Retain existing modules in JacksonAutoConfiguration

Previously, the default Jackson2ObjectMapperBuilderCustomizer
implementation did set the list of modules to use. This had the effect
of removing any modules that were registered programmatically by a
customizer with higher precedence.

This commit uses the variant of modulesToInstall that retain any
existing modules. It also adds a note in the documentation as this
behavior can be easily missed.

See 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 +73,8 @@ 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.Ordered;
+import org.springframework.core.annotation.Order;
 import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -324,6 +326,16 @@ class JacksonAutoConfigurationTests {
 		});
 	}
 
+	@Test
+	void customModulesRegisteredByBuilderCustomizerWithHighestPrecedenceShouldBeRetained() {
+		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("customizer-module");
+			});
+	}
+
 	@Test
 	void defaultSerializationInclusion() {
 		this.contextRunner.run((context) -> {
@@ -592,6 +604,17 @@ class JacksonAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class CustomModuleBuilderCustomizerConfig {
+
+		@Bean
+		@Order(Ordered.HIGHEST_PRECEDENCE)
+		Jackson2ObjectMapperBuilderCustomizer customModuleCustomizer() {
+			return (builder) -> builder.modulesToInstall(new SimpleModule("customizer-module"));
+		}
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class ObjectMapperBuilderConsumerConfig {