Android Networking

1. Overview of network access on Android

1.1. Accessing the network in Android

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

The Android system crashes your application with a NetworkOnMainThreadException exception, if you access network is accessed from the main thread.

Android contains the standard Java network java.net package which can be used to access network resources. The base class for HTTP network access in the java.net package is the HttpURLConnection class.

Performing network operations with standard Java API can be cumbersome. You have to open and close a connections, enable caches and ensure to perform the network operation in a background thread.

To simplify these operations several popular Open Source libraries are available. The most popular ones are the following:

OkHttp for efficient HTTP access
Retrofit for REST based clients
Glide for image processing

Prefer using OkHttp over the usage of HttpURLConnection. It is faster than the standard Java library and has a better API.

1.2. Permission to access the network

To access the Internet your application requires the android.permission.INTERNET permission. On modern Android API versions, this permission is automatically granted to your application.

1.3. Check the network availability

The network on an Android device is not always available. To check the network state your application requires the android.permission.ACCESS_NETWORK_STATE permission. 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.

2. Web Sockets

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/.