Configure ProblemDetailJacksonMixin on JsonMapper

Prior to this commit, Spring Boot would use Framework's
`Jackson2ObjectMapperBuilder` to configure the `ObjectMapper` instance.
This builder would configure the `ProblemDetail` mixin automatically.

With the introduction of Jackson 3.x support, Spring Framework removed
its builder in favor of the native Jackson builder. As a result, the
mixin is not registered with the `JsonMapper` aymore.

This commit ensures that the mixin is registered if the `ProblemDetail`
class is present in the classpath.

Closes gh-47298

Author: bclozel

module/spring-boot-jackson/build.gradle

@@ -28,9 +28,8 @@ dependencies {
 	api(project(":core:spring-boot"))
 	api("tools.jackson.core:jackson-databind")
 
-	implementation("org.springframework:spring-web")
-
 	optional(project(":core:spring-boot-autoconfigure"))
+	optional("org.springframework:spring-web")
 	optional("tools.jackson.dataformat:jackson-dataformat-cbor")
 	optional("tools.jackson.dataformat:jackson-dataformat-xml")
 	optional("tools.jackson.module:jackson-module-jakarta-xmlbind-annotations")

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

@@ -59,6 +59,8 @@
 import org.springframework.context.annotation.Primary;
 import org.springframework.context.annotation.Scope;
 import org.springframework.core.Ordered;
+import org.springframework.http.ProblemDetail;
+import org.springframework.http.converter.json.ProblemDetailJacksonMixin;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
@@ -305,6 +307,26 @@ private void configureConstructorDetector(JsonMapper.Builder builder) {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	@ConditionalOnClass(ProblemDetail.class)
+	static class ProblemDetailsConfiguration {
+
+		@Bean
+		ProblemDetailJsonMapperBuilderCustomizer problemDetailJsonMapperBuilderCustomizer() {
+			return new ProblemDetailJsonMapperBuilderCustomizer();
+		}
+
+		static final class ProblemDetailJsonMapperBuilderCustomizer implements JsonMapperBuilderCustomizer {
+
+			@Override
+			public void customize(JsonMapper.Builder builder) {
+				builder.addMixIn(ProblemDetail.class, ProblemDetailJacksonMixin.class);
+			}
+
+		}
+
+	}
+
 	static class JacksonAutoConfigurationRuntimeHints implements RuntimeHintsRegistrar {
 
 		@Override

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

@@ -67,6 +67,7 @@
 import org.springframework.context.annotation.Import;
 import org.springframework.context.annotation.Primary;
 import org.springframework.core.annotation.Order;
+import org.springframework.http.ProblemDetail;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
@@ -484,6 +485,17 @@ void shouldRegisterPropertyNamingStrategyHints() {
 		assertThat(RuntimeHintsPredicates.reflection().onType(PropertyNamingStrategies.class)).accepts(hints);
 	}
 
+	@Test
+	void shouldRegisterProblemDetailsMixin() {
+		this.contextRunner.run((context) -> {
+			ObjectMapper mapper = context.getBean(ObjectMapper.class);
+			ProblemDetail problemDetail = ProblemDetail.forStatus(404);
+			problemDetail.setProperty("spring", "boot");
+			String json = mapper.writeValueAsString(problemDetail);
+			assertThat(json).isEqualTo("{\"status\":404,\"title\":\"Not Found\",\"spring\":\"boot\"}");
+		});
+	}
+
 	static class MyDateFormat extends SimpleDateFormat {
 
 		MyDateFormat() {