Android TCP app hanging on inStream.readline()

Question

This is a continuation of this question because it my orginal question was answered, but it did not solve the bug.

Question:

• How do I fix the code hanging

Answer 1

You need to check if there is data available:

if (InStream.available > 0) {                                                      
   String modifiedSentence = InStream.readLine();
   sendMessageToAllUI(0, MAINACTIVITY_SET_TEXT_STATE, "appendText" , "IN FROM SERVER: " + modifiedSentence); 
}

But to be honest, even that is not ideal because you have no gurantee that the eond-of-line will have been received. If the server sends a few bytes but never sends the end-of-line then you will still be blocking forever. Production socket code should never rely on readLine but instead read into a buffer and check that buffer for end-of-line (or whatever criteria your protocol needs).

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Didn't read closely enough, I thought InStream was an InputStream instance. InputStream has available. InputStreamReader has ready (which in turn calls InputStream.available. As long as you keep a refernce to either of these then you can see if data is available to be read.

Answer 2

Your server is specifically designed to receive exactly one line from a client and send exactly one line back. Look at the code:

    while (true) {
        Socket connectionSocket = welcomeSocket.accept();
        BufferedReader inFromClient = new BufferedReader(
                new InputStreamReader(connectionSocket.getInputStream()));
        DataOutputStream outToClient = new DataOutputStream(
                connectionSocket.getOutputStream());

        clientSentence = inFromClient.readLine();
        String ip = connectionSocket.getInetAddress().toString()
                .substring(1);
        System.out.println("In from client (" + ip + "): "
                + clientSentence);
        if (clientSentence != null) {
            capitalizedSentence = clientSentence.toUpperCase() + '\n';
            System.out.println("Out to client (" + ip + "): "
                    + capitalizedSentence);
            outToClient.writeBytes(capitalizedSentence + "\n");
        }

Notice that inside the loop it accepts a new connection, reads exactly one line, and then writes exactly one line. It doesn't close the connection. It doesn't sanely end the conversation. It just stops reading.

A client that worked with this server would have to connect, send exactly one line, read exactly one line back, and then the client would have to close the connection. Your client doesn't do that. Why? Because you had no idea that's what you had to do. Why? Because you had no design ... no plan.

So that's your specific issue. But please, let me urge you to take a huge step back and totally change your approach. Before you write a single line of code, please actually design and specify a protocol at the byte level. The protocol should say what data is sent, how messages are delimited, who sends when, who closes the connection, and so on.

Otherwise, it's impossible to debug your code. Looking at the server code above, is it correct? Well, who knows. Because it's unclear what it's supposed to do. When you wrote the client, you assumed the server behaved one way. Was that assumption valid? Is the server broken? Who knows, because there's no specification of what the server is supposed to do.