Example: Android bi-directional network socket using AsyncTask

Question

Most of the network socket examples I found for Android were one directional only. I needed a solution for a bi-directional data stream. I eventually learned of the AsyncTask. This example shows how to get data from a socket and send data back to it. Due to the blocking nature of a socket that is receiving data, that blocking needs to run in a thread other than the UI thread.

For the sake of example, this code connects to a webserver. Pressing the "Start AsyncTask" button will open the socket. Once the socket is open, the web server waits for a request. Pressing the "Send Message" button will send a request to the server. Any response from the server will be displayed in the TextView. In the case of http, a web server will disconnect from the client once all the data has been sent. For other TCP data streams, the connection will stay up until one side disconnects.

Screenshot:

AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android"       package="com.exampleasynctask"       android:versionCode="1"       android:versionName="1.0">     <uses-sdk android:minSdkVersion="8" />     <uses-permission android:name="android.permission.INTERNET" />     <application android:icon="@drawable/icon" android:label="@string/app_name">         <activity android:name=".MainActivity"                   android:label="@string/app_name">             <intent-filter>                 <action android:name="android.intent.action.MAIN" />                 <category android:name="android.intent.category.LAUNCHER" />             </intent-filter>         </activity>     </application> </manifest> 

res\layout\main.xml:

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"     android:orientation="vertical"     android:layout_width="fill_parent"     android:layout_height="fill_parent"     > <Button android:id="@+id/btnStart" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Start AsyncTask"></Button> <Button android:id="@+id/btnSend" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Send Message"></Button> <TextView android:id="@+id/textStatus" android:textSize="24sp" android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="Status Goes Here" /> </LinearLayout> 

src\com.exampleasynctask\MainActivity.java:

package com.exampleasynctask;  import java.io.IOException; import java.io.InputStream; import java.io.OutputStream; import java.net.InetSocketAddress; import java.net.Socket; import java.net.SocketAddress;  import android.app.Activity; import android.os.AsyncTask; import android.os.Bundle; import android.util.Log; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.TextView;  public class MainActivity extends Activity {     Button btnStart, btnSend;     TextView textStatus;     NetworkTask networktask;      @Override     public void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.main);         btnStart = (Button)findViewById(R.id.btnStart);         btnSend = (Button)findViewById(R.id.btnSend);         textStatus = (TextView)findViewById(R.id.textStatus);         btnStart.setOnClickListener(btnStartListener);         btnSend.setOnClickListener(btnSendListener);         networktask = new NetworkTask(); //Create initial instance so SendDataToNetwork doesn't throw an error.     }      private OnClickListener btnStartListener = new OnClickListener() {         public void onClick(View v){             btnStart.setVisibility(View.INVISIBLE);             networktask = new NetworkTask(); //New instance of NetworkTask             networktask.execute();         }     };     private OnClickListener btnSendListener = new OnClickListener() {         public void onClick(View v){             textStatus.setText("Sending Message to AsyncTask.");             networktask.SendDataToNetwork("GET / HTTP/1.1\r\n\r\n");         }     };      public class NetworkTask extends AsyncTask<Void, byte[], Boolean> {         Socket nsocket; //Network Socket         InputStream nis; //Network Input Stream         OutputStream nos; //Network Output Stream          @Override         protected void onPreExecute() {             Log.i("AsyncTask", "onPreExecute");         }          @Override         protected Boolean doInBackground(Void... params) { //This runs on a different thread             boolean result = false;             try {                 Log.i("AsyncTask", "doInBackground: Creating socket");                 SocketAddress sockaddr = new InetSocketAddress("192.168.1.1", 80);                 nsocket = new Socket();                 nsocket.connect(sockaddr, 5000); //10 second connection timeout                 if (nsocket.isConnected()) {                      nis = nsocket.getInputStream();                     nos = nsocket.getOutputStream();                     Log.i("AsyncTask", "doInBackground: Socket created, streams assigned");                     Log.i("AsyncTask", "doInBackground: Waiting for inital data...");                     byte[] buffer = new byte[4096];                     int read = nis.read(buffer, 0, 4096); //This is blocking                     while(read != -1){                         byte[] tempdata = new byte[read];                         System.arraycopy(buffer, 0, tempdata, 0, read);                         publishProgress(tempdata);                         Log.i("AsyncTask", "doInBackground: Got some data");                         read = nis.read(buffer, 0, 4096); //This is blocking                     }                 }             } catch (IOException e) {                 e.printStackTrace();                 Log.i("AsyncTask", "doInBackground: IOException");                 result = true;             } catch (Exception e) {                 e.printStackTrace();                 Log.i("AsyncTask", "doInBackground: Exception");                 result = true;             } finally {                 try {                     nis.close();                     nos.close();                     nsocket.close();                 } catch (IOException e) {                     e.printStackTrace();                 } catch (Exception e) {                     e.printStackTrace();                 }                 Log.i("AsyncTask", "doInBackground: Finished");             }             return result;         }          public void SendDataToNetwork(String cmd) { //You run this from the main thread.             try {                 if (nsocket.isConnected()) {                     Log.i("AsyncTask", "SendDataToNetwork: Writing received message to socket");                     nos.write(cmd.getBytes());                 } else {                     Log.i("AsyncTask", "SendDataToNetwork: Cannot send message. Socket is closed");                 }             } catch (Exception e) {                 Log.i("AsyncTask", "SendDataToNetwork: Message send failed. Caught an exception");             }         }          @Override         protected void onProgressUpdate(byte[]... values) {             if (values.length > 0) {                 Log.i("AsyncTask", "onProgressUpdate: " + values[0].length + " bytes received.");                 textStatus.setText(new String(values[0]));             }         }         @Override         protected void onCancelled() {             Log.i("AsyncTask", "Cancelled.");             btnStart.setVisibility(View.VISIBLE);         }         @Override         protected void onPostExecute(Boolean result) {             if (result) {                 Log.i("AsyncTask", "onPostExecute: Completed with an Error.");                 textStatus.setText("There was a connection error.");             } else {                 Log.i("AsyncTask", "onPostExecute: Completed.");             }             btnStart.setVisibility(View.VISIBLE);         }     }      @Override     protected void onDestroy() {         super.onDestroy();         networktask.cancel(true); //In case the task is currently running     } } 

Answer 1

The SendDataToNetwork task runs in the main ui thread, meaning it will crash a Honeycomb or higher app due to NetworkOnMainThreadException Fatal exception. Here's what my SendDataToNetwork looks like to avoid this issue:

public boolean sendDataToNetwork(final byte[] cmd) {      if (_nsocket.isConnected()) {         Log.i(TAG, "SendDataToNetwork: Writing received message to socket");         new Thread(new Runnable() {             public void run() {                 try {                     _nos.write(cmd);                 } catch (Exception e) {                     e.printStackTrace();                     Log.i(TAG, "SendDataToNetwork: Message send failed. Caught an exception");                 }             }         }).start();          return true;     }      Log.i(TAG, "SendDataToNetwork: Cannot send message. Socket is closed");     return false; }