Locked socket when performing data exchange

Question

I am trying to carry out server and client code in Java for the client to send a String, the server process and return it and the client receive.

The customer is kept always listening.

But after receiving the String he hangs.

Server class

// porta do servidor
    int serverDoor = 4000;

    // numero maximo de conexões que o servidor aceita
    int maxConnections = 10;

    // servidor socket
    ServerSocket server = null;

    // conexão socket
    Socket connection = null;

    // saida dos dados
    OutputStream output = null;

    // entrada dos dados
    InputStream input = null;

    try {

            server = new ServerSocket(serverDoor, maxConnections);

            while (true) {

            System.out.println("Esperando cliente");

            connection = server.accept();

            // abrindo o stream de saida
            output = connection.getOutputStream();
            output.flush();

            // abrindo o stream de entrada
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(connection.getInputStream()));

            // recebendo
            String teste = bufferedReader.readLine();

            output.write("Teste".getBytes());
            output.flush();

        }

    } catch (IOException e) {

        e.printStackTrace();

    } finally {

        try {

            if (output != null) {
                output.close();
            }

            if (input != null) {
                input.close();
            }

            if (connection != null) {
                connection.close();
            }

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

    }           
}

Customer class

Socket client;
OutputStream output;
InputStream input;

// Step 1: Create a Socket to make connection
client = new Socket(InetAddress.getByName(url), porta);
//client.setSoTimeout(15000); // 5 segundos

// Step 2: Get the input and output streams
output = client.getOutputStream();
output.flush();
input = client.getInputStream();

// Step 3: Process connection
output.write(msg.getBytes());
output.flush();

// recebendo
System.out.println("Antes de receber cliente");
StringBuilder sb = new StringBuilder();
int temp;
while ((temp = input.read()) > -1)
    sb.append((char) temp);
System.out.println("Depois de receber cliente");

// Step 4: Close connection
if (output != null)
    output.close();
if (input != null)
    input.close();
if (client != null)
    client.close();

return sb.toString();

}

After the method of receiving the String it to, I use Windows 8, I don’t know if this can represent something.

Answer 1

You need to close the output on the server so the client understands that the message is over:

// recebendo
String teste = bufferedReader.readLine();

output.write("Teste".getBytes());
output.flush();
output.close();

And the server can also crash if the client does not return an n at the end of the message according to the documentation of the Bufferedreader#readline()

// Step 3: Process connection
output.write(msg.getBytes());
output.write('\n');
output.flush();

A suggestion to improve your code would be to use PrintWriter and BufferedReader client and server. You are using different ways of reading and writing.

Here’s a detailed example.

Answer 2

In the server class the closure of the output stream (output) is in an improper location, Finally. This Finally will only be executed when the scope of Try comes to an end, that is, when the looping ends, but it is infinite, and "never" will be closed and for this reason the client "hangs".

The client is "locked", or better, waiting for more bytes in this stretch:

...
// recebendo
System.out.println("Antes de receber cliente");
StringBuilder sb = new StringBuilder();
int temp;
while ((temp = input.read()) > -1)
        sb.append((char) temp);
System.out.println("Depois de receber cliente");
...

The only condition of termination of the while is read() return -1 (or less), which will normally only occur when there is an end to the data stream via close() on the server (and network problems, etc.). Thus, the input.read() method call freezes, because the server output is still open and as a consequence the client input is waiting for more data and has the impression that "locked";

Finally, the purpose of looping in this question is to treat a connection for each client, as a consequence input and output are instantiated to each Accept() of the Serversocket instance and therefore must be closed at each iteration. To this end, you can close the Connection, as it will be in charge of closing everything that is pending.

Following this reasoning the source code for the server would look like this:

// porta do servidor
int serverDoor = 4000;

// numero maximo de conexões que o servidor aceita
int maxConnections = 10;

// servidor socket
ServerSocket server = null;

try {

    server = new ServerSocket(serverDoor, maxConnections);

    while (true) {

        System.out.println("Esperando cliente");

        // conexão socket, dentro do looping
        Socket connection = null;

        // saida dos dados, dentro do looping
        OutputStream output = null;

        // aceitando a conexão
        connection = server.accept();

        // abrindo o stream de saida
        output = connection.getOutputStream();
        output.flush();

        // abrindo o stream de entrada
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(connection.getInputStream()));

        // recebendo
        String teste = bufferedReader.readLine();

        output.write("Teste".getBytes());
        output.flush();

        // fechando tudo!
        connection.close();
    }

} catch (IOException e) {

    e.printStackTrace();

}