Java：TCP套接字连接。 接收null / readline（）的客户端返回null

Question

I am new to java TCP socket. I tried to implement a server and a client. So the server should check input (do something) and send string to client. The client should send string to the server and look for an input string from the server (and do something). Both should loop checking and sending all the time if something new is available. The client can send data to the server, the server receives it an can display/process this data. But the data from the server isn't displayed by the client. Can someone tell me why the client isn't receiving the string from the server? Any better ideas to do endless loop? There will be only one client and one server.

 while true:
 server out------>  send String----->   in client            
        in<-----  sent String <------   out

this is the simplified server part:

public class MainActivity extends Activity {
Socket client;
ServerSocket server;
int serverport = 54321;
String inputData = null;
BufferedReader in;
PrintWriter out;
String outputData;

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Thread(setupConnection).start();    
}

private Runnable setupConnection = new Thread() {
public void run() {
try {
server = new ServerSocket(serverport);
    while (true) {
    client = server.accept();
    in = new BufferedReader(new InputStreamReader(client.getInputStream()));
    out = new PrintWriter(client.getOutputStream(), true);
    inputData = in.readLine();
    InputStream inStream = new ByteArrayInputStream(inputData.getBytes());
    in.close();
        if (inputData != null) {
        System.out.println(TAG + "-----Incoming Message---- " + inputData);
        //this is working String is shown           
        }       }
    out.write("nothing to do?");
    out.flush();
    out.close();
    }

} catch (SocketException e) {
Log.v(TAG, "SocketException: " + e);
e.printStackTrace();
} catch (IOException e) {
Log.v(TAG, "IOException: " + e);
e.printStackTrace();
}
}

the simplified client looks like this:

public class testClass {
public static void main(String[] args) throws IOException, InterruptedException {
Socket socket = null;
String host = "127.0.0.1";
int port = 54321;
PrintWriter out = null;
BufferedReader in = null;

while (true) {
try {
    socket = new Socket(host, port);
    out = new PrintWriter(socket.getOutputStream(), true);
    in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
} catch (UnknownHostException e) {
    System.out.println(TAG + "Error: " + e);
    System.err.println("Don't know about host: localhost.");
    System.exit(1);
} catch (IOException e) {
    System.out.println(TAG + "Error: " + e);
    System.err.println("Couldn't get I/O for " + "the connection to: localhost.");
    System.exit(1);
}

out.println("Hello, is it me you're looking for...");
out.flush();


String input = in.readLine();
System.out.println("Input: " + input);

in.close();
out.close();

}
}
}

Answer 1

If readLine() returns null,the peer has closed the connection, and you must do likewise. And stop reading.

Answer 2

if you want implement this code in android , you faces many problems: you can find the solution in this link: Post JSON in android

in the following code may be fix this problem:

    HttpPost post = new HttpPost("http://xxxxxx");
    DefaultHttpClient httpClient = new DefaultHttpClient();
    post.setEntity(new ByteArrayEntity(json.toString().getBytes()));
    HttpResponse response = httpClient.execute(post);
    return EntityUtils.toString(response.getEntity());

} catch (UnsupportedEncodingException e) {
    e.printStackTrace();
    System.out.println(">>>>>>>" + e.getMessage());
} catch (ClientProtocolException e) {
    e.printStackTrace();
    System.out.println(">>>>>>>" + e.getMessage());
} catch (IOException e) {
    e.printStackTrace();
    System.out.println(">>>>>>>" + e.getMessage());
}