grpc.io/content/en/docs/languages/java/basics.md

---
title: Basics tutorial
description: A basic tutorial introduction to gRPC in Java.
weight: 50
---

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 `.proto` file.
- 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](/docs/what-is-grpc/introduction/) and are familiar
with [protocol
buffers](https://protobuf.dev/overview). Note
that the example in this tutorial uses the
[proto3](https://github.com/google/protobuf/releases) version of the protocol
buffers language: you can find out more in the [proto3 language
guide](https://protobuf.dev/programming-guides/proto3) and [Java
generated code
guide](https://protobuf.dev/reference/java/java-generated).

### Why use gRPC?

{{< why-grpc >}}

### Example code and setup

The example code for our tutorial is in
[grpc/grpc-java/examples/src/main/java/io/grpc/examples/routeguide](https://github.com/grpc/grpc-java/tree/master/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:

```sh
git clone -b {{< param grpc_vers.java >}} --depth 1 https://github.com/grpc/grpc-java
```

Then change your current directory to `grpc-java/examples`:

```sh
cd grpc-java/examples
```

### Defining the service

Our first step (as you'll know from the [Introduction to gRPC](/docs/what-is-grpc/introduction/)) is to
define the gRPC *service* and the method *request* and *response* types using
[protocol
buffers](https://protobuf.dev/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`:

```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:

```proto
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.

  ```proto
  // 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 `stream`
  keyword before the *response* type.

  ```proto
  // 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 `stream` keyword before the *request* type.

  ```proto
  // 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 `stream`
  keyword before both the request and the response.

  ```proto
  // 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:

```proto
// 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](https://github.com/google/protobuf/releases) 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.java` which contains (along with some other useful code):
  - a base class for `RouteGuide` servers to implement,
    `RouteGuideGrpc.RouteGuideImplBase`, with all the methods defined in the
    `RouteGuide` service.
  - *stub* classes that clients can use to talk to a `RouteGuide` server.

### Creating the server {#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:

- 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](https://github.com/grpc/grpc-java/blob/master/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:

```java
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`.

```java
@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 request
- `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 `checkFeature()` method.
2. We use the response observer's `onNext()` method to return the `Feature`.
3. 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 `Feature`s to our client.

```java
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 `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 `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 `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 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 `Point`s.

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 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 `onNext()` with our `RouteSummary`, and then
  call its `onCompleted()` method to finish the call from the server side.

##### Bidirectional streaming RPC

Finally, let's look at our bidirectional streaming RPC `RouteChat()`.

```java
@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:

```java
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:

1. Specify the address and port we want to use to listen for client requests
   using the builder's `forPort()` method.
1. Create an instance of our service implementation class `RouteGuideService`
   and pass it to the builder's `addService()` method.
1. Call `build()` and `start()` on the builder to create and start an RPC server
   for our service.

### Creating the client {#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](https://github.com/grpc/grpc-java/blob/master/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:

```java
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`.

```java
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.

```java
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 `Feature`s:

```java
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 `Feature`s.

##### Client-side streaming RPC

Now for something a little more complicated: the client-side streaming method
`RecordRoute`, where we send a stream of `Point`s 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](#server) some of this may look very
familiar - asynchronous streaming RPCs are implemented in a similar way on both
sides.

```java
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 a `RouteSummary` to the message stream.
- Override the `onCompleted()` method (called when the *server* has completed
  the call on its side) to reduce a `CountDownLatch` that 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
`Point`s 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()`.

```java
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.

[example directory README]: https://github.com/grpc/grpc-java/blob/master/examples/README.md
[grpc-java README]: https://github.com/grpc/grpc-java/blob/master/README.md