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Quick Start

This guide gets you started with gRPC in Android Java with a simple working example.


Prerequisites

  • 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

Note

gRPC Java does not support running a server on an Android device. For this quick start, the Android client app will connect to a server running on your local (non-Android) computer.

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 v1.32.1 https://github.com/grpc/grpc-java
    
  2. Change to the examples directory:

    $ cd grpc-java/examples
    

Run the example

  1. Compile the server:

    $ ./gradlew installDist
    
  2. Run the server:

    $ ./build/install/examples/bin/hello-world-server
    INFO: Server started, listening on 50051
    
  3. From another terminal, build the client and install it on your device:

    $ (cd android/helloworld; ../../gradlew installDebug)
    
  4. Launch the client app from your device.

  5. In the client app, enter the server’s Host and Port information. The values you enter depend on the device kind (real or virtual) — for details, see Connecting to the server below.

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

    Hello Alice
    

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

Note

We’ve omitted timestamps from the client and server trace output shown in this page.

Update a 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;
}
  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;
    }
    
  2. Make the same change to android/helloworld/app/src/main/proto/helloworld.proto.

Remember to save the files!

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

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

Update the client

Follow these steps:

  1. Open HelloworldActivity.java from the android/helloworld/app/src/main/java/io/grpc/helloworldexample folder.

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

    HelloReply reply = stub.sayHello(request);
    return reply.getMessage();
    
  3. Add a call to sayHelloAgain() in the return statement expression like this:

    return reply.getMessage() + "\n" + stub.sayHelloAgain(request).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 server:

    $ ./gradlew installDist
    
  2. Run the server:

    $ ./build/install/examples/bin/hello-world-server
    INFO: Server started, listening on 50051
    
  3. From another terminal, build the client and install it on your device:

    $ (cd android/helloworld; ../../gradlew installDebug)
    
  4. Launch the client app from your device.

  5. In the client app, enter the server’s Host and Port information. The values you enter depend on the device kind (real or virtual) — for details, see Connecting to the server below.

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

    Hello Alice
    Hello again Alice
    

Connecting to the server

Connecting from a virtual device

Run the Hello World app on your Android Virtual Device and use the following values:

  • Host: 10.0.2.2
  • Port: 50051

Connecting from a physical device

To run the app on a physical device via USB debugging, you must configure USB port forwarding using the adb command as follows:

$ adb reverse tcp:8080 tcp:50051

This sets up port forwarding from port 8080 on the device to port 50051 on the connected computer, which is the port that the Hello World server is listening on.

In the app, use the following values:

  • Host: localhost
  • Port: 8080

What’s next