Add ProblemDetailJacksonMixin

Closes gh-28665

spring-web/src/main/java/org/springframework/http/ProblemDetail.java

@@ -24,12 +24,20 @@ import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
- * Representation of an RFC 7807 problem detail, including all RFC-defined
- * properties.
+ * Representation for an RFC 7807 problem detail. Includes spec-defined
+ * properties, and a {@link #getProperties() properties} map for additional,
+ * non-standard properties.
  *
- * <p>For an extended response, create a subclass with additional properties.
- * A subclass can use the {@link ProblemDetail#ProblemDetail(ProblemDetail)}
- * copy constructor to extend an existing {@code ProblemDetail} instance.
+ * <p>For an extended response, an application can add to the
+ * {@link #getProperties() properties} map. When using the Jackson library, the
+ * {@code properties} map is expanded as top level JSON properties through the
+ * {@link org.springframework.http.converter.json.ProblemDetailJacksonMixin}.
+ *
+ * <p>For an extended response, an application can also create a subclass with
+ * additional properties. Subclasses can use the protected copy constructor to
+ * re-create an existing {@code ProblemDetail} instance as the subclass, e.g.
+ * from an {@code @ControllerAdvice} such as
+ * {@link org.springframework.web.servlet.mvc.method.annotation.ResponseEntityExceptionHandler}.
  *
  * @author Rossen Stoyanchev
  * @since 6.0
@@ -203,7 +211,9 @@ public class ProblemDetail {
 	}
 
 	/**
-	 * Return a generic map of properties that are not known ahead of time.
+	 * Return a generic map of properties that are not known ahead of time,
+	 * possibly {@code null} if no properties have been added. To add a property,
+	 * use {@link #setProperty(String, Object)}.
 	 */
 	@Nullable
 	public Map<String, Object> getProperties() {

spring-web/src/main/java/org/springframework/http/converter/json/Jackson2ObjectMapperBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -60,6 +60,7 @@ import org.springframework.beans.BeanUtils;
 import org.springframework.beans.FatalBeanException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.KotlinDetector;
+import org.springframework.http.ProblemDetail;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
@@ -730,6 +731,7 @@ public class Jackson2ObjectMapperBuilder {
 			objectMapper.setFilterProvider(this.filters);
 		}
 
+		objectMapper.addMixIn(ProblemDetail.class, ProblemDetailJacksonMixin.class);
 		this.mixIns.forEach(objectMapper::addMixIn);
 
 		if (!this.serializers.isEmpty() || !this.deserializers.isEmpty()) {

spring-web/src/main/java/org/springframework/http/converter/json/ProblemDetailJacksonMixin.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2002-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.http.converter.json;
+
+import java.util.Map;
+
+import com.fasterxml.jackson.annotation.JsonAnyGetter;
+import com.fasterxml.jackson.annotation.JsonAnySetter;
+import com.fasterxml.jackson.annotation.JsonInclude;
+
+import org.springframework.http.ProblemDetail;
+
+import static com.fasterxml.jackson.annotation.JsonInclude.Include.NON_EMPTY;
+
+/**
+ * An interface to associate Jackson annotations with
+ * {@link org.springframework.http.ProblemDetail} to avoid a hard dependency on
+ * the Jackson library.
+ *
+ * <p>The annotations ensure the {@link ProblemDetail#getProperties() properties}
+ * map is unwrapped and rendered as top level JSON properties, and likewise that
+ * the {@code properties} map contains unknown properties from the JSON.
+ *
+ * <p>{@link Jackson2ObjectMapperBuilder} automatically registers this as a
+ * "mix-in" for {@link ProblemDetail}, which means it always applies, unless
+ * an {@code ObjectMapper} is instantiated directly and configured for use.
+ *
+ * @author Rossen Stoyanchev
+ * @since 6.0
+ */
+@JsonInclude(NON_EMPTY)
+public interface ProblemDetailJacksonMixin {
+
+	@JsonAnySetter
+	void setProperty(String name, Object value);
+
+	@JsonAnyGetter
+	Map<String, Object> getProperties();
+
+}