How to send a String body with Retrofit

Retrofit logo.

Retrofit uses converters like those for GSON or Moshi to convert a data object to the response body (serialize on some way, typically to JSON format). All that a developer needs to do is pass that data object to a Retrofit service, which would perform the serialization internally.

However, sometimes we would want to provide a String body to Retrofit directly, whether it is because we have already serialized the body beforehand, or the API expects some non-standard body format and we need to build the body manually. This post explains how to do it.

First of all, we need to add a Scalars Converter to the list of our Gradle dependencies, which would take care of converting java.lang.String objects to text/plain request bodies, as per their readme. Please check this link for the latest version.

dependencies {
    implementation “com.squareup.retrofit2:converter-scalars:2.4.0”
    ..
}

Then, we need to pass a correct converter factory to our Retrofit builder. It will later tell Retrofit how to convert the @Body parameter passed to the service.

val retrofit = Retrofit.Builder()
        .client(okHttpClient)
        .baseUrl("/")
        .addConverterFactory(ScalarsConverterFactory.create())
        .build()

After this, it is time for setting up a Retrofit service with a String body parameter.

@POST("test")
fun submit(@Body body: String): Call<Void>

At this stage, we are almost done. However, the Scalars converter would try to send the body as text/plain by default since it is expecting us to send the body in plain text. In case when a data object is serialized as a JSON String and the API endpoint expects a JSON body, we need to explicitly tell Retrofit to add an application/json content type header. Our complete interface method then would look the following way:

@Headers("Content-Type: application/json")
@POST("test")
fun submit(@Body body: String): Call<Void>

This is it! Now you are equipped with a minimalistic example of sending a String body using Retrofit. Thank you for reading this post.