Prerequisites

• JDK version 7 or higher

Get the example code

The example code is part of the grpc-java repo.

1. Download the repo as a zip file and unzip it, or clone the repo:

   $ git clone -b v1.60.0 --depth 1 https://github.com/grpc/grpc-java
   

2. Change to the examples directory:

   $ cd grpc-java/examples
   

Run the example

From the examples directory:

1. Compile the client and server

   $ ./gradlew installDist
   

2. Run the server:

   $ ./build/install/examples/bin/hello-world-server
   INFO: Server started, listening on 50051
   

3. From another terminal, run the client:

   $ ./build/install/examples/bin/hello-world-client
   INFO: Will try to greet world ...
   INFO: Greeting: Hello world
   

Congratulations! You’ve just run a client-server application with gRPC.

Update the gRPC service

In this section you’ll update the application by adding an extra server method. The gRPC service is defined using protocol buffers. To learn more about how to define a service in a .proto file see Basics tutorial. For now, all you need to know is that both the server and the client stub have a SayHello() RPC method that takes a HelloRequest parameter from the client and returns a HelloReply from the server, and that the method is defined like this:

// The greeting service definition.
service Greeter {
  // 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;
}

Open src/main/proto/helloworld.proto and add a new SayHelloAgain() method with the same request and response types as SayHello():

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
  // Sends another greeting
  rpc SayHelloAgain (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;
}

Remember to save the file!

Update the app

When you build the example, the build process regenerates GreeterGrpc.java, which contains the generated gRPC client and server classes. This also regenerates classes for populating, serializing, and retrieving our request and response types.

However, you still need to implement and call the new method in the hand-written parts of the example app.

Update the server

In the same directory, open src/main/java/io/grpc/examples/helloworld/HelloWorldServer.java. Implement the new method like this:

private class GreeterImpl extends GreeterGrpc.GreeterImplBase {

  @Override
  public void sayHello(HelloRequest req, StreamObserver<HelloReply> responseObserver) {
    HelloReply reply = HelloReply.newBuilder().setMessage("Hello " + req.getName()).build();
    responseObserver.onNext(reply);
    responseObserver.onCompleted();
  }

  @Override
  public void sayHelloAgain(HelloRequest req, StreamObserver<HelloReply> responseObserver) {
    HelloReply reply = HelloReply.newBuilder().setMessage("Hello again " + req.getName()).build();
    responseObserver.onNext(reply);
    responseObserver.onCompleted();
  }
}

Update the client

In the same directory, open src/main/java/io/grpc/examples/helloworld/HelloWorldClient.java. Call the new method like this:

public void greet(String name) {
  logger.info("Will try to greet " + name + " ...");
  HelloRequest request = HelloRequest.newBuilder().setName(name).build();
  HelloReply response;
  try {
    response = blockingStub.sayHello(request);
  } catch (StatusRuntimeException e) {
    logger.log(Level.WARNING, "RPC failed: {0}", e.getStatus());
    return;
  }
  logger.info("Greeting: " + response.getMessage());
  try {
    response = blockingStub.sayHelloAgain(request);
  } catch (StatusRuntimeException e) {
    logger.log(Level.WARNING, "RPC failed: {0}", e.getStatus());
    return;
  }
  logger.info("Greeting: " + response.getMessage());
}

Run the updated app

Run the client and server like you did before. Execute the following commands from the examples directory:

1. Compile the client and server:

   $ ./gradlew installDist
   

2. Run the server:

   $ ./build/install/examples/bin/hello-world-server
   INFO: Server started, listening on 50051
   

3. From another terminal, run the client:

   $ ./build/install/examples/bin/hello-world-client
   INFO: Will try to greet world ...
   INFO: Greeting: Hello world
   INFO: Greeting: Hello again world