Synchronous backends

There are several synchronous backend implementations. Sending a request using these backends is a blocking operation, and results in a sttp.client4.Response[T].

Using HttpClient

The default synchronous backend. To use, you don’t need any extra dependencies, core is enough:

"com.softwaremill.sttp.client4" %% "core" % "4.0.0-M18"

Create the backend using:

import sttp.client4.httpclient.HttpClientSyncBackend

val backend = HttpClientSyncBackend()

or, if you’d like to instantiate the HttpClient yourself:

import sttp.client4.httpclient.HttpClientSyncBackend
import java.net.http.HttpClient
val httpClient: HttpClient = ???
val backend = HttpClientSyncBackend.usingClient(httpClient)

This backend is based on the built-in java.net.http.HttpClient available from Java 11 onwards.

Host header override is supported in environments running Java 12 onwards, but it has to be enabled by system property:

-Djdk.httpclient.allowRestrictedHeaders=host

Using HttpURLConnection

To use, you don’t need any extra dependencies, core is enough:

"com.softwaremill.sttp.client4" %% "core" % "4.0.0-M18"

Create the backend using:

import sttp.client4.httpurlconnection.HttpURLConnectionBackend

val backend = HttpURLConnectionBackend()

This backend supports host header override, but it has to be enabled by system property:

-Dsun.net.http.allowRestrictedHeaders=true

Using OkHttp

To use, add the following dependency to your project:

"com.softwaremill.sttp.client4" %% "okhttp-backend" % "4.0.0-M18"

Create the backend using:

import sttp.client4.okhttp.OkHttpSyncBackend

val backend = OkHttpSyncBackend()

or, if you’d like to instantiate the OkHttpClient yourself:

import sttp.client4.okhttp.OkHttpSyncBackend
import okhttp3._

val okHttpClient: OkHttpClient = ???
val backend = OkHttpSyncBackend.usingClient(okHttpClient)

This backend depends on OkHttp and fully supports HTTP/2.

Streaming

Synchronous backends don’t support non-blocking streaming.

Websockets

Both HttpClient and OkHttp backends support regular websockets.

Server-sent events

Ox is a Scala 3 toolkit that allows you to handle concurrency and resiliency in direct-style, leveraging Java 21 virtual threads. If you’re using Ox with sttp, you can handle SSE as a Source[ServerSentEvent]:

// sbt dependency
"com.softwaremill.sttp.client4" %% "ox" % "4.0.0-M18",

import ox.*
import ox.channels.Source
import sttp.client4.*
import sttp.client4.impl.ox.sse.OxServerSentEvents
import sttp.model.sse.ServerSentEvent
import java.io.InputStream

def handleSse(is: InputStream)(using IO): Unit =
  supervised {
    OxServerSentEvents.parse(is).foreach(event => println(s"Received event: $event"))
  }

val backend = DefaultSyncBackend()
IO.unsafe:
  basicRequest
    .get(uri"https://postman-echo.com/server-events/3")
      .response(asInputStreamAlways(handleSse))
      .send(backend)