Automatically add Spring Modulith event externalization dependency

If a message broker *and* Spring Modulith are declared as project
dependencies, the event externalization of Spring Modulith for
configured broker are now declared automatically as well. Also, the
Spring Modulith Events API dependency is declared in compile scope to
allow event types to be marked for externalization.

See gh-1528

start-site/src/main/java/io/spring/start/site/extension/dependency/springmodulith/SpringModulithBuildCustomizer.java

@@ -41,6 +41,8 @@ class SpringModulithBuildCustomizer implements BuildCustomizer<Build> {
 
 	private static final Collection<String> PERSISTENCE = List.of("jdbc", "jpa", "mongodb");
 
+	private static final Collection<String> BROKERS = List.of("activemq", "amqp", "artemis", "kafka");
+
 	@Override
 	public void customize(Build build) {
 		DependencyContainer dependencies = build.dependencies();
@@ -52,6 +54,9 @@ public void customize(Build build) {
 					modulithDependency("observability").scope(DependencyScope.RUNTIME));
 		}
 		addEventPublicationRegistryBackend(build);
+		if (addEventExternalizationDependency(build)) {
+			dependencies.add("modulith-events-api", modulithDependency("events-api"));
+		}
 		dependencies.add("modulith-starter-test",
 				modulithDependency("starter-test").scope(DependencyScope.TEST_COMPILE));
 	}
@@ -75,4 +80,29 @@ private Builder<?> modulithDependency(String name) {
 		return Dependency.withCoordinates("org.springframework.modulith", "spring-modulith-" + name);
 	}
 
+	private boolean addEventExternalizationDependency(Build build) {
+		DependencyContainer dependencies = build.dependencies();
+		return BROKERS.stream()
+			.filter(dependencies::has)
+			.map(this::getModulithBrokerKey)
+			.peek((it) -> dependencies.add("modulith-events-" + it,
+					modulithDependency("events-" + it).scope(DependencyScope.RUNTIME)))
+			.findAny()
+			.isPresent();
+	}
+
+	private String getModulithBrokerKey(String broker) {
+
+		switch (broker) {
+			case "kafka":
+			case "amqp":
+				return broker;
+			case "artemis":
+			case "activemq":
+				return "jms";
+			default:
+				throw new IllegalArgumentException("Unsupported broker!");
+		}
+	}
+
 }

start-site/src/test/java/io/spring/start/site/extension/dependency/springmodulith/SpringModulithBuildCustomizerTests.java

@@ -74,6 +74,24 @@ void presenceOfSpringDataModuleAddsModuleEventStarter(String store) {
 		assertThat(build.dependencies().ids()).doesNotContain("modulith-starter-core");
 	}
 
+	@ParameterizedTest
+	@ValueSource(strings = { "amqp", "kafka" })
+	void addsExternalizationDependency(String broker) {
+		Build build = createBuild("modulith", broker);
+		this.customizer.customize(build);
+		assertThat(build.dependencies().ids()).contains("modulith-events-" + broker);
+		assertThat(build.dependencies().ids()).contains("modulith-events-api");
+	}
+
+	@ParameterizedTest
+	@ValueSource(strings = { "activemq", "artemis" })
+	void addsJmsExternalizationDependency(String broker) {
+		Build build = createBuild("modulith", broker);
+		this.customizer.customize(build);
+		assertThat(build.dependencies().ids()).contains("modulith-events-jms");
+		assertThat(build.dependencies().ids()).contains("modulith-events-api");
+	}
+
 	private Build createBuild(String... dependencies) {
 		InitializrMetadata metadata = getMetadata();
 		MavenBuild build = new MavenBuild(new MetadataBuildItemResolver(metadata, getDefaultPlatformVersion(metadata)));