public class LoginActivity extends Activity {
    EditText ET_NAME, ET_PASS;
    String login_name, login_pass;

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

        ET_NAME = findViewById(R.id.user_name);
        ET_PASS = findViewById(R.id.user_pass);
    }

    public void userLogin(View view) {
        login_name = ET_NAME.getText().toString();
        login_pass = ET_PASS.getText().toString();
        String method = "login";
        BackgroundTask backgroundTask = new BackgroundTask(this);
        backgroundTask.execute(method, login_name, login_pass);
    }

    public void userReg(View view) {
        startActivity(new Intent(this, RegisterActivity.class));
    }
}

public class BackgroundTask extends AsyncTask<String, Void, String> {
    AlertDialog alertDialog;
    Context ctx;

    public BackgroundTask(Context ctx) {
        this.ctx = ctx;
    }

    @Override
    protected void onPreExecute() {
        alertDialog = new AlertDialog.Builder(ctx).create();
        alertDialog.setTitle("Login information");
    }

    @Override
    protected String doInBackground(String... params) {
        String reg_url = "http://192.168.111.1/webapp/register.php";
        String login_url = "http://192.168.111.1/webapp/login.php";
        String method = params[0];
        if (method.equals("register")) {
            String name = params[1];
            String user_name = params[2];
            String user_pass = params[3];

            try {
                URL url = new URL(reg_url);
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                httpURLConnection.setRequestMethod("POST");
                httpURLConnection.setDoOutput(true);
                OutputStream OS = httpURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(OS, "UTF-8"));
                String data = URLEncoder.encode("user", "UTF-8") + "=" + URLEncoder.encode(name, "UTF-8") + "&" +
                        URLEncoder.encode("user_name", "UTF-8") + "=" + URLEncoder.encode(user_name, "UTF-8") + "&" +
                        URLEncoder.encode("user_pass", "UTF-8") + "=" + URLEncoder.encode(user_pass, "UTF-8");
                bufferedWriter.write(data);
                bufferedWriter.flush();
                bufferedWriter.close();
                OS.close();
                InputStream IS = httpURLConnection.getInputStream();
                IS.close();
                return "Register Success...";
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        } else if (method.equals("login")) {
            String login_name = params[1];
            String login_pass = params[2];
            try {
                URL url = new URL(login_url);
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                httpURLConnection.setRequestMethod("POST");
                httpURLConnection.setDoOutput(true);
                httpURLConnection.setDoInput(true);
                OutputStream outputStream = httpURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String data = URLEncoder.encode("login_name", "UTF-8") + "=" + URLEncoder.encode(login_name, "UTF-8") + "&" +
                        URLEncoder.encode("login_pass", "UTF-8") + "=" + URLEncoder.encode(login_pass, "UTF-8");
                bufferedWriter.write(data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();

                InputStream inputStream = httpURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String response = "";
                String line = "";
                while ((line = bufferedReader.readLine()) != null) {
                    response += line;
                }
                bufferedReader.close();
                inputStream.close();
                httpURLConnection.disconnect();
                return response;
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onPostExecute(String result) {
        if (result.equals("Register Success..."))
            Toast.makeText(ctx, result, Toast.LENGTH_SHORT).show();
        else {
            alertDialog.setMessage(result);
            alertDialog.show();
        }
    }
}

public class MainActivity extends AppCompatActivity {
    String JSON_STRING;
    String json_string;


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

    public void getJSON(View view) {
        BackgroundTask backgroundTask = new BackgroundTask(this);
        backgroundTask.execute();
    }

    public void parseJSON(View view) {
        if (json_string == null) {
            Toast.makeText(getApplicationContext(), "First Get JSON", Toast.LENGTH_SHORT).show();
        } else {
            Intent intent = new Intent(getApplicationContext(), DisplayListView.class);
            intent.putExtra("json_data", json_string);
            startActivity(intent);
        }
    }

    class BackgroundTask extends AsyncTask<Void, Void, String> {
        String json_url;
        Context context;

        public BackgroundTask(Context context) {
            this.context = context;
        }

        @Override
        protected String doInBackground(Void... aVoid) {
            try {
                URL url = new URL(json_url);
                HttpURLConnection hucon = (HttpURLConnection) url.openConnection();
                InputStream is = hucon.getInputStream();
                BufferedReader br = new BufferedReader(new InputStreamReader(is));
                StringBuilder sb = new StringBuilder();
                while ((JSON_STRING = br.readLine()) != null) {
                    sb.append(JSON_STRING + "n");
                }

                br.close();
                is.close();
                hucon.disconnect();
                return sb.toString().trim();
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPreExecute() {
            json_url = "http://192.168.111.1/jsonparsing/json_get_data.php";
        }

        @Override
        protected void onProgressUpdate(Void... values) {
            super.onProgressUpdate(values);
        }

        @Override
        protected void onPostExecute(String result) {
            TextView textView = (TextView) findViewById(R.id.tv_json_value);
            textView.setText(result);
            json_string = result;
        }
    }
}

<? php

        $host = "localhost";
    $user="root";
    $password="";
    $db="app_db";

    $con=mysqli_connect($host,$user,$password,$db);


    $sql="select * from user;";

    $result=mysqli_query($con,$sql);
    $response=array();

    while($row=mysqli_fetch_array($result)){
    array_push($response,array('full_name'=>$row[1],'user_name'=>$row[2],'email'=>$row[3],'phone'=>$row[4]));
    }

    mysqli_close($con);
    echo json_encode(array("server_response"=>$response));
    ?>