How to connect Node Js server with Android Studio app?

Question

I'm trying to establish connection between an android studio and a node js server, but I'm kinda stuck. I pretty sure I've done many things wrong, but my one error message I can start searching for solution from is a listen EADDRINUSE :::3000 error on the Node server side. Here are all the codes from both sides, I really hope some more experienced eyes can point out the issue and give me a direction to continue at. Thanks in advance!

Node JS code:

var express = require('express');
var app = express();

app.on('listening',function(){
    console.log('ok, server is running');
});

app.post('/postdata', (req, res) => {
    var data = req.body.data; // your data
    // do something with that data (write to a DB, for instance)
    console.log(data)
    res.status(200).json({
        message: "Data received successfully"
    });
});
app.listen(3000);

Android Studio code(MainActivity.java):

package com.example.tbg_studio;

import androidx.appcompat.app.AppCompatActivity;

import android.app.DownloadManager;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonObjectRequest;
import com.android.volley.toolbox.Volley;

import org.json.JSONException;
import org.json.JSONObject;

import java.util.HashMap;

public class MainActivity extends AppCompatActivity {
    Button RequestButton; // button which on clicking, sends the request
    TextView DisplayText; // a text field to display the request response
    EditText DataField; // a text field where the data to be sent is entered

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

        RequestButton = (Button) findViewById(R.id.RequestButton);
        DataField = (EditText) findViewById(R.id.DataField);
        DisplayText = (TextView) findViewById(R.id.DisplayText);

        final RequestQueue queue = Volley.newRequestQueue(this);
        final String url = "http://serverdomainorip/postdata"; // your URL

        queue.start();
        RequestButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                HashMap<String, String> params = new HashMap<String,String>();
                params.put("data", DataField.getText().toString()); // the entered data as the body.

                JsonObjectRequest jsObjRequest = new
                        JsonObjectRequest(Request.Method.POST,
                        url,
                        new JSONObject(params),
                        new Response.Listener<JSONObject>() {
                            @Override
                            public void onResponse(JSONObject response) {
                                try {
                                    DisplayText.setText(response.getString("message"));
                                } catch (JSONException e) {
                                    e.printStackTrace();
                                }
                            }
                        }, new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        DisplayText.setText("That didn't work!");
                    }
                });
                queue.add(jsObjRequest);
            }
        });
    }

}

and the Node JS error message:

Error: listen EADDRINUSE :::3000
at Object._errnoException (util.js:1022:11)
at _exceptionWithHostPort (util.js:1044:20)
at Server.setupListenHandle [as _listen2] (net.js:1367:14)
at listenInCluster (net.js:1408:12)
at Server.listen (net.js:1492:7)
at Function.listen (/home/leventecsoba/node_modules/express/lib/application.js:618:24)
at Object. (/home/leventecsoba/Asztal/Node JS/tbg_studio.js:16:5)
at Module._compile (module.js:652:30)
at Object.Module._extensions..js (module.js:663:10)
at Module.load (module.js:565:32)

AS the error says.You have already one server running on port 3000 — Swayangjit
I killed the running process, got rid of the error message. But the server isn't running(theres no ok, its running log message). — leventecsoba

Yves Gurcan Yves Gurcan · Accepted Answer · 2019-12-21T19:27:19

It looks like you already have something (probably another instance of a Node server) running at :3000, so the code itself does not really matter. If killing processes does not make it for you, simply restart your computer (I know, it sounds silly, but it usually works!). With a fresh start, you'll be able to take control of port 3000 again and run the Node server you want there.

Alternatively, you change this line in your Node server to have it listen for requests on port 3001:

app.listen(3001)

You'll have to also update the code in Android Studio to have your mobile app send requests to port 3001.

How to connect Node Js server with Android Studio app?

2 Answers