Question

I’m developing a server-client application.
The server is done in Java (PC) and the client in Java. (Android)

I’m having trouble with the following implementation:

Server grabs bitmap -> raw bytes -> TCP -> Client (Async Streams

Now the byte array is delivered in multiple packets of different lengths in the client. So to handle this properly, I should use the prefix method.

To use prefix mode you need to send the length of the message in bytes
as four bytes and then the message

My code

public void sendScreenshot(byte[] buffer) throws IOException {
    OutputStream os = socket.getOutputStream();
    os.write(buffer.length + 1);
    os.write((byte) 0);
    os.write(buffer, 0, buffer.length);
    os.flush();
}

In VB.net, this is achieved in the following code:

Private Sub dat(ByVal dat As String)
    Dim nstream As NetworkStream = sock.GetStream()
    Dim bit As Byte() = System.Text.Encoding.UTF8.GetBytes(dat)
    Dim bw As New BinaryWriter(sock.GetStream())
    bw.Write(bit.Length + 1)
    bw.Write((byte)command)
    bw.Write(bit, 0, bit.length)

End Sub

Any help implementing it in Java is welcome?

Answer 1

Use a DataOutputStream:

DataOutputStream out = new DataOutputStream(os);
out.writeInt(buffer.length + 1);
// This writes a single byte
out.write(0);
out.write(buffer);
out.flush();

The .writeInt() here comes from this part of the text you quoted:

you need to send the length of the message in bytes as four bytes

which means an int. Note that this will write the int in network order. While this is unspecified in your extract, I suppose this is what is expected.

Similarly, on the receiving end, you can use a DataInputStream, read the length as an int and then the payload.