Quick start

Prerequisites

• Kotlin version 1.3 or higher

• JDK version 7 or higher

• Android SDK, API level 16 or higher

  1. Install Android Studio or the Android command-line tools.

  2. Let other tools and scripts know where to find your Android SDK by setting the following environment variable:

     $ export ANDROID_SDK_ROOT="<path-to-your-android-sdk>"
     

• An android device set up for USB debugging or an Android Virtual Device

Get the example code

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

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

   $ git clone https://github.com/grpc/grpc-kotlin
   

2. Change to the examples directory:

   $ cd grpc-kotlin/examples
   

Run the example

1. Compile the server:

   $ ./gradlew installDist
   

2. Run the server:

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

3. From another terminal, build the client and install it on your device:

   $ ./gradlew :android:installDebug
   

4. Launch the client app from your device.

5. Type “Alice” in the Name box and click Send. You’ll see the following response:

   Hello Alice
   

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 helloworld/hello_world.proto from the protos/src/main/proto/io/grpc/examples folder, and add a new SayHelloAgain() method, with the same request and response types:

// 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 HelloWorldGrpcKt.kt, 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

Follow the instructions given in Update the server of the Kotlin quick start page.

Update the client

Follow these steps:

1. Open helloworld/MainActivity.kt from the client/src/main/kotlin/io/grpc/examples folder.

2. Locate the function containing the call to sayHello(). You’ll see these lines of code:

   val response = greeter.sayHello(request)
   responseText.text = response.message
   

3. Add a call to sayHelloAgain() and change how the response message is created. Replace the lines of code above with the following:

   val response = greeter.sayHello(request)
   val againResponse = greeter.sayHelloAgain(request)
   val message = "${response.message}\n${againResponse.message}"
   responseText.text = message
   

Run the updated app

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

1. Compile the server:

   $ ./gradlew installDist
   

2. Run the server:

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

3. From another terminal, build the client and install it on your device:

   $ ./gradlew :android:installDebug
   

4. Launch the client app from your device.

5. Type “Alice” in the Message box and click Send. You’ll see the following response:

   Hello Alice
   Hello again Alice
   

What’s next

• Learn how gRPC works in Introduction to gRPC and Core concepts.

• Explore the API reference.