Getting started with Spine in Java

This guide will walk you through a minimal client-server application in Java which handles one command to print some text on behalf of the current computer user. The document goes through already written code which is quite simple. So, it won’t take long.

What we’ll do

We’ll go through the example which shows a Bounded Context called “Hello”. The context has one ProcessManager, which handles the Print command sent from the client-side code to the server-side code hosting the context. We’ll go through the production code of the example suite, and through the code which tests the Hello context.

What you’ll need

  1. JDK version 8 or higher.
  2. Git.
  3. The source code of the Hello World example.

    git clone [email protected]:spine-examples/hello.git

Run the code

To check that you’ve got everything installed, please run the following command:

./gradlew :sayHello

If you’re under Windows, it would be:

gradlew.bat :sayHello

This would build and execute the example. The process should finish with the output which looks like this:

> Task :sayHello
Dec 18, 2020 5:32:11 PM io.spine.base.Environment setCurrentType
INFO: `Environment` set to `io.spine.base.Production`.
Dec 18, 2020 5:32:12 PM io.spine.server.Server lambda$start$1
INFO: In-process server started with the name `a7c62b63-2cc6-4679-bb92-072591142275`.
[savik] Hello World!
The client received the event: io.spine.helloworld.hello.event.Printed{"username":"savik","text":"Hello World!"}
Dec 18, 2020 5:32:15 PM io.spine.server.Server shutdown
INFO: Shutting down the server...
Dec 18, 2020 5:32:15 PM io.spine.server.Server shutdown
INFO: Server shut down.

The first line tells which Gradle task we run. The following couple of lines is the server-side logging that informs us that the server was started.

The line with the Environment tells that we’re running the application in the Production environment. The line with “Hello World!” text is the “meat” of this example suite. It is what our ProcessManager (called Console) does in response to the Print command received from the Client. The text in between brackets is the name of the current computer user. The name was passed as the argument of the Print command.

We opted to show a ProcessManager — instead of an Aggregate — because the console output is similar to an “External System”. Dealing with things like that is the job of Process Managers. We also want to highlight the importance of using this architectural pattern.

The output that follows is the logging produced by the Client class as it receives the Printed event from the server.

Then, the server shuts down concluding the example.

Now, let’s dive into the code.

Project structure

For the sake of simplicity, this example is organised as a single-module Gradle project. Most likely, a project for a real world application would be multi-module.

The root directory

The root of the project contains the following files:

  • LICENSE — the text of the Apache v2 license under which the framework and this example are licensed.
  • — a brief intro for the example.
  • gradlew and gradlew.bat — scripts for running Gradle Wrapper.
  • build.gradle — the project configuration. We’ll review this file later in details.

The root directory also contains “invisible” files, names of which start with the dot (e.g. .gitattributes and .travis.yml). These files configure Git and CI systems we use. They are not directly related to the subject of the example and this guide. If you’re interested in this level of details, please look into the code and comments in these files.

Here are the directories of interest in the project root:

  • gradle — this directory contains the code of Gradle Wrapper and two Gradle scripts used in the project configuration.
  • generated — this directory contains the code generated by Protobuf Compiler and Spine Model Compiler. This directory and code it contains is created automatically when a domain model changes. This directory is excluded from version control.
  • src — contains the handcrafted source code.

Let’s see how the source code is structured.

The src directory

The source code directory follows standard Gradle conventions and has two sub-directories:

  • main — the production code;
  • test — the tests for the production code.

The production code consists of two parts allocated by sub-directories:

  • proto — contains the definition of data structures in Google Protobuf. A domain model definition starts from adding the code to this directory. Then, the Protobuf code is compiled to the languages of the project. The output of this process is placed into the generated directory, with a sub-directory for each language. This example uses only Java.

  • java — this directory contains the model behavior and other server- and client-side code. A real project would have these parts in separate modules or projects. We put it all together for the sake of simplicity.

Now, let’s review the code in details, starting with how to add Spine to a Gradle project.

Adding Spine to a Gradle project

Let’s open build.gradle from the root of the project. The simplest and recommended way for adding Spine dependencies to a project is the Bootstrap plugin:

plugins {

Once the plugin is added, we can use its features:


This enables Java in the module and adds necessary dependencies and configurations.

Calling spine.enableJava().server() adds both server- and client-side dependencies. This way a module of a Bounded Context “A” may be a client for a Bounded Context “B”. Client-side applications or modules should call: spine.enableJava().client().

Other project configuration

The rest of the build.gradle file does the following:

  1. Sets the version of Java to 8.

  2. Adds JUnit dependencies by applying the tests.gradle script plugin (which we extracted for the sake of simplicity).

  3. Defines the sayHello task which runs the Example application, which orchestrates the demo.

We are not reviewing these parts of the project configuration deeper because they are not related to the use of the Spine framework. If you’re interested in more details, please look into the mentioned .gradle files.

Now, let’s look into the data structure of the Hello context.

Hello context data

The data types of the Hello context are defined under the src/main/proto/hello directory with the following files:

  • commands.proto — this file defines the Print command.

    By convention, commands are defined in a file with the commands suffix in its name. It can be, for example, order_commands.proto or just commands.proto like in our example.

  • events.proto — this file defines the Printed event.

    Similarly to commands, events are defined in proto files having the events suffix in their names.

These two files define signals used by the Hello context. There’s also data of the Console Process Manager, which is defined in the package server in the file console.proto.

We arrange the sub-package server to highlight the fact that this is server-only data. It is not a convention used by the framework. We find the clarity of this infix useful when creating cloud applications. So, we share it as a recommendation in this example.

Let’s review the context data definition in details.

The commands.proto file

After the copyright header the file starts with the syntax declaration. The framework supports only this version of the Protobuf dialect:

syntax = "proto3";

Then follows the import statement for custom options used when defining data types. This import is required for all proto files of a Spine-based project.

import "spine/options.proto";

The following file-wide option defines the prefix for type names used in this file.

option (type_url_prefix) = "";

This prefix is needed for recognizing binary data when it’s unpacked. Most likely, all types in a system would use the same prefix.

Then we see the standard Protobuf option for defining a Java package for the generated code:

option java_package="io.spine.helloworld.hello.command";

There are three parts of interest in this package name:

  • io.spine.helloworld — this part represents the location of our Hello World “solution” as if it were hosted on the web at

  • hello — this is the package of the Hello context. In this example we have only one, but a real world app would have more. Each Bounded Context goes into a dedicated package.

  • command — this part gathers commands of the context under one package. We have only one command in this example, but in real world scenarios, with dozens of commands, it is convenient to gather them under a package.

The following standard proto file option defines the name for the outer class generated by Protobuf Compiler for this .proto file:

option java_outer_classname = "CommandsProto";

Outer classes are used by Protobuf implementation internally. When the java_outer_classname option is omitted, Protobuf Compiler would calculate the Java class name taking the name of the corresponding .proto file. We recommend setting the name directly to make it straight. This also avoids possible name clashes with the handcrafted code.

The next standard option instructs the Protobuf Compiler to put each generated Java type into a separate file. This way it would be easier to analyze dependencies of the code which uses these generated types.

option java_multiple_files = true;

The command for printing a text in a console is defined this way:

message Print {

    // The login name of the computer user.
    string username = 1;

    // The text to print.
    string text = 2 [(required) = true];

By convention, the first field of a command is the ID of the target entity. This field is required to have a non-empty value so that the command can be dispatched to the entity. In our case, we identify a console by the login name of the computer user.

The second field is marked as (required) using the custom option imported in the spine/options.proto file above. This command does not make much sense if there is no text to print.

In Protobuf a data type is either a message (we can send it) or an enum. If you’re new to this language, you may want to look at the Proto3 Language Guide.

Now, let’s see how to define events.

The events.proto file

Events are declared similarly to commands. The header of the file has:

  • The proto3 syntax declaration.
  • The hello package declaration.
  • The import statement for custom Protobuf options used by the framework:

     import "spine/options.proto";
  • The same (type_url_prefix) we use in this project.
  • A separate Java package for events of the context:

     option java_package="io.spine.helloworld.hello.event";
  • The outer Java class for all types in this file:

     option java_outer_classname = "EventsProto";
  • The instruction to put Java types into separate files.

     option java_multiple_files = true;

The sole event in this project is declared this way:

message Printed {

    // The login name of the user.
    string username = 1 [(required) = true];

    // The printed text.
    string text = 2 [(required) = true];

The event tells which text was printed for a user. Both of the fields are marked as (required) because the event does not make much sense if one of them is empty.

Unlike for commands, the framework does not assume that the first event field is always populated. This is so because default routing rules for commands and events are different. When an event is produced by some entity, it remembers the ID of this producer entity. By default, the framework uses the producer ID to route events to their target entities — if they have identifiers of the same type. If the type of producer ID does not match one of the target entity, then event fields are analyzed. It is also possible to set custom routing rules.

Now, let’s see the server-side data of the Hello context.

The console.proto file

The header of the file is similar to those we saw in commands.proto and events.proto. The difference is that we use server for the proto and Java package names to make sure the server-only is not used by the client code.

This file defines a single data type. It is the state of the entity handling the Print command:

message Output {
    option (entity) = { kind: PROCESS_MANAGER };

    // The login name of the computer user.
    string username = 1;

    // Text lines of the screen.
    repeated string lines = 2;

The option (entity) tells us that this type is going to be used by a ProcessManager. The first field of the type holds the ID of this entity. The framework assumes such fields as implicitly (required). Then goes the declaration of the remote screen output. The value of this field is empty until something is printed on the screen. Therefore, it is not marked (required).

Now, let’s see how this data is used at the server-side.

The Console class

The class is declared this way:

final class Console extends ProcessManager<String, Output, Output.Builder> {

The generic arguments passed to ProcessManager are:

  1. String — the type of the ID of the entity. Remember the type of the first field of the Print command?

  2. Output — the type of the entity state, which we reviewed in the previous section.

  3. Output.Builder — the type used to modify the state of the entity. In Protobuf for Java, each message type has a specific builder type.

Handling the Print command

The command is handled by this method:

Printed handle(Print command) {
    String username = command.getUsername();
    String text = command.getText();
    println(username, text);
    return Printed.newBuilder()

Let’s review the method in details.

The @Assign annotation tells that we assign this method to handle the command which the method accepts as the parameter. The method returns the event message Printed.

The following code obtains the name of the user, and the text to print from the received command, and then applies them to the state of the Process Manager. Instances of the Console class store the text printed for each user.

String username = command.getUsername();
String text = command.getText();

Then we print the text to System.out so that it becomes visible on the screen:

println(username, text);

This is done by the method the Console class declares:

private void println(String userName, String text) {
    String output = format("[%s] %s", userName, text);

Then, the command-handling method concludes by producing the event message:

return Printed.newBuilder()

The vBuild() call validates and builds the message. This method is generated by Spine Model Compiler. For instructions on adding validation attributes to your model please see Validation User Guide.

After the event is generated, it is posted to the EventBus and delivered to subscribers automatically. You don’t need to write any code for this.

Now, let’s see how the Console Process Manager is exposed to the outer world so that it can receive commands.

Assembling the Hello context

Let’s open the HelloContext class. The first thing of interest in this class is the declaration of the name of the Bounded Context:

static final String NAME = "Hello";

This constant is used for creating the context (we will review it in a minute) and when annotating the server-side code which belongs to the Hello context. This is done in using the @BoundedContext annotation:

package io.spine.helloworld.server.hello;

The framework assumes that all entity classes belonging to this and nested packages belong to the Bounded Context with the name specified in the argument of the annotation. This arrangement is needed for routing events.

The second thing the HelloContext does is creating a Builder for the Bounded Context:

public static BoundedContextBuilder newBuilder() {
    return BoundedContext

The context we create is single-tenant. It contains one entity type which we pass to the builder.

If an entity uses default routing rules for the incoming events and commands, its type can be added to BoundedContextBuilder directly. If custom routing rules are needed, they are specified by a custom Repository class. In this case, an instance of such Repository is passed to BoundedContextBuilder instead of the entity type managed by this Repository.

Once we assembled the Bounded Context, let’s test it.

Testing the Hello context

Let’s open the HelloContextTest suite. It is based on JUnit 5 and spine-testutil-server library.

We already added JUnit dependency when defining the Gradle project. The testImplementation dependency for spine-testutil-server is automatically added when you enable Spine in your project using spine.enableJava().server(). So, we’re good to go testing.

The class of the test suite extends the abstract base called ContextAwareTest:

@DisplayName("Hello context should")
class HelloContextTest extends ContextAwareTest {

The base class is responsible for creation of a test fixture for a Bounded Context under the test before each test, and for disposing the fixture after. The fixture is accessed using the context() method.

We pass the Hello Context for testing using its builder by implementing the abstract method contextBuilder() inherited from ContextAwareTest:

protected BoundedContextBuilder contextBuilder() {
    return HelloContext.newBuilder();

Now we can get down to the tests. The suite verifiers the outcome of the Print command. The test methods are gathered under the nested class called PrintCommand. The class holds the reference to the command as its field:

@DisplayName("handle the `Print` command")
class PrintCommand {

    private Print command;

Sending the command

The command is created and sent to the test fixture before each test method:

void sendCommand() {
    command = Print.newBuilder()

For test values we use statically imported randomString() method of the TestValues utility class provided by the spine-testutil-server library. We use the context() method provided by ContextAwareTest for obtaining the reference of the test fixture of the Bounded Context under the test.

Now we need to test that handling the command produces the event, and the handling entity updates its state.

Testing creation of the event

Testing that an event was generated is quite simple. We create the expected event and assert it with the test fixture:

@Test @DisplayName("emitting the `Printed` event")
void event() {
    Printed expected = Printed.newBuilder()

Testing entity state update

For testing the state of the Console Process Manager was updated, we construct the expected state and pass it to the assertState() method of the test fixture:

@Test @DisplayName("updating the `Console` entity")
void entity() {
    Output expected = Output.newBuilder()
    context().assertState(command.getUsername(), expected);

The first argument passed to the assertState() method is the ID of the entity the state of which we test. The second argument is the expected state.

Now as we’ve checked that our Bounded Context works correctly, let’s expose it in the server-side application.

The server-side application

Let’s open the Server class of our example application suite. The static initialization of the class configures the server environment:

static {

Configuring the environment

The configureEnvironment() method initializes the Production environment of this example with the settings that are normally used for testing:

private static void configureEnvironment() {

A real-world application would use StorageFactory and TransportFactory instances that correspond to a database, and a messaging system used by the application.

The constructor

The implementation of the Server class wraps around the class io.spine.server.Server provided by the framework. This API is for exposing BoundedContexts in a server-side application. This is what our Server class does in the constructor:

public Server(String serverName) {
    this.server = inProcess(serverName)

The constructor accepts the name which is used for connecting clients. This name is passed to the inProcess() factory method of the io.spine.server.Server class.

In-process gRPC communications are normally used for testing. This example uses in-process client/server arrangement in the production code for the sake of simplicity. A real-world application would use a Server instance exposed via a TCP/IP port.

Once we have the Server.Builder instance returned by the inProcess() method, we add the Hello Context via its builder to the constructed Server instance.

Start and shutdown

The remaining code of our Server class declares start() and shutdown() methods that simply delegate calls to the wrapped io.spine.server.Server instance.

Now we have the server, but how does the client side look like?

The client code

Similarly to Server, the Client class of our example application wraps around the io.spine.client.Client API provided by the spine-client library:

public final class Client {

    private final io.spine.client.Client client;

The io.spine:spine-client library is provided to the example application project as a transitive dependency of the io.spine:spine-server library, which is added to the project when you do spine.enableJava().server() in your Gradle project.

Then, the Client class declares a field for keeping subscriptions to the results of a command execution. We’ll see how this field is used in a minute.

private @Nullable ImmutableSet<Subscription> subscriptions;

First of all, let’s see how the Client instances are created.

The constructor

public Client(String serverName) {
    this.client = inProcess(serverName)
            .shutdownTimeout(2, TimeUnit.SECONDS)

Again, similarly to a Server, a Client is created using in-process connection to a named server. The shutdownTimeout parameter configures the amount of time allowed for completing ongoing client-server communications.

Now, let’s review the main thing the Client class does, sending the Print command.

Sending the command

public void sendCommand() {
    String userName = System.getProperty("");
    Print commandMessage =
                 .setText("Hello World!")
    this.subscriptions =
                  .observe(Printed.class, this::onPrinted)

The method does three things:

  1. Creates a command message using the login name of the current computer user.
  2. Subscribes to the Printed event which will be generated as the result of handling the command we are going to post (and only this instance of the command, not all Print commands).
  3. Posts the command.

Let’s review subscribing and posting in details.

The client.asGuest() call starts composing a client request. For the sake of simplicity, we do this on behalf of a user who is not logged in. A real-world application would send most of the commands using client.onBehalfOf(UserId).

The command(commandMessage) call tells we want to send a command with the passed message.

Then, we subscribe to the Printed event which would be generated as the result of handling the command. The events obtained from the server would be passed to the onPrinted() method of the Client class.

The post() method sends the command to the server, returning the set with one Subscription to the Printed event. We store the returned set in the field to use later for cancelling the subscription.

Handling the event

When the client code receives the Printed event, it prints its data and then cancels the subscription:

private void onPrinted(Printed event) {

There’s not much exciting about the printing part.

private void printEvent(EventMessage e) {
    String out = format(
            "The client received the event: %s%s",

The only interesting thing here is the statically imported Json.toCompactJson() call which converts the event message to a String. The method is available from the Json utility class provided by the framework. The utility does heavy lifting of the conversion which involves knowing all Protobuf types available in the project.

Cancelling the subscription, if any, iterates through the set passing each of them to the Client.subscriptions() API for the cancellation:

private void cancelSubscriptions() {
    if (subscriptions != null) {
        subscriptions.forEach(s -> client.subscriptions().cancel(s));
        subscriptions = null;

Once we finish with the cancellation, we clear the subscriptions field.

Closing the client

The close() method simply delegates to the method of the io.spine.client.Client class. We also need to tell the calling code if the client has finished its job. This is what the isDone() method is for:

public boolean isDone() {
    return client

The method checks active subscriptions. If there are none, we got the event and cleared its subscription.

Now, let’s put it all together.

Orchestrating the example

Let’s review the Example class and its main() method. It simulates client-server communication scenario.

public static void main(String[] args) {
    String serverName = Identifier.newUuid();
    Server server = new Server(serverName);
    Client client = null;
    try {
        client = new Client(serverName);
        while (!client.isDone()) {
    } catch (IOException e) {
    } finally {
        if (client != null) {

The first thing the method does is creating a Server using a random name.

Then, the server is started. It is done in the try/catch/finally block because the start() method may throw IOException in case of communication problems reported by gRPC. The catch block calls the onError() method of the Example class which simply prints the exception. A real-world application would use more sophisticated exception handling.

After the Server is started, we create a Client using the name of the server generated before. Then, the client sends the command, and we wait until the client finishes its job using the statically imported method sleepUninterruptibly() provided by Guava.

The main() method finishes by closing the client, and shutting down the server.


Although the “business case” of this example is trivial, it shows basic steps of development of a solution based on Spine Event Engine framework.

The development starts with the discovery of the business using EventStorming or another learning approach.

Then, we select a Bounded Context and define events, commands, Value Objects, and entity states using Protobuf. Using these .proto files Spine Model Compiler generates the code implementing the defined data types.

After that, we add the business logic for handling commands or events in entity classes derived from Aggregate, ProcessManager, or Projection.

Then, these entity types are assembled into a Bounded Context and tested as a whole. A test suite sends signals (i.e. commands or events) to the implementation of the Bounded Context and verifies the changes manifested as events or new entity states.

Once the Bounded Context is tested, it is added to a Server which gathers all the Bounded Contexts of the solution and accepts incoming requests from the client applications.

Client applications send commands to modify the business model and subscribe to messages like events that reflect the changes of the model.

Once handling of all commands and events of the selected Bounded Context is done, another context is selected for the development. The process is repeated until all the contexts are implemented.