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 on this line inStream.readline()

My Intent:

1. This is in a thread that will loop through checking if there is an outMessage, if there is, it will send the message.
2. Next it will check it if there is anything in the in-stream, if there is, it will send it to the handler in my main activity.
3. Lastly, it will sleep for 1 second, then check again.
4. This should allow me to read/write multiple times without needing to close and open the socket.

Problem:

• It is reading and writing better, but still not working properly

What is happening now:

• If outMessage is initialized with a value, upon connection with the server, the socket:
  1. writes and flushes the value (server receives & responds)
  2. updates value of outMessage (to null or to "x" depending on how i have it hard-coded)
  3. reads and shows the response message from the server
  4. re-enters for the next loop
  5. IF i set outMessage to null, it skips over that if statements correctly then hangs; otherwise, if i set outMessage to a string (lets say "x"), it goes through the whole if statement, then hangs.
     • The code it hangs on is either of the inStream.readline() calls (I currently have one commented out).

Additional info: - once connected, I can type in the "send" box, submit (updates the outMessage value), then disconnect. Upon re-connecting, it will read the value and do the sequence again until it get stuck on that same line.

Changes since the referenced question: - Made outMessage and connectionStatus both 'volatile' - added end-of-line delimiters in neccesary places.

Code:

        public void run() { 
            while (connectionStatus != TCP_SOCKET_STATUS_CONNECTED) {
                try {
                    Thread.sleep(500);  
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            while (connectionStatus == TCP_SOCKET_STATUS_CONNECTED) {
                try {   
                    if (outMessage != null){                                            
                        OutStream.writeBytes(outMessage + "\n");                    
                        OutStream.flush();                                          
                        sendMessageToAllUI(0, MAINACTIVITY_SET_TEXT_STATE, "appendText" , "OUT TO SERVER: " + outMessage);
                        outMessage = "x";                                           
                    }                                                           
                    Thread.sleep(100);
 //             if (InStream.readLine().length() > 0) {                             
                        String modifiedSentence = InStream.readLine();              
                        sendMessageToAllUI(0, MAINACTIVITY_SET_TEXT_STATE, "appendText" , "IN FROM SERVER: " + modifiedSentence);
//                  }                                                   
                    Thread.sleep(1000);
                } catch (IOException e) {                               
                    connectionLost();
                    break;
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }                               
        }

The thread that makes the socket:

public void run() {
        setName("AttemptConnectionThread");
        connectionStatus = TCP_SOCKET_STATUS_CONNECTING;
        try {
            SocketAddress sockaddr = new InetSocketAddress(serverIP, port);
            tempSocketClient = new Socket(); // Create an unbound socket

            // This method will block no more than timeoutMs. If the timeout occurs, SocketTimeoutException is thrown.
            tempSocketClient.connect(sockaddr, timeoutMs);
            OutStream = new DataOutputStream(tempSocketClient.getOutputStream());
            InStream = new BufferedReader(new InputStreamReader(tempSocketClient.getInputStream()));
            socketClient = tempSocketClient;
            socketClient.setTcpNoDelay(true);
            connected(); 
        } catch (UnknownHostException e) {
            connectionFailed();
        } catch (SocketTimeoutException e) {
            connectionFailed();
        } catch (IOException e) {
            // Close the socket
            try {
                tempSocketClient.close();
            } catch (IOException e2) {
            }
            connectionFailed();
            return;
        }
    } 

Server:

public static void main(String[] args) throws IOException {
    String clientSentence;
    String capitalizedSentence;
    try {
        ServerSocket welcomeSocket = new ServerSocket(8888);
        SERVERIP = getLocalIpAddress();
        System.out.println("Connected and waiting for client input!\n Listening on IP: " + SERVERIP +"\n\n");
        Socket connectionSocket = welcomeSocket.accept();
        BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
        DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
        while(true)
        {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            clientSentence = inFromClient.readLine();
            System.out.println("clientSentance == " + clientSentence);
            String ip = connectionSocket.getInetAddress().toString().substring(1);
            if(clientSentence != null)
            {
                System.out.println("In from client ("+ip+")("+ System.currentTimeMillis() +"): "+clientSentence);
                capitalizedSentence = clientSentence.toUpperCase() + '\n';
                outToClient.writeBytes(capitalizedSentence + '\n');
                System.out.println("Out to client ("+ip+"): "+capitalizedSentence);
            }
        }
    } catch (IOException e) {
        //if server is already running, it will not open new port but instead re-print the open ports information
         SERVERIP = getLocalIpAddress();
         System.out.println("Connected and waiting for client input!\n");
         System.out.println("Listening on IP: " + SERVERIP +"\n\n");

    }
}

Thanks in advance!

Edits:

• added the server code after updating
• I tried messing around with setting the SoTimout for the socket but took that back out

Answer 1

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.

Answer 2

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).

---

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.