Wed. Jun 12th, 2024

API Declaration

Annotations on the interface methods and its parameters indicate how a request will be handled.

REQUEST METHOD

Every method must have an HTTP annotation that provides the request method and relative URL. There are eight built-in annotations: HTTPGETPOSTPUTPATCHDELETEOPTIONS and HEAD. The relative URL of the resource is specified in the annotation.

@GET("users/list")

You can also specify query parameters in the URL.

@GET("users/list?sort=desc")

URL MANIPULATION

A request URL can be updated dynamically using replacement blocks and parameters on the method. A replacement block is an alphanumeric string surrounded by { and }. A corresponding parameter must be annotated with @Path using the same string.

@GET("group/{id}/users")
Call<List<User>> groupList(@Path("id") int groupId);

Query parameters can also be added.

@GET("group/{id}/users")
Call<List<User>> groupList(@Path("id") int groupId, @Query("sort") String sort);

For complex query parameter combinations a Map can be used.

@GET("group/{id}/users")
Call<List<User>> groupList(@Path("id") int groupId, @QueryMap Map<String, String> options);

REQUEST BODY

An object can be specified for use as an HTTP request body with the @Body annotation.

@POST("users/new")
Call<User> createUser(@Body User user);

The object will also be converted using a converter specified on the Retrofit instance. If no converter is added, only RequestBody can be used.

FORM ENCODED AND MULTIPART

Methods can also be declared to send form-encoded and multipart data.

Form-encoded data is sent when @FormUrlEncoded is present on the method. Each key-value pair is annotated with @Field containing the name and the object providing the value.

@FormUrlEncoded
@POST("user/edit")
Call<User> updateUser(@Field("first_name") String first, @Field("last_name") String last);

Multipart requests are used when @Multipart is present on the method. Parts are declared using the @Part annotation.

@Multipart
@PUT("user/photo")
Call<User> updateUser(@Part("photo") RequestBody photo, @Part("description") RequestBody description);

Multipart parts use one of Retrofit‘s converters or they can implement RequestBody to handle their own serialization.

HEADER MANIPULATION

You can set static headers for a method using the @Headers annotation.

@Headers("Cache-Control: max-age=640000")
@GET("widget/list")
Call<List<Widget>> widgetList();

@Headers({
    "Accept: application/vnd.github.v3.full+json",
    "User-Agent: Retrofit-Sample-App"
})
@GET("users/{username}")
Call<User> getUser(@Path("username") String username);

Note that headers do not overwrite each other. All headers with the same name will be included in the request.

A request Header can be updated dynamically using the @Header annotation. A corresponding parameter must be provided to the @Header. If the value is null, the header will be omitted. Otherwise, toString will be called on the value, and the result used.

@GET("user")
Call<User> getUser(@Header("Authorization") String authorization)

Similar to query parameters, for complex header combinations, a Map can be used.

@GET("user")
Call<User> getUser(@HeaderMap Map<String, String> headers)

Headers that need to be added to every request can be specified using an OkHttp interceptor.

SYNCHRONOUS VS. ASYNCHRONOUS

Call instances can be executed either synchronously or asynchronously. Each instance can only be used once, but calling clone() will create a new instance that can be used.

On Android, callbacks will be executed on the main thread. On the JVM, callbacks will happen on the same thread that executed the HTTP request.

By Rajashekar

I’m (Rajashekar) a core Android developer with complimenting skills as a web developer from India. I cherish taking up complex problems and turning them into beautiful interfaces. My love for decrypting the logic and structure of coding keeps me pushing towards writing elegant and proficient code, whether it is Android, PHP, Flutter or any other platforms. You would find me involved in cuisines, reading, travelling during my leisure hours.

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