Reaching a network device by IP and port using the Android emulator

Question

Why does my Java app for Android not connect to the server?

I run the application in Android emulator, and the server which is on port 9999 and host 127.0.0.1 in my pc, but it will just not connect and I think this method isn't good for Sndroid app.

Update: I work with API 8 and Android 2.2

This is my source code:

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
//Java imports
//import android.util.Log;
import java.io.*;
import java.net.*;
import java.util.*;
import java.util.logging.Level;
import java.util.logging.Logger;


public class MainActivity extends Activity{
//Variaveis Interface
private Button ligar;
private Button enviar;
private EditText text1;
private TextView text2;
//Variaveis
static Socket cSocket;
static PrintWriter out;
static BufferedReader in;
   

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    
    //Declaração butões
    ligar = (Button) findViewById(R.id.ligar);
    enviar = (Button) findViewById(R.id.enviar);
    text1 = (EditText) findViewById(R.id.text1);
    text2 = (TextView) findViewById(R.id.text2);

    //Interacao
    ligar.setOnClickListener(new OnClickListener(){
        public void onClick(View arg0){
             connect();
        }
    });
    enviar.setOnClickListener(new OnClickListener(){
        public void onClick(View arg0){
               out.println("Hello");
               text2.setText("");
        }
    });
  }
  //Outras Funcoes

public void connect(){
//Funcao ligar
cSocket = null;
out = null;
in = null;

try{
   cSocket = new Socket("10.0.2.2",4444);
   out = new PrintWriter(cSocket.getOutputStream(), true);
   in = new BufferedReader(new InputStreamReader(cSocket.getInputStream()));
   text2.setText("Estas conectado com sucesso.");
   }
   catch (IOException ex) {
   //Logger.getLogger(client.class.getName()).log(Level.SEVERE, null, ex);
   text2.setText("Erro! Na conexão");
   }                
   }
//
}

Answer 1

See here:

Host machine can be reached using IP address 10.0.2.2 from the emulator.

**edit, answer to your comment:*

For completeness and to better understand my answer, read the Android Emulator documentation.

These are the IP addresses as reached from the emulator:

• 10.0.2.1, Router/gateway address.
• 10.0.2.2, Special alias to your host loopback interface (i.e., 127.0.0.1 on your development machine)
• 10.0.2.3, First DNS server
• 10.0.2.4 / 10.0.2.5 / 10.0.2.6, Optional second, third and fourth DNS server (if any)
• 10.0.2.15, The emulated device's own network/ethernet interface
• 127.0.0.1, The emulated device's own loopback interface

That said, we have:

• Common mistake 1: accessing 127.0.0.1 from the emulator trying to reach your host machine. Use 10.0.2.2, as I said.
• Common mistake 2: Trying to access an emulator service on HostComputerIP:appServicePort. It won't work since your host computer itself (Windows, Linux, OS etc.) is not running a service in that port. You need to redirect a port on the emulator console to a port on an emulated Android instance itself (see 2 below).

Common networking needs:

1- Emulator app as client and local computer as server

Because the emulator is NAT'd, I believe you can connect to any computer on your local network directly. I mean, since the virtual router has access to both networks, it should be able to handle outgoing (i.e., emulator->real lan) connections just fine.

Example: on my network (192.168.0.x), I can connect from the emulator to my real router (192.168.0.254) just pointing the emulator web browser to http://192.168.0.254:port. I use different services on it (hail to Tomato!), and I can access all of them on each port. No need to handle port forwarding, as expected.

By the looks of your code, I believe you need:

// I assume 192.168.0.114 is your server, which is
// located on your local network, running a server application
// on port 9999.
cSocket = new Socket("192.168.0.114",9999);

2- Local computer as client and emulator app as server

Now that's a different story. You need to setup port redirections on the virtual router. The easiest way is:

Telnet into the "management" system (this is not the emulator), from your host (your computer, console on linux or command prompt on Windows):

telnet localhost 5554

After that, use:

adb forward tcp:localPort tcp:emulatorPort

After this, you will be able to have a service on emulatorPort and you will be able to connect to it from computers in the local network by accessing hostComputerIP:localPort.

This is the way people (including me) use, for example, SSHDroid inside an emulator.

Anything else?