how to create Socket connection in Android?

Socket connections in Android are the same as in Java: http://www.oracle.com/technetwork/java/socket-140484.html

Things you need to be aware of:

1. If phone goes to sleep your app will no longer execute, so socket will eventually timeout. You can prevent this with wake lock. This will eat devices battery tremendously - I know I wouldn't use that app.
2. If you do this constantly, even when your app is not active, then you need to use Service.
3. Activities and Services can be killed off by OS at any time, especially if they are part of an inactive app.

Take a look at AlarmManager, if you need scheduled execution of your code.

Do you need to run your code and receive data even if user does not use the app any more (i.e. app is inactive)?

Here, in this post you will find the detailed code for establishing socket between devices or between two application in the same mobile.

You have to create two application to test below code.

In both application's manifest file, add below permission

<uses-permission android:name="android.permission.INTERNET" />

1st App code: Client Socket

activity_main.xml

<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent">    <TableRow        android:id="@+id/tr_send_message"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:gravity="center"        android:layout_alignParentLeft="true"        android:layout_alignParentStart="true"        android:layout_alignParentTop="true"        android:layout_marginTop="11dp">        <EditText            android:id="@+id/edt_send_message"            android:layout_width="0dp"            android:layout_height="wrap_content"            android:layout_weight="1"            android:layout_marginRight="10dp"            android:layout_marginLeft="10dp"            android:hint="Enter message"            android:inputType="text" />        <Button            android:id="@+id/btn_send"            android:layout_width="wrap_content"            android:layout_height="wrap_content"            android:layout_marginRight="10dp"            android:text="Send" />    </TableRow>    <ScrollView        android:layout_width="match_parent"        android:layout_height="match_parent"        android:layout_alignParentLeft="true"        android:layout_alignParentStart="true"        android:layout_below="@+id/tr_send_message"        android:layout_marginTop="25dp"        android:id="@+id/scrollView2">        <TextView            android:id="@+id/tv_reply_from_server"            android:layout_width="match_parent"            android:layout_height="wrap_content"            android:orientation="vertical" />    </ScrollView></RelativeLayout>

MainActivity.java

import android.os.Bundle;import android.os.Handler;import android.support.v7.app.AppCompatActivity;import android.view.View;import android.widget.Button;import android.widget.EditText;import android.widget.TextView;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStreamReader;import java.io.OutputStream;import java.io.PrintWriter;import java.net.Socket;/** * Created by Girish Bhalerao on 5/4/2017. */public class MainActivity extends AppCompatActivity implements View.OnClickListener {    private TextView mTextViewReplyFromServer;    private EditText mEditTextSendMessage;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        Button buttonSend = (Button) findViewById(R.id.btn_send);        mEditTextSendMessage = (EditText) findViewById(R.id.edt_send_message);        mTextViewReplyFromServer = (TextView) findViewById(R.id.tv_reply_from_server);        buttonSend.setOnClickListener(this);    }    @Override    public void onClick(View v) {        switch (v.getId()) {            case R.id.btn_send:                sendMessage(mEditTextSendMessage.getText().toString());                break;        }    }    private void sendMessage(final String msg) {        final Handler handler = new Handler();        Thread thread = new Thread(new Runnable() {            @Override            public void run() {                try {                    //Replace below IP with the IP of that device in which server socket open.                    //If you change port then change the port number in the server side code also.                    Socket s = new Socket("xxx.xxx.xxx.xxx", 9002);                    OutputStream out = s.getOutputStream();                    PrintWriter output = new PrintWriter(out);                    output.println(msg);                    output.flush();                    BufferedReader input = new BufferedReader(new InputStreamReader(s.getInputStream()));                    final String st = input.readLine();                    handler.post(new Runnable() {                        @Override                        public void run() {                            String s = mTextViewReplyFromServer.getText().toString();                            if (st.trim().length() != 0)                                mTextViewReplyFromServer.setText(s + "\nFrom Server : " + st);                        }                    });                    output.close();                    out.close();                    s.close();                } catch (IOException e) {                    e.printStackTrace();                }            }        });        thread.start();    }}

2nd App Code - Server Socket

activity_main.xml

<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent">    <Button        android:id="@+id/btn_stop_receiving"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="STOP Receiving data"        android:layout_alignParentTop="true"        android:enabled="false"        android:layout_centerHorizontal="true"        android:layout_marginTop="89dp" />    <ScrollView        android:layout_width="match_parent"        android:layout_height="match_parent"        android:layout_below="@+id/btn_stop_receiving"        android:layout_marginTop="35dp"        android:layout_alignParentLeft="true"        android:layout_alignParentStart="true">        <TextView            android:id="@+id/tv_data_from_client"            android:layout_width="match_parent"            android:layout_height="wrap_content"            android:orientation="vertical" />    </ScrollView>    <Button        android:id="@+id/btn_start_receiving"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="START Receiving data"        android:layout_alignParentTop="true"        android:layout_centerHorizontal="true"        android:layout_marginTop="14dp" /></RelativeLayout>

MainActivity.java

import android.os.Bundle;import android.os.Handler;import android.support.v7.app.AppCompatActivity;import android.view.View;import android.widget.Button;import android.widget.TextView;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStreamReader;import java.io.PrintWriter;import java.net.ServerSocket;import java.net.Socket;/** * Created by Girish Bhalerao on 5/4/2017. */public class MainActivity extends AppCompatActivity implements View.OnClickListener {    final Handler handler = new Handler();    private Button buttonStartReceiving;    private Button buttonStopReceiving;    private TextView textViewDataFromClient;    private boolean end = false;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        buttonStartReceiving = (Button) findViewById(R.id.btn_start_receiving);        buttonStopReceiving = (Button) findViewById(R.id.btn_stop_receiving);        textViewDataFromClient = (TextView) findViewById(R.id.tv_data_from_client);        buttonStartReceiving.setOnClickListener(this);        buttonStopReceiving.setOnClickListener(this);    }    private void startServerSocket() {        Thread thread = new Thread(new Runnable() {            private String stringData = null;            @Override            public void run() {                try {                    ServerSocket ss = new ServerSocket(9002);                    while (!end) {                        //Server is waiting for client here, if needed                        Socket s = ss.accept();                        BufferedReader input = new BufferedReader(new InputStreamReader(s.getInputStream()));                        PrintWriter output = new PrintWriter(s.getOutputStream());                        stringData = input.readLine();                        output.println("FROM SERVER - " + stringData.toUpperCase());                        output.flush();                        try {                            Thread.sleep(1000);                        } catch (InterruptedException e) {                            e.printStackTrace();                        }                        updateUI(stringData);                        if (stringData.equalsIgnoreCase("STOP")) {                            end = true;                            output.close();                            s.close();                            break;                        }                        output.close();                        s.close();                    }                    ss.close();                } catch (IOException e) {                    e.printStackTrace();                }            }        });        thread.start();    }    private void updateUI(final String stringData) {        handler.post(new Runnable() {            @Override            public void run() {                String s = textViewDataFromClient.getText().toString();                if (stringData.trim().length() != 0)                    textViewDataFromClient.setText(s + "\n" + "From Client : " + stringData);            }        });    }    @Override    public void onClick(View v) {        switch (v.getId()) {            case R.id.btn_start_receiving:                startServerSocket();                buttonStartReceiving.setEnabled(false);                buttonStopReceiving.setEnabled(true);                break;            case R.id.btn_stop_receiving:                //stopping server socket logic you can add yourself                buttonStartReceiving.setEnabled(true);                buttonStopReceiving.setEnabled(false);                break;        }    }}

Simple socket server app example

I've already posted a client example at: https://stackoverflow.com/a/35971718/895245 , so here goes a server example.

This example app runs a server that returns a ROT-1 cypher of the input.

You would then need to add an Exit button + some sleep delays, but this should get you started.

To play with it:

• install the app
• get your phone and PC on a LAN
• find your phone's IP with https://android.stackexchange.com/a/130468/126934
• run netcat $PHONE_IP 12345
• type some lines

Android sockets are the same as Java's, except we have to deal with some permission issues.

src/com/cirosantilli/android_cheat/socket/Main.java

package com.cirosantilli.android_cheat.socket;import android.app.Activity;import android.app.IntentService;import android.content.Intent;import android.os.Bundle;import android.util.Log;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStreamReader;import java.io.PrintStream;import java.net.ServerSocket;import java.net.Socket;public class Main extends Activity {    static final String TAG = "AndroidCheatSocket";    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Log.d(Main.TAG, "onCreate");        Main.this.startService(new Intent(Main.this, MyService.class));    }    public static class MyService extends IntentService {        public MyService() {            super("MyService");        }        @Override        protected void onHandleIntent(Intent intent) {            Log.d(Main.TAG, "onHandleIntent");            final int port = 12345;            ServerSocket listener = null;            try {                listener = new ServerSocket(port);                Log.d(Main.TAG, String.format("listening on port = %d", port));                while (true) {                    Log.d(Main.TAG, "waiting for client");                    Socket socket = listener.accept();                    Log.d(Main.TAG, String.format("client connected from: %s", socket.getRemoteSocketAddress().toString()));                    BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));                    PrintStream out = new PrintStream(socket.getOutputStream());                    for (String inputLine; (inputLine = in.readLine()) != null;) {                        Log.d(Main.TAG, "received");                        Log.d(Main.TAG, inputLine);                        StringBuilder outputStringBuilder = new StringBuilder("");                        char inputLineChars[] = inputLine.toCharArray();                        for (char c : inputLineChars)                            outputStringBuilder.append(Character.toChars(c + 1));                        out.println(outputStringBuilder);                    }                }            } catch(IOException e) {                Log.d(Main.TAG, e.toString());            }        }    }}

We need a Service or other background method or else: How do I fix android.os.NetworkOnMainThreadException?

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?><manifest xmlns:android="http://schemas.android.com/apk/res/android"      package="com.cirosantilli.android_cheat.socket"      android:versionCode="1"      android:versionName="1.0">    <uses-sdk android:minSdkVersion="22" />    <uses-permission android:name="android.permission.INTERNET" />    <application android:label="AndroidCheatsocket">        <activity android:name="Main">            <intent-filter>                <action android:name="android.intent.action.MAIN" />                <category android:name="android.intent.category.LAUNCHER" />            </intent-filter>        </activity>        <service android:name=".Main$MyService" />    </application></manifest>

We must add: <uses-permission android:name="android.permission.INTERNET" /> or else: Java socket IOException - permission denied

On GitHub with a build.xml: https://github.com/cirosantilli/android-cheat/tree/92de020d0b708549a444ebd9f881de7b240b3fbc/socket