Spring Web

Spring 5.0 introduced compatibility with Reactive Streams, a library interoperability standardization effort co-lead by Lightbend (with Akka Streams) along with Kaazing, Netflix, Pivotal, Red Hat, Twitter and many others.

Thanks to adopting Reactive Streams, multiple libraries can now inter-op since the same interfaces are implemented by all these libraries. Akka Streams by-design, hides the raw reactive-streams types from end-users, since it allows for detaching these types from RS and allows for a painless migration to java.util.concurrent.Flow which was introduced in Java 9.

This Alpakka module makes it possible to directly return a Source in your Spring Web endpoints.

Artifacts

sbt

libraryDependencies += "com.lightbend.akka" %% "akka-stream-alpakka-spring-web" % "0.15"

Maven

<dependency>
  <groupId>com.lightbend.akka</groupId>
  <artifactId>akka-stream-alpakka-spring-web_2.12</artifactId>
  <version>0.15</version>
</dependency>

Gradle

dependencies {
  compile group: 'com.lightbend.akka', name: 'akka-stream-alpakka-spring-web_2.12', version: '0.15'
}

Usage

Using Akka Streams in Spring Web (or Boot for that matter) is very simple, as Alpakka provides autoconfiguration to the framework, which means that Spring is made aware of Sources and Sinks etc.

All you need to do is include the above dependency (akka-stream-alpakka-spring-web), start your app as usual:

Java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {

  public static void main(String[] args) {
    SpringApplication.run(DemoApplication.class, args);
  }

}

And you’ll be able to return Akka Streams in HTTP endpoints directly:

Java

import akka.NotUsed;
import akka.actor.ActorSystem;
import akka.stream.Materializer;
import akka.stream.javadsl.Source;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class SampleController {

  @RequestMapping("/")
  public Source<String, NotUsed> index() {
    return
      Source.repeat("Hello world!")
        .intersperse("\n")
        .take(10);
  }

}

Both javadsl and scaladsl Akka Stream types are supported.

In fact, since Akka supports Java 9 and the java.util.concurrent.Flow.* types already, before Spring, you could use it to adapt those types in your applications as well.

The provided configuration

The automatically enabled configuration is as follows:

Java

import akka.actor.ActorSystem;
import akka.stream.ActorMaterializer;
import akka.stream.Materializer;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.ReactiveAdapterRegistry;
import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;

@Configuration
@ConditionalOnClass(akka.stream.javadsl.Source.class)
public class SpringWebAkkaStreamsConfiguration {

    private final ActorSystem system;
    private final ActorMaterializer mat;

    @Autowired
    public SpringWebAkkaStreamsConfiguration(RequestMappingHandlerAdapter requestMappingHandlerAdapter) {
        final ReactiveAdapterRegistry registry = requestMappingHandlerAdapter.getReactiveAdapterRegistry();

        system = ActorSystem.create("SpringWebAkkaStreamsSystem");
        mat = ActorMaterializer.create(system);
        new AkkaStreamsRegistrar(mat).registerAdapters(registry);
    }

    @Bean
    @ConditionalOnMissingBean(ActorSystem.class)
    public ActorSystem getActorSystem() {
        return system;
    }

    @Bean
    @ConditionalOnMissingBean(Materializer.class)
    public ActorMaterializer getMaterializer() {
        return mat;
    }
}

In case you’d like to manually configure it slightly differently.

Shameless plug: Akka HTTP

While the integration presented here works, it’s not quite the optimal way of using Akka in conjunction with serving HTTP apps. If you’re new to reactive systems and picking technologies, you may want to have a look at Akka HTTP.

If, for some reason, you decided use Spring MVC this integration should help you achieve the basic streaming scenarios though.