Basics tutorial
A basic tutorial introduction to gRPC in Java.
Basics tutorial
This tutorial provides a basic Java programmer’s introduction to working with gRPC.
By walking through this example you’ll learn how to:
- Define a service in a
.protofile. - Generate server and client code using the protocol buffer compiler.
- Use the Java gRPC API to write a simple client and server for your service.
It assumes that you have read the Introduction to gRPC and are familiar with protocol buffers. Note that the example in this tutorial uses the proto3 version of the protocol buffers language: you can find out more in the proto3 language guide and Java generated code guide.
Why use gRPC?
Our example is a simple route mapping application that lets clients get information about features on their route, create a summary of their route, and exchange route information such as traffic updates with the server and other clients.
With gRPC we can define our service once in a .proto file and generate clients
and servers in any of gRPC’s supported languages, which in turn can be run in
environments ranging from servers inside a large data center to your own tablet —
all the complexity of communication between different languages and environments is
handled for you by gRPC. We also get all the advantages of working with protocol
buffers, including efficient serialization, a simple IDL, and easy interface
updating.
Example code and setup
The example code for our tutorial is in
grpc/grpc-java/examples/src/main/java/io/grpc/examples/routeguide.
To download the example, clone the latest release in grpc-java repository by
running the following command:
git clone -b v1.68.1 --depth 1 https://github.com/grpc/grpc-java
Then change your current directory to grpc-java/examples:
cd grpc-java/examples
Defining the service
Our first step (as you’ll know from the Introduction to gRPC) is to
define the gRPC service and the method request and response types using
protocol
buffers. You can
see the complete .proto file in
grpc-java/examples/src/main/proto/route_guide.proto.
As we’re generating Java code in this example, we’ve specified a java_package
file option in our .proto:
option java_package = "io.grpc.examples.routeguide";
This specifies the package we want to use for our generated Java classes. If no
explicit java_package option is given in the .proto file, then by default the
proto package (specified using the “package” keyword) will be used. However,
proto packages generally do not make good Java packages since proto packages are
not expected to start with reverse domain names. If we generate code in another
language from this .proto, the java_package option has no effect.
To define a service, we specify a named service in the .proto file:
service RouteGuide {
...
}
Then we define rpc methods inside our service definition, specifying their
request and response types. gRPC lets you define four kinds of service methods,
all of which are used in the RouteGuide service:
A simple RPC where the client sends a request to the server using the stub and waits for a response to come back, just like a normal function call.
// Obtains the feature at a given position. rpc GetFeature(Point) returns (Feature) {}A server-side streaming RPC where the client sends a request to the server and gets a stream to read a sequence of messages back. The client reads from the returned stream until there are no more messages. As you can see in our example, you specify a server-side streaming method by placing the
streamkeyword before the response type.// Obtains the Features available within the given Rectangle. Results are // streamed rather than returned at once (e.g. in a response message with a // repeated field), as the rectangle may cover a large area and contain a // huge number of features. rpc ListFeatures(Rectangle) returns (stream Feature) {}A client-side streaming RPC where the client writes a sequence of messages and sends them to the server, again using a provided stream. Once the client has finished writing the messages, it waits for the server to read them all and return its response. You specify a client-side streaming method by placing the
streamkeyword before the request type.// Accepts a stream of Points on a route being traversed, returning a // RouteSummary when traversal is completed. rpc RecordRoute(stream Point) returns (RouteSummary) {}A bidirectional streaming RPC where both sides send a sequence of messages using a read-write stream. The two streams operate independently, so clients and servers can read and write in whatever order they like: for example, the server could wait to receive all the client messages before writing its responses, or it could alternately read a message then write a message, or some other combination of reads and writes. The order of messages in each stream is preserved. You specify this type of method by placing the
streamkeyword before both the request and the response.// Accepts a stream of RouteNotes sent while a route is being traversed, // while receiving other RouteNotes (e.g. from other users). rpc RouteChat(stream RouteNote) returns (stream RouteNote) {}
Our .proto file also contains protocol buffer message type definitions for all
the request and response types used in our service methods - for example, here’s
the Point message type:
// Points are represented as latitude-longitude pairs in the E7 representation
// (degrees multiplied by 10**7 and rounded to the nearest integer).
// Latitudes should be in the range +/- 90 degrees and longitude should be in
// the range +/- 180 degrees (inclusive).
message Point {
int32 latitude = 1;
int32 longitude = 2;
}
Generating client and server code
Next we need to generate the gRPC client and server interfaces from our .proto
service definition. We do this using the protocol buffer compiler protoc with
a special gRPC Java plugin. You need to use the
proto3 compiler (which supports
both proto2 and proto3 syntax) in order to generate gRPC services.
When using Gradle or Maven, the protoc build plugin can generate the necessary
code as part of the build. You can refer to the grpc-java README for
how to generate code from your own .proto files.
The following classes are generated from our service definition:
Feature.java,Point.java,Rectangle.java, and others which contain all the protocol buffer code to populate, serialize, and retrieve our request and response message types.RouteGuideGrpc.javawhich contains (along with some other useful code):- a base class for
RouteGuideservers to implement,RouteGuideGrpc.RouteGuideImplBase, with all the methods defined in theRouteGuideservice. - stub classes that clients can use to talk to a
RouteGuideserver.
- a base class for
Creating the server
First let’s look at how we create a RouteGuide server. If you’re only
interested in creating gRPC clients, you can skip this section and go straight
to Creating the client (though you might find it interesting
anyway!).
There are two parts to making our RouteGuide service do its job:
- Overriding the service base class generated from our service definition: doing the actual “work” of our service.
- Running a gRPC server to listen for requests from clients and return the service responses.
You can find our example RouteGuide server in
grpc-java/examples/src/main/java/io/grpc/examples/routeguide/RouteGuideServer.java.
Let’s take a closer look at how it works.
Implementing RouteGuide
As you can see, our server has a RouteGuideService class that extends the
generated RouteGuideGrpc.RouteGuideImplBase abstract class:
private static class RouteGuideService extends RouteGuideGrpc.RouteGuideImplBase {
...
}
Simple RPC
RouteGuideService implements all our service methods. Let’s
look at the simplest method first, GetFeature(), which just gets a Point from
the client and returns the corresponding feature information from its database
in a Feature.
@Override
public void getFeature(Point request, StreamObserver<Feature> responseObserver) {
responseObserver.onNext(checkFeature(request));
responseObserver.onCompleted();
}
...
private Feature checkFeature(Point location) {
for (Feature feature : features) {
if (feature.getLocation().getLatitude() == location.getLatitude()
&& feature.getLocation().getLongitude() == location.getLongitude()) {
return feature;
}
}
// No feature was found, return an unnamed feature.
return Feature.newBuilder().setName("").setLocation(location).build();
}
The getFeature() method takes two parameters:
Point: the requestStreamObserver<Feature>: a response observer, which is a special interface for the server to call with its response.
To return our response to the client and complete the call:
- We construct and populate a
Featureresponse object to return to the client, as specified in our service definition. In this example, we do this in a separate privatecheckFeature()method. - We use the response observer’s
onNext()method to return theFeature. - We use the response observer’s
onCompleted()method to specify that we’ve finished dealing with the RPC.
Server-side streaming RPC
Next let’s look at one of our streaming RPCs. ListFeatures is a server-side
streaming RPC, so we need to send back multiple Features to our client.
private final Collection<Feature> features;
...
@Override
public void listFeatures(Rectangle request, StreamObserver<Feature> responseObserver) {
int left = min(request.getLo().getLongitude(), request.getHi().getLongitude());
int right = max(request.getLo().getLongitude(), request.getHi().getLongitude());
int top = max(request.getLo().getLatitude(), request.getHi().getLatitude());
int bottom = min(request.getLo().getLatitude(), request.getHi().getLatitude());
for (Feature feature : features) {
if (!RouteGuideUtil.exists(feature)) {
continue;
}
int lat = feature.getLocation().getLatitude();
int lon = feature.getLocation().getLongitude();
if (lon >= left && lon <= right && lat >= bottom && lat <= top) {
responseObserver.onNext(feature);
}
}
responseObserver.onCompleted();
}
Like the simple RPC, this method gets a request object (the Rectangle in which
our client wants to find Features) and a StreamObserver response observer.
This time, we get as many Feature objects as we need to return to the client
(in this case, we select them from the service’s feature collection based on
whether they’re inside our request Rectangle), and write them each in turn to
the response observer using its onNext() method. Finally, as in our simple
RPC, we use the response observer’s onCompleted() method to tell gRPC that
we’ve finished writing responses.
Client-side streaming RPC
Now let’s look at something a little more complicated: the client-side streaming
method RecordRoute(), where we get a stream of Points from the client and
return a single RouteSummary with information about their trip.
@Override
public StreamObserver<Point> recordRoute(final StreamObserver<RouteSummary> responseObserver) {
return new StreamObserver<Point>() {
int pointCount;
int featureCount;
int distance;
Point previous;
long startTime = System.nanoTime();
@Override
public void onNext(Point point) {
pointCount++;
if (RouteGuideUtil.exists(checkFeature(point))) {
featureCount++;
}
// For each point after the first, add the incremental distance from the previous point
// to the total distance value.
if (previous != null) {
distance += calcDistance(previous, point);
}
previous = point;
}
@Override
public void onError(Throwable t) {
logger.log(Level.WARNING, "Encountered error in recordRoute", t);
}
@Override
public void onCompleted() {
long seconds = NANOSECONDS.toSeconds(System.nanoTime() - startTime);
responseObserver.onNext(RouteSummary.newBuilder().setPointCount(pointCount)
.setFeatureCount(featureCount).setDistance(distance)
.setElapsedTime((int) seconds).build());
responseObserver.onCompleted();
}
};
}
As you can see, like the previous method types our method gets a
StreamObserver response observer parameter, but this time it returns a
StreamObserver for the client to write its Points.
In the method body we instantiate an anonymous StreamObserver to return, in
which we:
- Override the
onNext()method to get features and other information each time the client writes aPointto the message stream. - Override the
onCompleted()method (called when the client has finished writing messages) to populate and build ourRouteSummary. We then call our method’s own response observer’sonNext()with ourRouteSummary, and then call itsonCompleted()method to finish the call from the server side.
Bidirectional streaming RPC
Finally, let’s look at our bidirectional streaming RPC RouteChat().
@Override
public StreamObserver<RouteNote> routeChat(final StreamObserver<RouteNote> responseObserver) {
return new StreamObserver<RouteNote>() {
@Override
public void onNext(RouteNote note) {
List<RouteNote> notes = getOrCreateNotes(note.getLocation());
// Respond with all previous notes at this location.
for (RouteNote prevNote : notes.toArray(new RouteNote[0])) {
responseObserver.onNext(prevNote);
}
// Now add the new note to the list
notes.add(note);
}
@Override
public void onError(Throwable t) {
logger.log(Level.WARNING, "Encountered error in routeChat", t);
}
@Override
public void onCompleted() {
responseObserver.onCompleted();
}
};
}
As with our client-side streaming example, we both get and return a
StreamObserver response observer, except this time we return values via our
method’s response observer while the client is still writing messages to their
message stream. The syntax for reading and writing here is exactly the same as
for our client-streaming and server-streaming methods. Although each side will
always get the other’s messages in the order they were written, both the client
and server can read and write in any order — the streams operate completely
independently.
Starting the server
Once we’ve implemented all our methods, we also need to start up a gRPC server
so that clients can actually use our service. The following snippet shows how we
do this for our RouteGuide service:
public RouteGuideServer(int port, URL featureFile) throws IOException {
this(ServerBuilder.forPort(port), port, RouteGuideUtil.parseFeatures(featureFile));
}
/** Create a RouteGuide server using serverBuilder as a base and features as data. */
public RouteGuideServer(ServerBuilder<?> serverBuilder, int port, Collection<Feature> features) {
this.port = port;
server = serverBuilder.addService(new RouteGuideService(features))
.build();
}
...
public void start() throws IOException {
server.start();
logger.info("Server started, listening on " + port);
...
}
As you can see, we build and start our server using a ServerBuilder.
To do this, we:
- Specify the address and port we want to use to listen for client requests
using the builder’s
forPort()method. - Create an instance of our service implementation class
RouteGuideServiceand pass it to the builder’saddService()method. - Call
build()andstart()on the builder to create and start an RPC server for our service.
Creating the client
In this section, we’ll look at creating a client for our RouteGuide
service. You can see our complete example client code in
grpc-java/examples/src/main/java/io/grpc/examples/routeguide/RouteGuideClient.java.
Instantiating a stub
To call service methods, we first need to create a stub, or rather, two stubs:
- a blocking/synchronous stub: this means that the RPC call waits for the server to respond, and will either return a response or raise an exception.
- a non-blocking/asynchronous stub that makes non-blocking calls to the server, where the response is returned asynchronously. You can make certain types of streaming call only using the asynchronous stub.
First we need to create a gRPC channel for our stub, specifying the server address and port we want to connect to:
public RouteGuideClient(String host, int port) {
this(ManagedChannelBuilder.forAddress(host, port).usePlaintext());
}
/** Construct client for accessing RouteGuide server using the existing channel. */
public RouteGuideClient(ManagedChannelBuilder<?> channelBuilder) {
channel = channelBuilder.build();
blockingStub = RouteGuideGrpc.newBlockingStub(channel);
asyncStub = RouteGuideGrpc.newStub(channel);
}
We use a ManagedChannelBuilder to create the channel.
Now we can use the channel to create our stubs using the newStub and
newBlockingStub methods provided in the RouteGuideGrpc class we generated
from our .proto.
blockingStub = RouteGuideGrpc.newBlockingStub(channel);
asyncStub = RouteGuideGrpc.newStub(channel);
Calling service methods
Now let’s look at how we call our service methods.
Simple RPC
Calling the simple RPC GetFeature on the blocking stub is as straightforward
as calling a local method.
Point request = Point.newBuilder().setLatitude(lat).setLongitude(lon).build();
Feature feature;
try {
feature = blockingStub.getFeature(request);
} catch (StatusRuntimeException e) {
logger.log(Level.WARNING, "RPC failed: {0}", e.getStatus());
return;
}
We create and populate a request protocol buffer object (in our case Point),
pass it to the getFeature() method on our blocking stub, and get back a
Feature.
If an error occurs, it is encoded as a Status, which we can obtain from the
StatusRuntimeException.
Server-side streaming RPC
Next, let’s look at a server-side streaming call to ListFeatures, which
returns a stream of geographical Features:
Rectangle request =
Rectangle.newBuilder()
.setLo(Point.newBuilder().setLatitude(lowLat).setLongitude(lowLon).build())
.setHi(Point.newBuilder().setLatitude(hiLat).setLongitude(hiLon).build()).build();
Iterator<Feature> features;
try {
features = blockingStub.listFeatures(request);
} catch (StatusRuntimeException e) {
logger.log(Level.WARNING, "RPC failed: {0}", e.getStatus());
return;
}
As you can see, it’s very similar to the simple RPC we just looked at, except
instead of returning a single Feature, the method returns an Iterator that
the client can use to read all the returned Features.
Client-side streaming RPC
Now for something a little more complicated: the client-side streaming method
RecordRoute, where we send a stream of Points to the server and get back a
single RouteSummary. For this method we need to use the asynchronous stub. If
you’ve already read Creating the server some of this may look very
familiar - asynchronous streaming RPCs are implemented in a similar way on both
sides.
public void recordRoute(List<Feature> features, int numPoints) throws InterruptedException {
info("*** RecordRoute");
final CountDownLatch finishLatch = new CountDownLatch(1);
StreamObserver<RouteSummary> responseObserver = new StreamObserver<RouteSummary>() {
@Override
public void onNext(RouteSummary summary) {
info("Finished trip with {0} points. Passed {1} features. "
+ "Travelled {2} meters. It took {3} seconds.", summary.getPointCount(),
summary.getFeatureCount(), summary.getDistance(), summary.getElapsedTime());
}
@Override
public void onError(Throwable t) {
Status status = Status.fromThrowable(t);
logger.log(Level.WARNING, "RecordRoute Failed: {0}", status);
finishLatch.countDown();
}
@Override
public void onCompleted() {
info("Finished RecordRoute");
finishLatch.countDown();
}
};
StreamObserver<Point> requestObserver = asyncStub.recordRoute(responseObserver);
try {
// Send numPoints points randomly selected from the features list.
Random rand = new Random();
for (int i = 0; i < numPoints; ++i) {
int index = rand.nextInt(features.size());
Point point = features.get(index).getLocation();
info("Visiting point {0}, {1}", RouteGuideUtil.getLatitude(point),
RouteGuideUtil.getLongitude(point));
requestObserver.onNext(point);
// Sleep for a bit before sending the next one.
Thread.sleep(rand.nextInt(1000) + 500);
if (finishLatch.getCount() == 0) {
// RPC completed or errored before we finished sending.
// Sending further requests won't error, but they will just be thrown away.
return;
}
}
} catch (RuntimeException e) {
// Cancel RPC
requestObserver.onError(e);
throw e;
}
// Mark the end of requests
requestObserver.onCompleted();
// Receiving happens asynchronously
finishLatch.await(1, TimeUnit.MINUTES);
}
As you can see, to call this method we need to create a StreamObserver, which
implements a special interface for the server to call with its RouteSummary
response. In our StreamObserver we:
- Override the
onNext()method to print out the returned information when the server writes aRouteSummaryto the message stream. - Override the
onCompleted()method (called when the server has completed the call on its side) to reduce aCountDownLatchthat we can check to see if the server has finished writing.
We then pass the StreamObserver to the asynchronous stub’s recordRoute()
method and get back our own StreamObserver request observer to write our
Points to send to the server. Once we’ve finished writing points, we use the
request observer’s onCompleted() method to tell gRPC that we’ve finished
writing on the client side. Once we’re done, we check our CountDownLatch to
check that the server has completed on its side.
Bidirectional streaming RPC
Finally, let’s look at our bidirectional streaming RPC RouteChat().
public void routeChat() throws Exception {
info("*** RoutChat");
final CountDownLatch finishLatch = new CountDownLatch(1);
StreamObserver<RouteNote> requestObserver =
asyncStub.routeChat(new StreamObserver<RouteNote>() {
@Override
public void onNext(RouteNote note) {
info("Got message \"{0}\" at {1}, {2}", note.getMessage(), note.getLocation()
.getLatitude(), note.getLocation().getLongitude());
}
@Override
public void onError(Throwable t) {
Status status = Status.fromThrowable(t);
logger.log(Level.WARNING, "RouteChat Failed: {0}", status);
finishLatch.countDown();
}
@Override
public void onCompleted() {
info("Finished RouteChat");
finishLatch.countDown();
}
});
try {
RouteNote[] requests =
{newNote("First message", 0, 0), newNote("Second message", 0, 1),
newNote("Third message", 1, 0), newNote("Fourth message", 1, 1)};
for (RouteNote request : requests) {
info("Sending message \"{0}\" at {1}, {2}", request.getMessage(), request.getLocation()
.getLatitude(), request.getLocation().getLongitude());
requestObserver.onNext(request);
}
} catch (RuntimeException e) {
// Cancel RPC
requestObserver.onError(e);
throw e;
}
// Mark the end of requests
requestObserver.onCompleted();
// Receiving happens asynchronously
finishLatch.await(1, TimeUnit.MINUTES);
}
As with our client-side streaming example, we both get and return a
StreamObserver response observer, except this time we send values via our
method’s response observer while the server is still writing messages to their
message stream. The syntax for reading and writing here is exactly the same as
for our client-streaming method. Although each side will always get the other’s
messages in the order they were written, both the client and server can read and
write in any order — the streams operate completely independently.
Try it out!
Follow the instructions in the example directory README to build and run the client and server.