Within an Android application you should avoid performing long running operations on the user interface thread. This includes file and network access.

As of Android 3.0 (Honeycomb) the system is configured to crash with a NetworkOnMainThreadException exception, if network is accessed in the user interface thread.

A typical setup for performing network access in a productive Android application is using a service. While it is possible to do network access from an activity or a fragment, using a service typical leads to a better overall design because you code in the activity becomes simpler.

StrictMode.ThreadPolicy policy = new StrictMode.
ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy); 

Java provides a general-purpose, lightweight HTTP client API to access resources via the HTTP or HTTPS protocol. The main classes to access the Internet are the java.net.URL class and the java.net.HttpURLConnection class.

The URL class can be used to define a pointer to a web resource while the HttpURLConnection class can be used to access a web resource.

HttpURLConnection allows you to create an InputStream.

Once you have accessed an InputStream you can read it similarly to an InputStream from a local file.

In the latest version HttpURLConnection supports the transparent response compression (via the header Accept-Encoding: gzip, Server Name Indication (extension of SSL and TLS) and a response cache.

The API is relatively straight forward. For example to retrieve the webpage www.vogella.com you can use the following example.

try {
  URL url = new URL("http://www.vogella.com");
  HttpURLConnection con = (HttpURLConnection) url
    .openConnection();
  readStream(con.getInputStream());
  } catch (Exception e) {
  e.printStackTrace();
}



private void readStream(InputStream in) {
  BufferedReader reader = null;
  try {
    reader = new BufferedReader(new InputStreamReader(in));
    String line = "";
    while ((line = reader.readLine()) != null) {
      System.out.println(line);
    }
  } catch (IOException e) {
    e.printStackTrace();
  } finally {
    if (reader != null) {
      try {
        reader.close();
      } catch (IOException e) {
        e.printStackTrace();
        }
    }
  }
} 

The Javadoc of HttpURLConnection suggest to not reuse on HttpURLConnection. If you use it this way, HttpURLConnection has no threading issues, as it will not be shared between different Threads.

Obviously the network on an Android device is not always available. You can check the network is currently available via the following code.

public boolean isNetworkAvailable() {
    ConnectivityManager cm = (ConnectivityManager) 
      getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = cm.getActiveNetworkInfo();
    // if no network is available networkInfo will be null
    // otherwise check if we are connected
    if (networkInfo != null && networkInfo.isConnected()) {
        return true;
    }
    return false;
} 

This requires the ACCESS_NETWORK_STATE permission.

This chapter is only relevant for you if you are testing with the Android simulator behind a proxy. You can set the proxy via the Settings class. For example you could add the following line to your onCreate method in your activity.

Settings.System.putString(getContentResolver(), Settings.System.HTTP_PROXY, "myproxy:8080"); 

To change the proxy settings you have to have the android.permission.WRITE_SETTINGS permission in your AndroidManifest.xml file.

Web Sockets are a standard based on HTTP for asynchronous message-based communication between a client and a server. To start a web socket communication, you create a HTTP GET request with a special HTTP headers. If the server accepts this request, the client and the server can send each other messages.

Messages can be text or binary data and should be relatively small, as the web socket protocol is intended to be used with small payloads in the data.

It is good practice to use JSON as data format for the messages.

You find a Java library for both the server and the client under http://java-websocket.org/.