Reading Socket InputStream in a loop when writing byte arrays

Question

This is what you usually do when sending text data

// Receiver code
while (mRun && (response = in.readLine()) != null && socket.isConnected()) {
    // Do stuff                 
}

// Sender code
printWriter.println(mMessage);
printWriter.flush();

but when working with DataOutputStream#write(byte[]) to send byte[], how do you write a while loop to receive sent data.

All I have found is this, but it doesn't loop, so I'm guessing this will just run on the first sent message:

int length = in.readInt();
byte[] data = new byte[length];
in.readFully(data);

How can I achieve this?

PS: yep, I'm new to socket programming.

EDIT: I'm sending a byte array each 3 to 5 seconds. This is what I've got so far.

// In the client side, in order to send byte[]. This is executed each 3 seconds.
if(out != null) {
    try {
        out.writeInt(encrypted.length);
        out.write(encrypted);
        out.writeInt(0);
        out.flush();
        return true;

    } catch (IOException e) {
        e.printStackTrace();

        return false;
    }
}


 // In the serverside, in order to receive byte[] sent from client (also executed 3 to 5 seconds due to bytes being sent at said rate.  "client" being the Socket instance.
while(true && client.isConnected()) {

    byte[] data = null;

    while(true) {
        int length = in.readInt();
        if(length == 0)
            break;

        data = new byte[length];
        in.readFully(data);
    }

    if(data != null) {
        String response = new String(data);

        if(listener != null) {
            listener.onMessageReceived(response);
        }
    }
}

have you even tried the code? why wont it work? your question isnt very clear — user1231232141214124
– user1231232141214124, Commented Jun 9, 2014 at 16:57
cause I don't know what expression to do with the while loop — Christopher Francisco
– Christopher Francisco, Commented Jun 9, 2014 at 16:58

Jon Skeet · Accepted Answer · 2014-06-09 17:12:37Z

2

Assuming you're trying to handle a stream of messages, sounds like what you're missing is a way of specifying (in the stream) how big your messages are (or where they end).

I suggest you just write a prefix before each message, specifying the length:

output.writeInt(data.length);
output.write(data);

Then when reading:

while (true)
{
    int length = input.readInt();
    byte[] buffer = new byte[length];
    input.readFully(buffer, 0, length);
    // Process buffer
}

You'll also need to work out a way of detecting the end of input. DataInputStream doesn't have a clean way of detecting that as far as I can tell. There are various options - the simplest may well be to write out a message of length 0, and break out of the loop if you read a length of 0.

edited Jun 9, 2014 at 17:12

answered Jun 9, 2014 at 17:00

Jon Skeet

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11 Comments

Christopher Francisco Over a year ago

I'm guessing when you do bufferedReader.readLine() it keeps waiting for a break line. So what you are saying is that I should loop until a character sequence is met?

Jon Skeet Over a year ago

@ChristopherFrancisco: No, I didn't say that at all - if you're writing a byte array, there's no such thing as a character sequence. You could use a byte sequence instead, but usually it's better to just use a length prefix instead.

Christopher Francisco Over a year ago

I see. Where should I put the if statement to break the loop when the length is 0? after readFully()?

Christopher Francisco Over a year ago

Ok, now then, in the sending part, I'd put output.writeInt(0) after output.write(byte[]) right?

Jon Skeet Over a year ago

@ChristopherFrancisco: Well you'd put it outside whatever loop you've got - basically when you've sent all the messages. (If you're only sending a single message, you don't need any of this...)

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