Detect accidental misconfiguration of JsonMixin annotation

See gh-42592

Closes gh-42592

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/jackson/JsonMixinModuleEntries.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -29,6 +29,7 @@ import org.springframework.context.annotation.ClassPathScanningCandidateComponen
 import org.springframework.core.annotation.MergedAnnotation;
 import org.springframework.core.annotation.MergedAnnotations;
 import org.springframework.core.type.filter.AnnotationTypeFilter;
+import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
@@ -89,7 +90,10 @@ public final class JsonMixinModuleEntries {
 		MergedAnnotation<JsonMixin> annotation = MergedAnnotations
 			.from(mixinClass, MergedAnnotations.SearchStrategy.TYPE_HIERARCHY)
 			.get(JsonMixin.class);
-		for (Class<?> targetType : annotation.getClassArray("type")) {
+		Class<?>[] types = annotation.getClassArray("type");
+		Assert.notEmpty(types,
+				"@JsonMixin annotation on class '" + mixinClass.getName() + "' does not specify any types");
+		for (Class<?> targetType : types) {
 			builder.and(targetType, mixinClass);
 		}
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/jackson/JsonMixinModuleTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -24,16 +24,19 @@ import com.fasterxml.jackson.databind.ObjectMapper;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.beans.factory.BeanCreationException;
 import org.springframework.boot.jackson.scan.a.RenameMixInClass;
 import org.springframework.boot.jackson.scan.b.RenameMixInAbstractClass;
 import org.springframework.boot.jackson.scan.c.RenameMixInInterface;
 import org.springframework.boot.jackson.scan.d.EmptyMixInClass;
+import org.springframework.boot.jackson.scan.f.EmptyMixIn;
 import org.springframework.boot.jackson.types.Name;
 import org.springframework.boot.jackson.types.NameAndAge;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.util.ClassUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
 /**
  * Tests for {@link JsonMixinModule}.
@@ -51,6 +54,14 @@ class JsonMixinModuleTests {
 		}
 	}
 
+	@Test
+	void jsonWithModuleEmptyMixInWithEmptyTypesShouldFailed() {
+		assertThatExceptionOfType(BeanCreationException.class).isThrownBy(() -> load(EmptyMixIn.class))
+			.withMessageContaining("Error creating bean with name 'jsonMixinModule'")
+			.withStackTraceContaining("@JsonMixin annotation on class "
+					+ "'org.springframework.boot.jackson.scan.f.EmptyMixIn' does not specify any types");
+	}
+
 	@Test
 	void jsonWithModuleWithRenameMixInClassShouldBeMixedIn() throws Exception {
 		load(RenameMixInClass.class);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/jackson/scan/f/EmptyMixIn.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2012-2024 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.boot.jackson.scan.f;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import org.springframework.boot.jackson.JsonMixin;
+
+@JsonMixin
+public interface EmptyMixIn {
+
+	@JsonProperty("username")
+	String getName();
+
+}