Merge pull request #42836 from nosan

* pr/42836:
  Polish "Retain existing modules in JacksonAutoConfiguration"
  Retain existing modules in JacksonAutoConfiguration

Closes gh-42836

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

@@ -307,7 +307,7 @@ public class JacksonAutoConfiguration {
 			}
 
 			private void configureModules(Jackson2ObjectMapperBuilder builder) {
-				builder.modulesToInstall(this.modules.toArray(new Module[0]));
+				builder.modulesToInstall((modules) -> modules.addAll(this.modules));
 			}
 
 			private void configureLocale(Jackson2ObjectMapperBuilder builder) {

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

@@ -73,6 +73,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
 import org.springframework.context.annotation.Primary;
+import org.springframework.core.annotation.Order;
 import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -324,6 +325,17 @@ class JacksonAutoConfigurationTests {
 		});
 	}
 
+	@Test
+	void customModulesRegisteredByBuilderCustomizerShouldBeRetained() {
+		this.contextRunner.withUserConfiguration(ModuleConfig.class, CustomModuleBuilderCustomizerConfig.class)
+			.run((context) -> {
+				ObjectMapper objectMapper = context.getBean(Jackson2ObjectMapperBuilder.class).build();
+				assertThat(context.getBean(CustomModule.class).getOwners()).contains(objectMapper);
+				assertThat(objectMapper.getRegisteredModuleIds()).contains("module-A", "module-B",
+						CustomModule.class.getName());
+			});
+	}
+
 	@Test
 	void defaultSerializationInclusion() {
 		this.contextRunner.run((context) -> {
@@ -592,6 +604,23 @@ class JacksonAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class CustomModuleBuilderCustomizerConfig {
+
+		@Bean
+		@Order(-1)
+		Jackson2ObjectMapperBuilderCustomizer highPrecedenceCustomizer() {
+			return (builder) -> builder.modulesToInstall((modules) -> modules.add(new SimpleModule("module-A")));
+		}
+
+		@Bean
+		@Order(1)
+		Jackson2ObjectMapperBuilderCustomizer lowPrecedenceCustomizer() {
+			return (builder) -> builder.modulesToInstall((modules) -> modules.add(new SimpleModule("module-B")));
+		}
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class ObjectMapperBuilderConsumerConfig {
 

spring-boot-project/spring-boot-docs/src/docs/antora/modules/how-to/pages/spring-mvc.adoc

@@ -118,6 +118,8 @@ Such customizer beans can be ordered (Boot's own customizer has an order of 0),
 Any beans of type javadoc:com.fasterxml.jackson.databind.Module[] are automatically registered with the auto-configured javadoc:org.springframework.http.converter.json.Jackson2ObjectMapperBuilder[] and are applied to any javadoc:com.fasterxml.jackson.databind.ObjectMapper[] instances that it creates.
 This provides a global mechanism for contributing custom modules when you add new features to your application.
 
+NOTE: If you wish to register additional modules programmatically using a javadoc:org.springframework.boot.autoconfigure.jackson.Jackson2ObjectMapperBuilderCustomizer[], make sure to use the `modulesToInstall` method that takes a consumer as the other variants are not additive.
+
 If you want to replace the default javadoc:com.fasterxml.jackson.databind.ObjectMapper[] completely, either define a javadoc:org.springframework.context.annotation.Bean[format=annotation] of that type or, if you prefer the builder-based approach, define a javadoc:org.springframework.http.converter.json.Jackson2ObjectMapperBuilder[] javadoc:org.springframework.context.annotation.Bean[format=annotation].
 When defining an javadoc:com.fasterxml.jackson.databind.ObjectMapper[] bean, marking it as javadoc:org.springframework.context.annotation.Primary[format=annotation] is recommended as the auto-configuration's javadoc:com.fasterxml.jackson.databind.ObjectMapper[] that it will replace is javadoc:org.springframework.context.annotation.Primary[format=annotation].
 Note that, in either case, doing so disables all auto-configuration of the javadoc:com.fasterxml.jackson.databind.ObjectMapper[].