How can I connect between android client and python-Flask server, using REST API, without localhost communication?

Question

I want to build a simple android mobile app, that will communicate through REST API with a python-Flask server -

So the android client will run on my Sumsung Galaxy J7 Prime phone (using USB connection through an android-studio script which runs on my computer), and the python-Flask server will run on my computer (using running python-Flask file through PyCharm workspace).

All i want to do is to pass a simple message between those client and server, with a regular connection (not localhost).

This is the simple server code:

from flask import Flask
app = Flask(__name__)

@app.route("/RESTfulExample/json/product/get")
def hello():
    return "Hello World !!"

if __name__ == "__main__":
    app.run(host='0.0.0.0', debug=True)

This code works well when i run this server, and enter the URL address with google chrome URL: http://127.0.0.1:5000/RESTfulExample/json/product/get

It works fine also when I run a script in Jave which sends this URL to the server (with localhost communication):

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class firstJava {

    public static void main(String[] args) {

      try {

        URL url = new URL("http://localhost:5000/RESTfulExample/json/product/get");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setRequestProperty("Accept", "application/json");

        if (conn.getResponseCode() != 200) {
            throw new RuntimeException("Failed : HTTP error code : "
                    + conn.getResponseCode());
        }

        BufferedReader br = new BufferedReader(new InputStreamReader(
            (conn.getInputStream())));

        String output;
        System.out.println("Output from Server .... \n");
        while ((output = br.readLine()) != null) {
            System.out.println(output);
        }

        conn.disconnect();

      } catch (MalformedURLException e) {

        e.printStackTrace();

      } catch (IOException e) {

        e.printStackTrace();

      }

    }

}

The output is:

Output from Server .... 

Hello World !!

The problem comes when i use the exact same code in the my andriod-studio project, except of changing the IP address from localhost(127.0.0.1) to the IP of my computer (10.0.0.54):

public void buttonClicked (View view){

        EditText editText = (EditText) findViewById(R.id.passwordEditText);
        try {

            URL url = new URL("http://10.0.0.54:5000/RESTfulExample/json/product/get");
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setRequestProperty("Accept", "application/json");

            System.out.println(conn.getResponseCode());
            if (conn.getResponseCode() != 200) {
                throw new RuntimeException("Failed : HTTP error code : "
                        + conn.getResponseCode());
            }

            BufferedReader br = new BufferedReader(new InputStreamReader(
                    (conn.getInputStream())));

            String output;
            System.out.println("Output from Server .... \n");
            while ((output = br.readLine()) != null) {
                editText.setText(output);
            }

            conn.disconnect();

        } catch (MalformedURLException e) {

            e.printStackTrace();

        } catch (IOException e) {

            e.printStackTrace();

        }

}

When I run this script in android studio, the application in my phone collapses, and in the console in the android-studio, the error lines are those:

I/System.out: (HTTPLog)-Static: isSBSettingEnabled false
I/System.out: (HTTPLog)-Static: isSBSettingEnabled false
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.owner.firstapp, PID: 25703
                  java.lang.IllegalStateException: Could not execute method for android:onClick
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                      at android.view.View.performClick(View.java:5721)
                      at android.widget.TextView.performClick(TextView.java:10931)
                      at android.view.View$PerformClick.run(View.java:22620)
                      at android.os.Handler.handleCallback(Handler.java:739)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:148)
                      at android.app.ActivityThread.main(ActivityThread.java:7409)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
                   Caused by: java.lang.reflect.InvocationTargetException
                      at java.lang.reflect.Method.invoke(Native Method)
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                      at android.view.View.performClick(View.java:5721) 
                      at android.widget.TextView.performClick(TextView.java:10931) 
                      at android.view.View$PerformClick.run(View.java:22620) 
                      at android.os.Handler.handleCallback(Handler.java:739) 
                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                      at android.os.Looper.loop(Looper.java:148) 
                      at android.app.ActivityThread.main(ActivityThread.java:7409) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 
                   Caused by: android.os.NetworkOnMainThreadException
                      at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1273)
                      at libcore.io.BlockGuardOs.connect(BlockGuardOs.java:110)
                      at libcore.io.IoBridge.connectErrno(IoBridge.java:137)
                      at libcore.io.IoBridge.connect(IoBridge.java:122)
                      at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:183)
                      at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:452)
                      at java.net.Socket.connect(Socket.java:884)
                      at com.android.okhttp.internal.Platform.connectSocket(Platform.java:117)
                      at com.android.okhttp.internal.http.SocketConnector.connectRawSocket(SocketConnector.java:438)
                      at com.android.okhttp.internal.http.SocketConnector.connectCleartext(SocketConnector.java:105)
                      at com.android.okhttp.Connection.connect(Connection.java:1333)
                      at com.android.okhttp.Connection.connectAndSetOwner(Connection.java:1412)
                      at com.android.okhttp.OkHttpClient$1.connectAndSetOwner(OkHttpClient.java:131)
                      at com.android.okhttp.internal.http.HttpEngine.nextConnection(HttpEngine.java:485)
                      at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:466)
                      at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:372)
                      at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:476)
                      at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getResponse(HttpURLConnectionImpl.java:418)
                      at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getResponseCode(HttpURLConnectionImpl.java:540)

                      at com.example.owner.firstapp.MainActivity.buttonClicked(MainActivity.java:44)

                      at java.lang.reflect.Method.invoke(Native Method) 
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                      at android.view.View.performClick(View.java:5721) 
                      at android.widget.TextView.performClick(TextView.java:10931) 
                      at android.view.View$PerformClick.run(View.java:22620) 
                      at android.os.Handler.handleCallback(Handler.java:739) 
                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                      at android.os.Looper.loop(Looper.java:148) 
                      at android.app.ActivityThread.main(ActivityThread.java:7409) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 
I/Process: Sending signal. PID: 25703 SIG: 9
Application terminated.

As the separated line above tells, the problem is with line 44 - System.out.println(conn.getResponseCode()); which means, that for some reason, the conn.getResponseCode() isn't work.

So i understand that the problem occures when i stop use the localhost connection and move to a regular connection between 2 devices. I just don't know how to fix it.

I didn't know i will stuck so much time with a simple server-client communication.. I will glad if someone here will help me solve this simple problem.

Thank you very much in advance, and sorry for my bad english :)

Answer 1

I know you posted this few months ago, but have you thought enabling internet permissions in your manifest file ?