GH-520 - Early reject invalid ApplicationModules bootstraps.

We now immediately reject the ApplicationModules bootstrap in case the initial scanning of the root packages yield no classes at all.

spring-modulith-core/src/main/java/org/springframework/modulith/core/ApplicationModules.java

@@ -115,6 +115,8 @@ public class ApplicationModules implements Iterable<ApplicationModule> {
 				.importPackages(packages) //
 				.that(not(ignored.or(IS_AOT_TYPE).or(IS_SPRING_CGLIB_PROXY)));
 
+		Assert.notEmpty(allClasses, () -> "No classes found in packages %s!".formatted(packages));
+
 		Classes classes = Classes.of(allClasses);
 
 		this.modules = packages.stream() //

spring-modulith-core/src/main/java/org/springframework/modulith/core/ApplicationModulesFactory.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2024 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.core;
+
+/**
+ * Factory interface to create {@link ApplicationModules} instances for application classes. The default version will
+ * simply delegate to {@link ApplicationModules#of(Class)} which will only look at production classes. Our test support
+ * provides an alternative implementation to bootstrap an {@link ApplicationModules} instance from test types as well,
+ * primarily for our very own integration test purposes.
+ *
+ * @author Oliver Drotbohm
+ * @since 1.2
+ */
+public interface ApplicationModulesFactory {
+
+	/**
+	 * Returns the {@link ApplicationModules} instance for the given application class.
+	 *
+	 * @param applicationClass must not be {@literal null}.
+	 * @return will never be {@literal null}.
+	 */
+	ApplicationModules of(Class<?> applicationClass);
+
+	/**
+	 * Creates the default {@link ApplicationModulesFactory} delegating to {@link ApplicationModules#of(Class)}
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public static ApplicationModulesFactory defaultFactory() {
+		return ApplicationModules::of;
+	}
+}

spring-modulith-core/src/test/java/example/empty/EmptyApplication.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2024 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 example.empty;
+
+import org.springframework.modulith.Modulithic;
+
+/**
+ * @author Oliver Drotbohm
+ */
+@Modulithic
+public class EmptyApplication {
+
+}