GH-103 - Introduce ApplicationModuleInitializer.

Application modules can now declare instances of ApplicationModuleInititalizer to execute code upon an ApplicationStartedEvent.

This is achieved by registering a corresponding ApplicationListener in SpringModulithRuntimeAutoConfiguration to invoke all instances of AMI sorted via the Comparator introduced in GH-102. To make sure that the invocation order follows topological order, the runtime module strongly depends on JGraphT.

spring-modulith-api/src/main/java/org/springframework/modulith/ApplicationModuleInitializer.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.modulith;
+
+import org.springframework.boot.context.event.ApplicationStartedEvent;
+
+/**
+ * An interface to be implemented by Spring components that are supposed to be initialized on
+ * {@link ApplicationStartedEvent}. They're {@link ApplicationModule} specific as they're executed in the order
+ * determined by the dependencies of a module. In other words, {@link ApplicationModuleInitializer} of fundamental
+ * application modules are executed first, followed by the ones that depend on it.
+ *
+ * @author Oliver Drotbohm
+ */
+public interface ApplicationModuleInitializer {
+
+	/**
+	 * Run business logic that's needed to initialize the {@link ApplicationModule}.
+	 */
+	void initialize();
+}

spring-modulith-runtime/pom.xml

@@ -22,6 +22,12 @@
 			<version>${project.version}</version>
 		</dependency>
 
+		<dependency>
+			<groupId>org.jgrapht</groupId>
+			<artifactId>jgrapht-core</artifactId>
+			<version>1.4.0</version>
+		</dependency>
+
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-autoconfigure</artifactId>

spring-modulith-runtime/src/main/java/org/springframework/modulith/runtime/autoconfigure/SpringModulithRuntimeAutoConfiguration.java

@@ -15,20 +15,27 @@
  */
 package org.springframework.modulith.runtime.autoconfigure;
 
+import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 
+import java.util.List;
 import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import java.util.function.Supplier;
 
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.context.event.ApplicationStartedEvent;
 import org.springframework.context.ApplicationContext;
+import org.springframework.context.ApplicationListener;
 import org.springframework.context.annotation.Bean;
+import org.springframework.modulith.ApplicationModuleInitializer;
 import org.springframework.modulith.model.ApplicationModule;
 import org.springframework.modulith.model.ApplicationModules;
+import org.springframework.modulith.model.FormatableType;
 import org.springframework.modulith.runtime.ApplicationModulesRuntime;
 import org.springframework.modulith.runtime.ApplicationRuntime;
+import org.springframework.util.ClassUtils;
 
 /**
  * Auto-configuration to register a {@link SpringBootApplicationRuntime} and {@link ApplicationModulesRuntime} as Spring
@@ -57,6 +64,50 @@ class SpringModulithRuntimeAutoConfiguration {
 		return new ApplicationModulesRuntime(toSupplier(modules), runtime);
 	}
 
+	@Bean
+	ApplicationListener<ApplicationStartedEvent> applicationModuleInitialzingListener(ApplicationModulesRuntime runtime,
+			List<ApplicationModuleInitializer> initializers) {
+
+		return event -> {
+
+			var modules = runtime.get();
+
+			initializers.stream() //
+					.sorted(modules.getComparator()) //
+					.map(it -> LOG.isDebugEnabled() ? new LoggingApplicationModuleInitializerAdapter(it, modules) : it)
+					.forEach(ApplicationModuleInitializer::initialize);
+		};
+	}
+
+	@Slf4j
+	@RequiredArgsConstructor
+	private static class LoggingApplicationModuleInitializerAdapter implements ApplicationModuleInitializer {
+
+		private final ApplicationModuleInitializer delegate;
+		private final ApplicationModules modules;
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.modulith.ApplicationModuleInitializer#initialize()
+		 */
+		@Override
+		public void initialize() {
+
+			var listenerType = ClassUtils.getUserClass(delegate);
+			var formattable = FormatableType.of(listenerType);
+
+			var formattedListenerType = modules.getModuleByType(listenerType)
+					.map(formattable::getAbbreviatedFullName)
+					.orElseGet(formattable::getAbbreviatedFullName);
+
+			LOG.debug("Initializing {}.", formattedListenerType);
+
+			delegate.initialize();
+
+			LOG.debug("{} done.", formattedListenerType);
+		}
+	}
+
 	private static ApplicationModules initializeApplicationModules(Class<?> applicationMainClass) {
 
 		LOG.debug("Obtaining Spring Modulith application modules…");