Connect to the network | Android Developers

To perform network operations in your application, your manifest must include
the following permissions:

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

Best practices for secure network communication

Before you add networking functionality to your app, you need to ensure that
data and information within your app stays safe when transmitting it over a
network. To do so, follow these networking security best practices:

• Minimize the amount of sensitive or personal user
  data that you transmit over the
  network.
• Send all network traffic from your app over
  SSL.
• Consider creating a network security
  configuration, which lets your app
  trust custom certificate authorities (CAs) or restrict the set of system CAs
  that it trusts for secure communication.

For more information on how to apply secure networking principles, see
networking security tips.

Choose an HTTP client

Most network-connected apps use HTTP to send and receive data. The Android
platform includes the
HttpsURLConnection client,
which supports TLS, streaming uploads and downloads, configurable timeouts,
IPv6, and connection pooling.

Third party libraries that offer higher-level APIs for networking operations are
also available. These support various convenience features, such as the
serialization of request bodies and deserialization of response bodies.

• Retrofit: a type-safe HTTP
  client for the JVM from Square, built on top of OkHttp. Retrofit lets you
  create a client interface declaratively, and has support for several
  serialization libraries.
• Ktor: an HTTP client from JetBrains, built
  entirely for Kotlin, and powered by coroutines. Ktor supports various engines,
  serializers, and platforms.

Resolve DNS queries

Devices that run Android 10 (API level 29) and higher have native support for
specialized DNS lookups through both cleartext lookups and DNS-over-TLS mode.
The DnsResolver API provides generic,
asynchronous resolution, which lets you look up SRV, NAPTR, and other
record types. Note that parsing the response is left to the app to perform.

On devices that run Android 9 (API level 28) and lower, the platform DNS
resolver supports only A and AAAA records. This lets you look up the IP
addresses associated with a name but doesn’t support any other record types.

For NDK-based apps, see
android_res_nsend.

Encapsulate network operations with a repository

To simplify the process of performing network operations and reduce code
duplication in various parts of your app, you can use the repository design
pattern. A repository is a class that handles data operations and provides a
clean API abstraction over some specific data or resource.

You can use Retrofit to declare an interface that specifies the HTTP method,
URL, arguments, and response type for network operations, as in the following
example:

Kotlin

interface UserService {
    @GET("/users/{id}")
    suspend fun getUser(@Path("id") id: String): User
}

Java

public interface UserService {
    @GET("/user/{id}")
    Call<User> getUserById(@Path("id") String id);
}

Within a repository class, functions can encapsulate network operations and
expose their results. This encapsulation ensures that the components that call
the repository don’t need to know how the data is stored. Any future changes to
how the data is stored are isolated to the repository class as well.

Kotlin

class UserRepository constructor(
    private val userService: UserService
) {
    suspend fun getUserById(id: String): User {
        return userService.getUser(id)
    }
}

Java

class UserRepository {
    private UserService userService;

    public UserRepository(
            UserService userService
    ) {
        this.userService = userService;
    }

    public Call<User> getUserById(String id) {
        return userService.getUser(id);
    }
}

To avoid creating an unresponsive UI, don’t perform network operations on the
main thread. By default, Android requires you to perform network operations on a
thread other than the main UI thread; if you don’t, a
NetworkOnMainThreadException
is thrown. In the UserRepository class shown in the previous code example, the
network operation isn’t actually triggered. The caller of the UserRepository
must implement the threading either using coroutines or using the enqueue()
function.

Survive configuration changes

When a configuration change occurs, such as a screen rotation, your fragment or
activity is destroyed and recreated. Any data that isn’t saved in the instance
state for your fragment or activity, which should hold only small amounts of
data, is lost and you might need to make your network requests again.

You can use a ViewModel to ensure
that your data survives configuration changes. A ViewModel is a component
that’s designed to store and manage UI-related data in a lifecycle-conscious
way. Using the UserRepository created above, the ViewModel can make the
necessary network requests and provide the result to your fragment or activity
using LiveData:

Kotlin

class MainViewModel constructor(
    savedStateHandle: SavedStateHandle,
    userRepository: UserRepository
) : ViewModel() {
    private val userId: String = savedStateHandle["uid"] ?:
        throw IllegalArgumentException("Missing user ID")

    private val _user = MutableLiveData<User>()
    val user = _user as LiveData<User>

    init {
        viewModelScope.launch {
            try {
                // Calling the repository is safe as it will move execution off
                // the main thread
                val user = userRepository.getUserById(userId)
                _user.value = user
            } catch (error: Exception) {
                // show error message to user
            }

        }
    }
}

Java

class MainViewModel extends ViewModel {

    private final MutableLiveData<User> _user = new MutableLiveData<>();
    LiveData<User> user = (LiveData<User>) _user;

    public MainViewModel(
            SavedStateHandle savedStateHandle,
            UserRepository userRepository
    ) {
        String userId = savedStateHandle.get("uid");
        Call<User> userCall = userRepository.getUserById(userId);
        userCall.enqueue(new Callback<User>() {
            @Override
            public void onResponse(Call<User> call, Response<User> response) {
                if (response.isSuccessful()) {
                    _user.setValue(response.body());
                }
            }

            @Override
            public void onFailure(Call<User> call, Throwable t) {
                // show error message to user
            }
        });
    }
}

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