len);          // int value 16
dos.writeUTF(Src.Value);        // String value "sourceid"
dos.writeInt(Dst.Type);         // int value 2
dos.writeInt(Dst.Len);          // int value 14
dos.writeUTF(Dst.Value);        // String value "destid"
dos.writeInt(Metadata.Type);        // int value 3
dos.writeInt(Metadata.Len);     // int value 16
dos.writeInt(Metadata.Status);      // int value 0
dos.writeInt(Metadata.Remaining);   // int value 0
dos.writeInt(Msg.Type);         // int value 6
dos.writeInt(Msg.Len);          // int value 27
dos.writeUTF(Msg.Value);        // String value "this is the message"
dos.flush();

InputStream is = client.getInputStream();
byte[] buf = new byte[Constants.ResponseSize];
is.read(buf, 0, Constants.ResponseSize);

client.close();

However, I get the following.

0000000 edac 0500 4f77 0000 0100 0000 1000 0800
0000010 [6f73 7275 6563 6469] 0000 0200 0000 0e00 *brackets have "sourceid"
0000020 0600 [6564 7473 6469] 0000 0300 0000 1000 *brackets have "destid"
0000030 0000 0000 0000 0000 0000 0600 0000 1b00
0000040 1300 [6854 7369 6920 2073 6874 2065 656d *brackets have "This is the message"
0000050 7373 6761 0065]                         
0000055

Java seems to be adding quite a bit of extra. Like the first two bytes dont go with anything I put in the stream. Thats followed with my value of 1 (32 bits) and 16 (the next 32 bits), but then it has 0800 before the "sourceid".

What the heck is Java doing?? And how can I get it to pack more like python?

Answer 1

Use DataOutputStream, not ObjectOutputStream.

ObjectOutputStream is used to ship Java objects from one JVM to another, and is primarily used by java object serialization framework.

DataOutputStream writes everything in Big Endian.

You may have to be careful, though, with writeUTF. By spec, it prepends the length of the byte sequence. This may, or may not, be what you want.