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.
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.
The service stub signature for a unary RPC method unaryExample:
public void unaryExample(
RequestType request,
StreamObserver<ResponseType> responseObserver)
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).
The service stub signature for a client-streaming RPC method
clientStreamingExample:
public StreamObserver<RequestType> clientStreamingExample(
StreamObserver<ResponseType> responseObserver)
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.
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.
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.
The asynchronous stub signature for a unary RPC method unaryExample:
public void unaryExample(
RequestType request,
StreamObserver<ResponseType> responseObserver)
The asynchronous stub signature for a server-streaming RPC method
serverStreamingExample:
public void serverStreamingExample(
RequestType request,
StreamObserver<ResponseType> responseObserver)
The asynchronous stub signature for a client-streaming RPC method
clientStreamingExample:
public StreamObserver<RequestType> clientStreamingExample(
StreamObserver<ResponseType> responseObserver)
The asynchronous stub signature for a bidirectional-streaming RPC method
bidirectionalStreamingExample:
public StreamObserver<RequestType> bidirectionalStreamingExample(
StreamObserver<ResponseType> responseObserver)
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.
The blocking stub signature for a unary RPC method unaryExample:
public ResponseType unaryExample(RequestType request)
The blocking stub signature for a server-streaming RPC method
serverStreamingExample:
public Iterator<ResponseType> serverStreamingExample(RequestType request)
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.
The future stub signature for a unary RPC method unaryExample:
public ListenableFuture<ResponseType> unaryExample(RequestType request)
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 https://grpc.io/docs/tutorials/basic/android.html#generating-client-code 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"