Java Quick Start

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

Prerequisites

• JDK version 7 or higher

Download the example

You’ll need a local copy of the example code to work through this quick start.Download the example code from our GitHub repository (the following commandclones the entire repository, but you just need the examples for this quick startand other tutorials):

# Clone the repository at the latest release to get the example code:
$ git clone -b v1.28.1 https://github.com/grpc/grpc-java
# Navigate to the Java examples:
$ cd grpc-java/examples

Run a gRPC application

From the examples directory:

• Compile the client and server

$ ./gradlew installDist

• Run the server:

$ ./build/install/examples/bin/hello-world-server

• From another terminal, run the client:

$ ./build/install/examples/bin/hello-world-client

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

Update a gRPC service

Now let’s look at how to update the application with an extra method on theserver for the client to call. Our gRPC service is defined using protocolbuffers; you can find out lots more about how to define a service in a .protofile ingRPC Basics: Java. For now all you need to know is that both theserver and the client “stub” have a SayHello RPC method that takes aHelloRequest parameter from the client and returns a HelloReply from theserver, and that this 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;
}

Let’s update this so that the Greeter service has two methods. Editsrc/main/proto/helloworld.proto and update it with a new SayHelloAgainmethod, 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 and run the application

When we recompile the example, normal compilation will regenerateGreeterGrpc.java, which contains our generated gRPC client and server classes.This also regenerates classes for populating, serializing, and retrieving ourrequest and response types.

However, we still need to implement and call the new method in the human-writtenparts of our example application.

Update the server

In the same directory, opensrc/main/java/io/grpc/examples/helloworld/HelloWorldServer.java. Implement thenew 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, opensrc/main/java/io/grpc/examples/helloworld/HelloWorldClient.java. Call the newmethod 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!

Just like we did before, from the examples directory:

• Compile the client and server:

$ ./gradlew installDist

• Run the server:

$ ./build/install/examples/bin/hello-world-server

• From another terminal, run the client:

$ ./build/install/examples/bin/hello-world-client

What’s next

• Read a full explanation of how gRPC works inWhat is gRPC?andgRPC Concepts.
• Work through a more detailed tutorial ingRPC Basics: Java.
• Explore the gRPC Java core API in itsreferencedocumentation.