无法将套接字从一个活动传递到另一个活动

Question

I was trying to pass my socket connection from one activity to another. Here is my code snippet.

public class MainActivity extends AppCompatActivity {

            //private EditText serverIp;

            private Button connectPhones;

            protected String SERVER_IP_ADDR = "172.16.1.103";

            protected String SERVER_PORT = "8889";

            private boolean connected = false;

            private Handler handler = new Handler();

            Socket socket;

            @Override
            protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.activity_main);

                //serverIp = (EditText) findViewById(R.id.server_ip);
                connectPhones = (Button) findViewById(R.id.connect);
                connectPhones.setOnClickListener(connectListener);
            }


            public Socket getSocket(){
                Log.wtf("MainActivity : ",socket.toString());
                return socket;
            }

            private View.OnClickListener connectListener = new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    if (!connected) {
                        //SERVER_IP_ADDR = SERVER_IP_ADDR.getText().toString();
                        if (!SERVER_IP_ADDR.equals("")) {
                            Thread cThread = new Thread(new ClientThread());
                            cThread.start();
                        }
                    }
                }
            };

            public class ClientThread implements Runnable {

                public void run() {
                    try {
                        InetAddress serverAddr = InetAddress.getByName(SERVER_IP_ADDR);
                        Log.d("ClientActivity", "C: Connecting...");
                        socket = new Socket(serverAddr, Integer.parseInt(SERVER_PORT));
                        connected = true;

                            try {
                                Log.d("ClientActivity", "C: Sending command.");
                                PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket
                                        .getOutputStream())), true);
                                out.println("Hey Server!");
                                Log.d("ClientActivity", "C: Sent.");
                                Intent intent = new Intent(getApplicationContext(), SocketInteraction.class);
                                startActivityForResult(intent,1);
                            } catch (Exception e) {
                                Log.e("ClientActivity", "S: Error", e);
                            }

                        //socket.close();
                        Log.d("ClientActivity", "C: Closed.");
                    } catch (Exception e) {
                        Log.e("ClientActivity", "C: Error", e);
                        connected = false;
                    }
                }

            }
        }

And in other activity, I am calling it like this

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.socketinteraction);
    MainActivity mainActivity = new MainActivity();
    final Socket socket = mainActivity.getSocket();   //returns null pointer

But I don't know why, Its returning a Nullpointerexception. Can someone please assist me on how should I deal with this ?

Answer 1

You need to use Singleton patterns class like the following:

public class SocketSingleton {


private static Socket socket;

public static void setSocket(Socket _socket){
    SocketSingleton.socket=_socket;
}

public static Socket getSocket(){
    return SocketSingleton.socket;
}

and then in your MainActivity use the following:

SocketSingleton.setSocket(socket);
Intent i = new Intent(getActivity().getApplicationContext(), NewActivity.class);
startActivity(intent);

and finally in your NewActivity use the following:

Socket socket = SocketSingleton.getSocket();

Answer 2

I am sorry to say this, but your code is a total mess,

Reasons

1. We should not create an instance of an activity, its the job of android framework to create, we can only start an activity.

2. The second activity has a different layout 'socketinteraction', and the code to create socket is written in the button click listener, which is in your MainActivity layout.

You might want to do something like this,

From your other activity,

1. Add a button in your layout 'socketinteraction'

2. On click of the button,start mainactivity like below,

   startActivityForResult(new Intent(this,MainActivity.class),0)

and always use Intent to communicate between activities. Refer this link, http://developer.android.com/training/basics/intents/result.html

3. Do your task in MainActivity, and once your job is done, set the Result using setResult method,

Refer to this link,

http://stackoverflow.com/questions/920306/sending-data-back-to-the-main-activity-in-android

Answer 3

You can do it quite easily, create and instance of your MainActivity like this.

    public class MainActivity extends Activity {
        public static MainActivity sInstance = null;

        protected void onCreate(Bundle bundle) {
             super.onCreate(...);
             setContentView(...);
             sInstance = this;
        }
    }

Now in your other activity you can call it simply like this.

    if (MainActivity.sInstance != null) {
         final Socket socket = MainActivity.sInstance.getSocket();
    }

this creates your MainActivity a Singleton while its running. Just don't finish it when you go to the next Activity and when you are done and when you are shutting your app down. Release the sInstance like this, Don't forget this part.

    sInstance = null;