Akka gRPC Quickstart with Java

Akka gRPC is a toolkit for building streaming gRPC servers and clients on top of Akka Streams. This guide will get you started building gRPC based systems with Java. If you prefer to use Akka gRPC with Scala, switch to the Akka gRPC Quickstart with Scala guide.

After trying this example the Akka gRPC documentation is a good next step to continue learning more about Akka gRPC.

Downloading the example

The Hello World example for Scala is a zipped project that includes a distribution of sbt, Maven and Gradle. You can choose any of these build tools. You can run it on Linux, MacOS, or Windows. The only prerequisite is Java 8.

Download and unzip the example:

  1. Download the zip file.
  2. Extract the zip file to a convenient location:
  • On Linux and OSX systems, open a terminal and use the command unzip akka-grpc-quickstart-java.zip. Note: On OSX, if you unzip using Archiver, you also have to make the build files executable:
$ chmod u+x ./sbt
$ chmod u+x ./sbt-dist/bin/sbt
Local installation of mvn is required.
$ chmod u+x ./gradlew
  • On Windows, use a tool such as File Explorer to extract the project.

Running the example

To run Hello World:

On OSX/Linux use ./sbt to start sbt in the instructions below, on Windows sbt.bat. On OSX/Linux use ./gradlew to start Gradle in the instructions below, on Windows ./gradlew.bat.

  1. In a console, change directories to the top level of the unzipped project.

    For example, if you used the default project name, akka-grpc-quickstart-java, and extracted the project to your root directory, from the root directory, enter: cd akka-grpc-quickstart-java

  2. Compile the project by entering:

    ./sbt compile
    mvn compile
    ./gradlew compileJava

    sbtMavenGradle downloads project dependencies, generates gRPC classes from protobuf, and compiles.

  3. Run the server:

    ./sbt "runMain com.example.helloworld.GreeterServer"
    mvn compile dependency:properties exec:exec@server
    ./gradlew runServer

    sbtMavenGradle runs the com.example.helloworld.GreeterServer main class that starts the gRPC server. The exec:exec@server execution is defined in the Maven pom.xml build definition. The runServer task is defined in build.gradle.

    The output should include something like:

    gRPC server bound to: /
  4. Run the client, open another console window and enter:

    ./sbt "runMain com.example.helloworld.GreeterClient"
    mvn compile dependency:properties exec:exec@client
    ./gradlew runClient

    sbtMavenGradle runs the com.example.helloworld.GreeterClient main class that starts the gRPC client. The exec:exec@client execution is defined in the Maven pom.xml build definition. The runClient task is defined in build.gradle.

    The output should include something like:

    Performing request: Alice
    Performing request: Bob
    HelloReply(Hello, Bob)
    HelloReply(Hello, Alice)

Congratulations, you just ran your first Akka gRPC server and client. Now take a look at what happened under the covers.

You can end the programs with ctrl-c.

What Hello World does

As you saw in the console output, the example outputs several greetings. Let’s take at the code and what happens at runtime.


First, the GreeterServer main class creates an akka.actor.typed.ActorSystem, a container in which Actors, Akka Streams and Akka HTTP run. Next, it defines a function from HttpRequest to CompletionStage<HttpResponse> using the GreeterServiceImpl. This function handles gRPC requests in the HTTP/2 with TLS server that is bound to port 8080 in this example.

import akka.actor.typed.ActorSystem; import akka.actor.typed.javadsl.Behaviors; import akka.http.javadsl.*; import akka.http.javadsl.model.HttpRequest; import akka.http.javadsl.model.HttpResponse; import akka.japi.function.Function; import com.typesafe.config.Config; import com.typesafe.config.ConfigFactory; import javax.net.ssl.KeyManagerFactory; import javax.net.ssl.SSLContext; import java.io.ByteArrayOutputStream; import java.io.IOException; import java.io.InputStream; import java.security.KeyFactory; import java.security.KeyStore; import java.security.PrivateKey; import java.security.SecureRandom; import java.security.cert.Certificate; import java.security.cert.CertificateFactory; import java.security.spec.PKCS8EncodedKeySpec; import java.util.Base64; import java.util.concurrent.CompletionStage; public class GreeterServer { public static void main(String[] args) throws Exception { // important to enable HTTP/2 in ActorSystem's config Config conf = ConfigFactory.parseString("akka.http.server.preview.enable-http2 = on") .withFallback(ConfigFactory.load()); ActorSystem<Void> system = ActorSystem.create(Behaviors.empty(), "GreeterServer", conf); new GreeterServer(system).run(); } final ActorSystem<?> system; public GreeterServer(ActorSystem<?> system) { this.system = system; } public CompletionStage<ServerBinding> run() throws Exception { Function<HttpRequest, CompletionStage<HttpResponse>> service = GreeterServiceHandlerFactory.create( new GreeterServiceImpl(system), system); CompletionStage<ServerBinding> bound = Http.get(system) .newServerAt("", 8080) .enableHttps(serverHttpContext()) .bind(service); bound.thenAccept(binding -> System.out.println("gRPC server bound to: " + binding.localAddress()) ); return bound; } }

GreeterServiceImpl is our implementation of the gRPC service, but first we must define the interface of the service in the protobuf file src/main/proto/helloworld.proto:

syntax = "proto3";

option java_multiple_files = true;
option java_package = "com.example.helloworld";
option java_outer_classname = "HelloWorldProto";

// The greeting service definition.
service GreeterService {
    // Sends a greeting
    rpc SayHello (HelloRequest) returns (HelloReply) {}


// The request message containing the user's name.
message HelloRequest {
    string name = 1;

// The response message containing the greetings
message HelloReply {
    string message = 1;

When compiling the project several things are generated from the proto definition. You can find the generated files in target/scala-2.13/akka-grpc/main/target/generated-sources/build/generated/source/proto/main/ if you are curious.

For the server the following classes are generated:

  • Message classes, such as HelloRequest and HelloReply
  • GreeterService interface of the service
  • GreeterServiceHandler utility to create the HttpRequest to HttpResponse function from the GreeterServiceImpl

The part that we have to implement on the server side is the GreeterServiceImpl which implements the generated GreeterService interface. It is this implementation that is bound to the HTTP server via the GreeterServiceHandler and it looks like this:

import akka.NotUsed; import akka.japi.Pair; import akka.actor.typed.ActorSystem; import akka.stream.javadsl.BroadcastHub; import akka.stream.javadsl.Keep; import akka.stream.javadsl.MergeHub; import akka.stream.javadsl.Sink; import akka.stream.javadsl.Source; import java.util.concurrent.CompletableFuture; import java.util.concurrent.CompletionStage; class GreeterServiceImpl implements GreeterService { final ActorSystem<?> system; public GreeterServiceImpl(ActorSystem<?> system) { this.system = system; } @Override public CompletionStage<HelloReply> sayHello(HelloRequest request) { return CompletableFuture.completedFuture( HelloReply.newBuilder() .setMessage("Hello, " + request.getName()) .build() ); } }


In this example we have the client in the same project as the server. That is common for testing purposes but for real usage you or another team would have a separate project (different service) that is using the client and doesn’t implement the server side of the service. Between such projects you would only share the proto file (by copying it).

From the same proto file that was used on the server side classes are generated for the client:

  • Message classes, such as HelloRequest and HelloReply
  • GreeterService interface of the service
  • GreeterServiceClient that implements the client side of the GreeterService

On the client side we don’t have to implement anything, the GreeterServiceClient is ready to be used as is.

We need an ActorSystem and then the GreeterServiceClient can be created and used like this:

import akka.Done;
import akka.NotUsed;
import akka.japi.Pair;
import akka.actor.typed.ActorSystem;
import akka.actor.typed.javadsl.Behaviors;
import akka.grpc.GrpcClientSettings;
import akka.stream.javadsl.Source;

import java.time.Duration;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.CompletionStage;

import static akka.NotUsed.notUsed;
class GreeterClient {

  public static void main(String[] args) {
    final ActorSystem<Void> system = ActorSystem.create(Behaviors.empty(), "GreeterClient");

    GreeterServiceClient client = GreeterServiceClient.create(
        GrpcClientSettings.fromConfig("helloworld.GreeterService", system),

    final List<String> names;
    if (args.length == 0) {
      names = Arrays.asList("Alice", "Bob");
    } else {
      names = Arrays.asList(args);

    names.forEach(name -> {
      System.out.println("Performing request: " + name);
      HelloRequest request = HelloRequest.newBuilder()
      CompletionStage<HelloReply> replyCS = client.sayHello(request);
      replyCS.whenComplete((reply, error) -> {
        if (error == null) {
        } else {

Note that clients and servers don’t have to be implemented with Akka gRPC. They can be implemented/used with other libraries or languages and interoperate according to the gRPC specification.

Other types of calls

In this first example we saw a gRPC service call for single request returning a CompletionStage reply. The parameter and return type of the calls may also be streams in 3 different combinations:

  • client streaming call - Source (stream) of requests from the client that returns a CompletionStage with a single response, see itKeepsTalking in above example
  • server streaming call - single request that returns a Source (stream) of responses, see itKeepsReplying in above example
  • client and server streaming call - Source (stream) of requests from the client that returns a Source (stream) of responses, see streamHellos in above example

As next step, let’s try the bidirectional streaming calls.