Java Generated Code Reference

Packages

For each service defined in a .proto file, the Java code generation produces a Java class. The class name is the service’s name suffixed by Grpc. The package for the generated code is specified in the .proto file using the java_package option.

For example, if ServiceName is defined in a .proto file containing the following:

package grpcexample;

option java_package = "io.grpc.examples";

Then the generated class will be io.grpc.examples.ServiceNameGrpc.

If java_package is not specified, the generated class will use the package as specified in the .proto file. This should be avoided, as proto packages usually do not begin with a reversed domain name.

Service Stub

The generated Java code contains an inner abstract class suffixed with ImplBase, such as ServiceNameImplBase. This class defines one Java method for each method in the service definition. It is up to the service implementer to extend this class and implement the functionality of these methods. Without being overridden, the methods return an error to the client saying the method is unimplemented.

The signatures of the stub methods in ServiceNameImplBase vary depending on the type of RPCs it handles. There are four types of gRPC service methods: unary, server-streaming, client-streaming, and bidirectional-streaming.

Unary

The service stub signature for a unary RPC method unaryExample:

public void unaryExample(
    RequestType request,
    StreamObserver<ResponseType> responseObserver)

Server-streaming

The service stub signature for a server-streaming RPC method serverStreamingExample:

public void serverStreamingExample(
    RequestType request,
    StreamObserver<ResponseType> responseObserver)

Notice that the signatures for unary and server-streaming RPCs are the same. A single RequestType is received from the client, and the service implementation sends its response(s) by invoking responseObserver.onNext(ResponseType response).

Client-streaming

The service stub signature for a client-streaming RPC method clientStreamingExample:

public StreamObserver<RequestType> clientStreamingExample(
    StreamObserver<ResponseType> responseObserver)

Bidirectional-streaming

The service stub signature for a bidirectional-streaming RPC method bidirectionalStreamingExample:

public StreamObserver<RequestType> bidirectionalStreamingExample(
    StreamObserver<ResponseType> responseObserver)

The signatures for client and bidirectional-streaming RPCs are the same. Since the client can send multiple messages to the service, the service implementation is reponsible for returning a StreamObserver<RequestType> instance. This StreamObserver is invoked whenever additional messages are received from the client.

Client Stubs

The generated class also contains stubs for use by gRPC clients to call methods defined by the service. Each stub wraps a Channel, supplied by the user of the generated code. The stub uses this channel to send RPCs to the service.

gRPC Java generates code for three types of stubs: asynchronous, blocking, and future. Each type of stub has a corresponding class in the generated code, such as ServiceNameStub, ServiceNameBlockingStub, and ServiceNameFutureStub.

Asynchronous Stub

RPCs made via an asynchronous stub operate entirely through callbacks on StreamObserver.

The asynchronous stub contains one Java method for each method from the service definition.

A new asynchronous stub is instantiated via the ServiceNameGrpc.newStub(Channel channel) static method.

Unary

The asynchronous stub signature for a unary RPC method unaryExample:

public void unaryExample(
    RequestType request,
    StreamObserver<ResponseType> responseObserver)

Server-streaming

The asynchronous stub signature for a server-streaming RPC method serverStreamingExample:

public void serverStreamingExample(
    RequestType request,
    StreamObserver<ResponseType> responseObserver)

Client-streaming

The asynchronous stub signature for a client-streaming RPC method clientStreamingExample:

public StreamObserver<RequestType> clientStreamingExample(
    StreamObserver<ResponseType> responseObserver)

Bidirectional-streaming

The asynchronous stub signature for a bidirectional-streaming RPC method bidirectionalStreamingExample:

public StreamObserver<RequestType> bidirectionalStreamingExample(
    StreamObserver<ResponseType> responseObserver)

Blocking Stub

RPCs made through a blocking stub, as the name implies, block until the response from the service is available.

The blocking stub contains one Java method for each unary and server-streaming method in the service definition. Blocking stubs do not support client-streaming or bidirectional-streaming RPCs.

A new blocking stub is instantiated via the ServiceNameGrpc.newBlockingStub(Channel channel) static method.

Unary

The blocking stub signature for a unary RPC method unaryExample:

public ResponseType unaryExample(RequestType request)

Server-streaming

The blocking stub signature for a server-streaming RPC method serverStreamingExample:

public Iterator<ResponseType> serverStreamingExample(RequestType request)

Future Stub

RPCs made via a future stub wrap the return value of the asynchronous stub in a GrpcFuture<ResponseType>, which implements the com.google.common.util.concurrent.ListenableFuture interface.

The future stub contains one Java method for each unary method in the service definition. Future stubs do not support streaming calls.

A new future stub is instantiated via the ServiceNameGrpc.newFutureStub(Channel channel) static method.

Unary

The future stub signature for a unary RPC method unaryExample:

public ListenableFuture<ResponseType> unaryExample(RequestType request)

Codegen

Typically the build system handles creation of the gRPC generated code.

For protobuf-based codegen, you can put your .proto files in the src/main/proto and src/test/proto directories along with an appropriate plugin.

A typical protobuf-maven-plugin configuration for generating gRPC and Protocol Buffers code would look like the following:

<build>
  <extensions>
    <extension>
      <groupId>kr.motd.maven</groupId>
      <artifactId>os-maven-plugin</artifactId>
      <version>1.4.1.Final</version>
    </extension>
  </extensions>
  <plugins>
    <plugin>
      <groupId>org.xolstice.maven.plugins</groupId>
      <artifactId>protobuf-maven-plugin</artifactId>
      <version>0.5.0</version>
      <configuration>
        <protocArtifact>com.google.protobuf:protoc:3.3.0:exe:${os.detected.classifier}</protocArtifact>
        <pluginId>grpc-java</pluginId>
        <pluginArtifact>io.grpc:protoc-gen-grpc-java:1.4.0:exe:${os.detected.classifier}</pluginArtifact>
      </configuration>
      <executions>
        <execution>
          <goals>
            <goal>compile</goal>
            <goal>compile-custom</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

Eclipse and NetBeans users should also look at os-maven-plugin’s IDE documentation.

A typical protobuf-gradle-plugin configuration would look like the following:

apply plugin: 'java'
apply plugin: 'com.google.protobuf'

buildscript {
  repositories {
    mavenCentral()
  }
  dependencies {
    // ASSUMES GRADLE 2.12 OR HIGHER. Use plugin version 0.7.5 with earlier
    // gradle versions
    classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.0'
  }
}

protobuf {
  protoc {
    artifact = "com.google.protobuf:protoc:3.2.0"
  }
  plugins {
    grpc {
      artifact = 'io.grpc:protoc-gen-grpc-java:1.4.0'
    }
  }
  generateProtoTasks {
    all()*.plugins {
      grpc {}
    }
  }
}

Bazel developers can use the java_grpc_library rule, typically as follows:

load("@grpc_java//:java_grpc_library.bzl", "java_grpc_library")

proto_library(
    name = "helloworld_proto",
    srcs = ["src/main/proto/helloworld.proto"],
)

java_proto_library(
    name = "helloworld_java_proto",
    deps = [":helloworld_proto"],
)

java_grpc_library(
    name = "helloworld_java_grpc",
    srcs = [":helloworld_proto"],
    deps = [":helloworld_java_proto"],
)

Android developers please see this for reference.

If you wish to invoke the protobuf plugin for gRPC Java directly, the command-line syntax is as follows:

$ protoc --plugin=protoc-gen-grpc-java \
  --grpc-java_out="$OUTPUT_FILE" --proto_path="$DIR_OF_PROTO_FILE" "$PROTO_FILE"