Add property to configure Jackson 3 with Boot's Jackson 2 defaults

Closes gh-

Author: wilkinsona

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

@@ -104,6 +104,8 @@ Note that, thanks to the use of xref:reference:features/external-config.adoc#fea
 
 This environment-based configuration is applied to the auto-configured javadoc:tools.jackson.databind.json.JsonMapper.Builder[] bean and applies to any mappers created by using the builder, including the auto-configured javadoc:tools.jackson.databind.json.JsonMapper[] bean.
 
+To ease the migration when working on an application that previously used Jackson 2, the auto-configured `JsonMapper` can be configured to use defaults that are as close as possible to those that Spring Boot used for Jackson 2. To enable these defaults, set configprop:spring.jackson.use-jackson2-defaults[] to `true`.
+
 The context's javadoc:tools.jackson.databind.json.JsonMapper.Builder[] can be customized by one or more javadoc:org.springframework.boot.jackson.autoconfigure.JsonMapperBuilderCustomizer[] beans.
 Such customizer beans can be ordered (Boot's own customizer has an order of 0), letting additional customization be applied both before and after Boot's customization.
 

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

@@ -36,6 +36,7 @@
 import tools.jackson.databind.PropertyNamingStrategies;
 import tools.jackson.databind.PropertyNamingStrategy;
 import tools.jackson.databind.cfg.ConstructorDetector;
+import tools.jackson.databind.cfg.DateTimeFeature;
 import tools.jackson.databind.json.JsonMapper;
 
 import org.springframework.aot.hint.ReflectionHints;
@@ -164,6 +165,11 @@ public int getOrder() {
 
 			@Override
 			public void customize(JsonMapper.Builder builder) {
+				if (this.jacksonProperties.isUseJackson2Defaults()) {
+					builder.configureForJackson2()
+						.disable(DateTimeFeature.WRITE_DATES_AS_TIMESTAMPS,
+								DateTimeFeature.WRITE_DURATIONS_AS_TIMESTAMPS);
+				}
 				if (this.jacksonProperties.getDefaultPropertyInclusion() != null) {
 					builder.changeDefaultPropertyInclusion((handler) -> handler
 						.withValueInclusion(this.jacksonProperties.getDefaultPropertyInclusion()));

module/spring-boot-jackson/src/main/java/org/springframework/boot/jackson/autoconfigure/JacksonProperties.java

@@ -109,6 +109,12 @@ public class JacksonProperties {
 	 */
 	private @Nullable Locale locale;
 
+	/**
+	 * Whether to configure Jackson 3 with the same defaults as Spring Boot previously
+	 * used for Jackson 2.
+	 */
+	private boolean useJackson2Defaults = false;
+
 	private final Datatype datatype = new Datatype();
 
 	private final Json json = new Json();
@@ -185,6 +191,14 @@ public void setLocale(@Nullable Locale locale) {
 		this.locale = locale;
 	}
 
+	public boolean isUseJackson2Defaults() {
+		return this.useJackson2Defaults;
+	}
+
+	public void setUseJackson2Defaults(boolean useJackson2Defaults) {
+		this.useJackson2Defaults = useJackson2Defaults;
+	}
+
 	public Datatype getDatatype() {
 		return this.datatype;
 	}

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

@@ -29,6 +29,8 @@
 import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 import tools.jackson.core.JsonGenerator;
+import tools.jackson.core.StreamReadFeature;
+import tools.jackson.core.StreamWriteFeature;
 import tools.jackson.core.json.JsonReadFeature;
 import tools.jackson.core.json.JsonWriteFeature;
 import tools.jackson.databind.DeserializationFeature;
@@ -539,6 +541,56 @@ void shouldRegisterProblemDetailsMixin() {
 		});
 	}
 
+	@Test
+	void whenUsingJackson2DefaultsShouldBeConfiguredUsingConfigureForJackson2() {
+		this.contextRunner.withPropertyValues("spring.jackson.use-jackson2-defaults=true").run((context) -> {
+			JsonMapper jsonMapper = context.getBean(JsonMapper.class);
+			JsonMapper jackson2ConfiguredJsonMapper = JsonMapper.builder().configureForJackson2().build();
+			for (DateTimeFeature feature : DateTimeFeature.values()) {
+				boolean expected = (feature == DateTimeFeature.WRITE_DATES_AS_TIMESTAMPS
+						|| feature == DateTimeFeature.WRITE_DURATIONS_AS_TIMESTAMPS) ? false
+								: jackson2ConfiguredJsonMapper.isEnabled(feature);
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name()).isEqualTo(expected);
+			}
+			for (DeserializationFeature feature : DeserializationFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (EnumFeature feature : EnumFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (JsonNodeFeature feature : JsonNodeFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (JsonReadFeature feature : JsonReadFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (JsonWriteFeature feature : JsonWriteFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (MapperFeature feature : MapperFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (SerializationFeature feature : SerializationFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (StreamReadFeature feature : StreamReadFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+			for (StreamWriteFeature feature : StreamWriteFeature.values()) {
+				assertThat(jsonMapper.isEnabled(feature)).as(feature.name())
+					.isEqualTo(jackson2ConfiguredJsonMapper.isEnabled(feature));
+			}
+		});
+	}
+
 	static class MyDateFormat extends SimpleDateFormat {
 
 		MyDateFormat() {