Polish "Retain existing modules in JacksonAutoConfiguration"

See gh-42836

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

@@ -73,7 +73,6 @@ 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;
 
@@ -327,12 +326,13 @@ class JacksonAutoConfigurationTests {
 	}
 
 	@Test
-	void customModulesRegisteredByBuilderCustomizerWithHighestPrecedenceShouldBeRetained() {
+	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("customizer-module");
+				assertThat(objectMapper.getRegisteredModuleIds()).contains("module-A", "module-B",
+						CustomModule.class.getName());
 			});
 	}
 
@@ -608,9 +608,15 @@ class JacksonAutoConfigurationTests {
 	static class CustomModuleBuilderCustomizerConfig {
 
 		@Bean
-		@Order(Ordered.HIGHEST_PRECEDENCE)
-		Jackson2ObjectMapperBuilderCustomizer customModuleCustomizer() {
-			return (builder) -> builder.modulesToInstall(new SimpleModule("customizer-module"));
+		@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")));
 		}
 
 	}

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[].