It assumes that you have read the [Getting started](https://github.com/grpc/grpc-common) guide and are familiar with [protocol buffers] (https://developers.google.com/protocol-buffers/docs/overview). Note that the example in this tutorial uses the [proto3](https://github.com/google/protobuf/releases) version of the protocol buffers language, which is currently in alpha release: you can see the [release notes](https://github.com/google/protobuf/releases) for the new version in the protocol buffers Github repository.
This isn't a comprehensive guide to using gRPC in Java: more reference documentation is coming soon.
## 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 implement clients and servers in any of gRPC's supported languages, which in turn can be run in environments ranging from servers inside Google 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](https://github.com/grpc/grpc-java/tree/master/examples/src/main/java/io/grpc/examples). To download the example, clone the `grpc-java` repository by running the following command:
Then change your current directory to `grpc-java/examples`:
```shell
$ cd grpc-java/examples
```
You also should have the relevant tools installed to generate the server and client interface code - if you don't already, follow the setup instructions in [the Java quick start guide](https://github.com/grpc/grpc-common/tree/master/java).
## Defining the service
Our first step (as you'll know from [Getting started](https://github.com/grpc/grpc-common)) is to define the gRPC *service* and the method *request* and *response* types using [protocol buffers] (https://developers.google.com/protocol-buffers/docs/overview). You can see the complete .proto file in [`grpc-java/examples/src/main/proto/route_guide.proto`](https://github.com/grpc/grpc-java/blob/master/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:
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:
Then we define `rpc` methods inside our service definition, specifying their request and response types. gRPC lets you define four kinds of service method, 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.
- 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 `stream` keyword before the *response* type.
- 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 server-side streaming method by placing the `stream` keyword before the *request* type.
- 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 `stream` keyword before both the request and the response.
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:
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](https://github.com/google/protobuf/releases) compiler in order to generate gRPC services
For simplicity, we've provided a [Gradle build file](https://github.com/grpc/grpc-java/blob/master/examples/build.gradle) that runs `protoc` for you with the appropriate plugin, input, and output (if you want to run this yourself, make sure you've installed protoc and followed the gRPC code [installation instructions](https://github.com/grpc/grpc-java) first):
```shell
gradle build
```
which actually runs:
[actual command]
Running this command generates the following files:
-`RouteGuideGrpc.java` which contains (along with some other useful code):
- an interface for `RouteGuide` servers to implement, `RouteGuideGrpc.Service`, with all the methods defined in the `RouteGuide` service.
- *stub* classes that clients can use to talk to a `RouteGuide` server. These also implement the `RouteGuide` interface.
<aname="server"></a>
## 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](#client) (though you might find it interesting anyway!).
There are two parts to making our `RouteGuide` service do its job:
- Implementing the service interface 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/RouteGuideServer.java](https://github.com/grpc/grpc-java/blob/master/examples/src/main/java/io/grpc/examples/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 implements the generated `RouteGuideGrpc.Service` interface:
```java
private static class RouteGuideService implements RouteGuideGrpc.RouteGuide {
`RouteGuideService` implements all our service methods. Let's look at the simplest type first, `GetFeature`, which just gets a `Point` from the client and returns the corresponding feature information from its database in a `Feature`.
```java
@Override
public void getFeature(Point request, StreamObserver<Feature> responseObserver) {
responseObserver.onValue(getFeature(request));
responseObserver.onCompleted();
}
...
private Feature getFeature(Point location) {
for (Feature feature : features) {
if (feature.getLocation().getLatitude() == location.getLatitude()
-`StreamObserver<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:
1. We construct and populate a `Feature` response object to return to the client, as specified in our service definition. In this example, we do this in a separate private `getFeature()` method.
2. We use the response observer's `onValue()` method to return the `Feature`.
3. We use the response observer's `onCompleted()` method to specify that we've finished dealing with the 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 `Feature`s to our client.
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.onValue(feature);
}
}
responseObserver.onCompleted();
}
```
Like the simple RPC, this method gets a request object (the `Rectangle` in which our client wants to find `Feature`s) 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 `Write()` method. Finally, as in our simple RPC, we use the response observer's `onCompleted()` method to tell gRPC that we've finished writing responses.
Now let's look at something a little more complicated: the client-side streaming method `RecordRoute`, where we get a stream of `Point`s from the client and return a single `RouteSummary` with information about their trip.
```java
@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 onValue(Point point) {
pointCount++;
if (RouteGuideUtil.exists(getFeature(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);
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 `Point`s.
In the method body we instantiate an anonymous `StreamObserver` to return, in which we:
- Override the `onValue()` method to get features and other information each time the client writes a `Point` to the message stream.
- Override the `onCompleted()` method (called when the *client* has finished writing messages) to populate and build our `RouteSummary`. We then call our method's own response observer's `onValue()` with our `RouteSummary`, and then call its `onCompleted()` method to finish the call from the server side.
// Respond with all previous notes at this location.
for (RouteNote prevNote : notes.toArray(new RouteNote[0])) {
responseObserver.onValue(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:
As you can see, we build and start our server using a `NettyServerBuilder`. This is a builder for servers based on the [Netty](http://netty.io/) transport framework.
1. Create an instance of our service implementation class `RouteGuideService` and pass it to the generated `RouteGuideGrpc` class's static `bindService()` method to get a service definition.
3. Specify the address and port we want to use to listen for client requests using the builder's `forPort()` method.
4. Register our service implementation with the builder by passing the service definition returned from `bindService()` to the builder's `addService()` method.
5. Call `build()` and `start()` on the builder to create and start an RPC server for our service.
<aname="client"></a>
## Creating the client
In this section, we'll look at creating a Java client for our `RouteGuide` service. You can see our complete example client code in [grpc-java/examples/src/main/java/io/grpc/examples/RouteGuideClient.java](https://github.com/grpc/grpc-java/blob/master/examples/src/main/java/io/grpc/examples/RouteGuideClient.java).
### Creating 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:
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.
Follow the instructions in the example directory [README](https://github.com/grpc/grpc-java/blob/master/examples/README.md) to build and run the client and server.
