#786 - Redesign of the support for the HTTP Problem spec (RFC-7807).

Problem has been redesigned for immutability. Extensions to the payload body are now implemented by wrapping a either an object exposing the properties to add in turn or a plain Map.

Added infrastructure configuration to make sure that Problem instances returned from controller methods (even if wrapped into a ResponseEntity<?>) cause the ContentType header to be set to application/problem+json. HyperMediaMappingInformation now allows to customize the root domain type to signal serialization support for.

Few cleanups regarding the ObjectMapper setup in test cases.

src/main/java/org/springframework/hateoas/MediaTypes.java

@@ -88,12 +88,12 @@ public class MediaTypes {
 	public static final MediaType VND_ERROR_JSON = MediaType.valueOf(VND_ERROR_JSON_VALUE);
 
 	/**
-	 * A String equivalent of {@link MediaTypes#PROBLEM_JSON_VALUE}.
+	 * A String equivalent of {@link MediaTypes#HTTP_PROBLEM_DETAILS_JSON_VALUE}.
 	 */
-	public static final String PROBLEM_JSON_VALUE = "application/problem+json";
+	public static final String HTTP_PROBLEM_DETAILS_JSON_VALUE = "application/problem+json";
 
 	/**
 	 * Public constant media type for {@code application/problem+json}.
 	 */
-	public static final MediaType PROBLEM_JSON = MediaType.parseMediaType(PROBLEM_JSON_VALUE);
+	public static final MediaType HTTP_PROBLEM_DETAILS_JSON = MediaType.parseMediaType(HTTP_PROBLEM_DETAILS_JSON_VALUE);
 }

src/main/java/org/springframework/hateoas/config/EnableHypermediaSupport.java

@@ -82,6 +82,8 @@ public @interface EnableHypermediaSupport {
 		 */
 		HAL_FORMS(MediaTypes.HAL_FORMS_JSON),
 
+		HTTP_PROBLEM_DETAILS(MediaTypes.HTTP_PROBLEM_DETAILS_JSON),
+
 		/**
 		 * Collection+JSON
 		 *

src/main/java/org/springframework/hateoas/config/HypermediaMappingInformation.java

@@ -18,6 +18,7 @@ package org.springframework.hateoas.config;
 import java.util.List;
 import java.util.Optional;
 
+import org.springframework.hateoas.RepresentationModel;
 import org.springframework.http.MediaType;
 import org.springframework.lang.Nullable;
 
@@ -40,6 +41,17 @@ public interface HypermediaMappingInformation {
 	 */
 	List<MediaType> getMediaTypes();
 
+	/**
+	 * Return the type that this hypermedia type is represented by. Default implementation returns
+	 * {@link RepresentationModel} as it's the base class most media type serializations work with.
+	 *
+	 * @return the type that this hypermedia type is represented by.
+	 * @since 1.1
+	 */
+	default Class<?> getRootType() {
+		return RepresentationModel.class;
+	}
+
 	/**
 	 * Configure an {@link ObjectMapper} and register custom serializers and deserializers for the supported media types.
 	 * If all you want to do is register a Jackson {@link Module}, prefer implementing {@link #getJacksonModule()}.

src/main/java/org/springframework/hateoas/config/WebConverters.java

@@ -38,8 +38,7 @@ class WebConverters {
 	private final List<HttpMessageConverter<?>> converters;
 
 	/**
-	 * Creates a new {@link WebConverters} from the given {@link ObjectMapper} and
-	 * {@link HypermediaMappingInformation}s.
+	 * Creates a new {@link WebConverters} from the given {@link ObjectMapper} and {@link HypermediaMappingInformation}s.
 	 *
 	 * @param mapper must not be {@literal null}.
 	 * @param mappingInformation must not be {@literal null}.
@@ -52,8 +51,7 @@ class WebConverters {
 	}
 
 	/**
-	 * Creates a new {@link WebConverters} from the given {@link ObjectMapper} and
-	 * {@link HypermediaMappingInformation}s.
+	 * Creates a new {@link WebConverters} from the given {@link ObjectMapper} and {@link HypermediaMappingInformation}s.
 	 *
 	 * @param mapper must not be {@literal null}.
 	 * @param mappingInformations must not be {@literal null}.
@@ -106,7 +104,7 @@ class WebConverters {
 	private static AbstractJackson2HttpMessageConverter createMessageConverter(HypermediaMappingInformation type,
 			ObjectMapper mapper) {
 
-		return new TypeConstrainedMappingJackson2HttpMessageConverter(RepresentationModel.class, type.getMediaTypes(),
+		return new TypeConstrainedMappingJackson2HttpMessageConverter(type.getRootType(), type.getMediaTypes(),
 				type.configureObjectMapper(mapper));
 	}
 }

src/main/java/org/springframework/hateoas/mediatype/problem/HttpProblemDetailsConfigurationProvider.java

@@ -0,0 +1,34 @@
+package org.springframework.hateoas.mediatype.problem;
+
+import java.util.Collection;
+
+import org.springframework.hateoas.MediaTypes;
+import org.springframework.hateoas.config.HypermediaMappingInformation;
+import org.springframework.hateoas.config.MediaTypeConfigurationProvider;
+import org.springframework.http.MediaType;
+
+/**
+ * {@link MediaTypeConfigurationProvider} for HAL.
+ *
+ * @author Oliver Drotbohm
+ */
+class HttpProblemDetailsConfigurationProvider implements MediaTypeConfigurationProvider {
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.hateoas.config.HyperMediaTypeProvider#getConfiguration()
+	 */
+	@Override
+	public Class<? extends HypermediaMappingInformation> getConfiguration() {
+		return HttpProblemDetailsMappingInformation.class;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.hateoas.config.HyperMediaTypeProvider#supportsAny(java.util.Collection)
+	 */
+	@Override
+	public boolean supportsAny(Collection<MediaType> mediaTypes) {
+		return mediaTypes.contains(MediaTypes.HTTP_PROBLEM_DETAILS_JSON);
+	}
+}

src/main/java/org/springframework/hateoas/mediatype/problem/HttpProblemDetailsMappingInformation.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2020 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.hateoas.mediatype.problem;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.springframework.hateoas.MediaTypes;
+import org.springframework.hateoas.config.HypermediaMappingInformation;
+import org.springframework.http.MediaType;
+
+/**
+ * {@link HypermediaMappingInformation} implementation to setup support for {@link Problem}.
+ *
+ * @author Oliver Drotbohm
+ */
+class HttpProblemDetailsMappingInformation implements HypermediaMappingInformation {
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.hateoas.config.HypermediaMappingInformation#getRootType()
+	 */
+	@Override
+	public Class<?> getRootType() {
+		return Problem.class;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.hateoas.config.HypermediaMappingInformation#getMediaTypes()
+	 */
+	@Override
+	public List<MediaType> getMediaTypes() {
+		return Collections.singletonList(MediaTypes.HTTP_PROBLEM_DETAILS_JSON);
+	}
+}

src/main/java/org/springframework/hateoas/mediatype/problem/Problem.java

@@ -15,147 +15,255 @@
  */
 package org.springframework.hateoas.mediatype.problem;
 
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.EqualsAndHashCode;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.ToString;
+import lombok.Value;
+import lombok.experimental.NonFinal;
+import lombok.experimental.Wither;
+
 import java.net.URI;
-import java.util.Objects;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.function.Consumer;
 
 import org.springframework.http.HttpStatus;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 
+import com.fasterxml.jackson.annotation.JsonAnyGetter;
+import com.fasterxml.jackson.annotation.JsonAnySetter;
 import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonInclude.Include;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.annotation.JsonUnwrapped;
 
 /**
  * Encapsulation of an RFC-7807 {@literal Problem} code. While it complies out-of-the-box, it may also be extended to
  * support domain-specific details.
- * 
+ *
  * @author Greg Turnquist
+ * @author Oliver Drotbohm
  */
-public class Problem<T extends Problem<? extends T>> {
+@Getter(onMethod = @__(@JsonProperty))
+@Wither
+@ToString
+@EqualsAndHashCode
+@JsonInclude(Include.NON_NULL)
+@NoArgsConstructor(force = true, access = AccessLevel.PROTECTED)
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+public class Problem {
 
-	private URI type;
-	private String title;
-	private HttpStatus status;
-	private String detail;
-	private URI instance;
+	private static Problem EMPTY = new Problem();
 
-	public Problem() {
-		this(null, null, null, null, null);
-	}
-
-	public Problem(URI type, String title, HttpStatus status, String detail, URI instance) {
-
-		this.type = type;
-		this.title = title;
-		this.status = status;
-		this.detail = detail;
-		this.instance = instance;
-	}
+	private final @Nullable URI type;
+	private final @Nullable String title;
+	private final @Nullable @Getter(onMethod = @__(@JsonIgnore)) HttpStatus status;
+	private final @Nullable String detail;
+	private final @Nullable URI instance;
 
 	@JsonCreator
 	public Problem(@JsonProperty("type") URI type, @JsonProperty("title") String title,
 			@JsonProperty("status") int status, @JsonProperty("detail") String detail,
 			@JsonProperty("instance") URI instance) {
+
 		this(type, title, HttpStatus.resolve(status), detail, instance);
 	}
 
 	/**
-	 * A {@link Problem} that reflects an {@link HttpStatus} code.
+	 * Returns an empty {@link Problem} instance.
 	 *
-	 * @see https://tools.ietf.org/html/rfc7807#section-4.2
+	 * @return an empty {@link Problem} instance.
 	 */
-	public Problem(HttpStatus httpStatus) {
-		this(URI.create("about:blank"), httpStatus.getReasonPhrase(), httpStatus, null, null);
+	public static Problem create() {
+		return EMPTY;
 	}
 
-	@SuppressWarnings("unchecked")
-	public T withType(URI type) {
-		this.type = type;
-		return (T) this;
+	/**
+	 * Returns an {@link ExtendedProblem} with the given payload as additional properties.
+	 *
+	 * @param <T>
+	 * @param payload must not be {@literal null}.
+	 * @return
+	 */
+	public static <T> ExtendedProblem<T> create(T payload) {
+
+		Assert.notNull(payload, "Payload must not be null!");
+
+		return EMPTY.withProperties(payload);
 	}
 
-	@SuppressWarnings("unchecked")
-	public T withTitle(String title) {
-		this.title = title;
-		return (T) this;
+	/**
+	 * Returns a {@link Problem} instance with the given {@link HttpStatus} and defaults as defined in
+	 * <a href="https://tools.ietf.org/html/rfc7807#section-4.2">RFC7807</a>.
+	 *
+	 * @param status must not be {@literal null}.
+	 * @return
+	 * @see <a href="https://tools.ietf.org/html/rfc7807#section-4.2">RFC7807</a>
+	 */
+	public static Problem statusOnly(HttpStatus status) {
+
+		Assert.notNull(status, "HttpStatus must not be null!");
+
+		return new Problem(URI.create("about:blank"), status.getReasonPhrase(), status, null, null);
 	}
 
-	@SuppressWarnings("unchecked")
-	public T withStatus(HttpStatus status) {
-		this.status = status;
-		return (T) this;
+	/**
+	 * Creates a new {@link ExtendedProblem} with the given payload as additional properties.
+	 *
+	 * @param <T>
+	 * @param payload must not be {@literal null}.
+	 * @return
+	 */
+	public <T> ExtendedProblem<T> withProperties(T payload) {
+		return new ExtendedProblem<>(type, title, status, detail, instance, payload);
 	}
 
-	@SuppressWarnings("unchecked")
-	public T withDetail(String detail) {
-		this.detail = detail;
-		return (T) this;
+	/**
+	 * Returns an {@link ExtendedProblem} with a {@link Map<String, Object>} populated by the given consumer as payload.
+	 *
+	 * @param consumer must not be {@literal null}.
+	 * @return
+	 */
+	public ExtendedProblem<Map<String, Object>> withProperties(Consumer<Map<String, Object>> consumer) {
+
+		Assert.notNull(consumer, "Consumer must not be null!");
+
+		Map<String, Object> map = new HashMap<>();
+		consumer.accept(map);
+
+		return withProperties(map);
 	}
 
-	@SuppressWarnings("unchecked")
-	public T withInstance(URI instance) {
-		this.instance = instance;
-		return (T) this;
+	/**
+	 * Returns an {@link ExtendedProblem} with the given {@link Map} unwrapping as additional properties.
+	 *
+	 * @param properties must not be {@literal null}.
+	 * @return
+	 */
+	public ExtendedProblem<Map<String, Object>> withProperties(Map<String, Object> properties) {
+
+		Assert.notNull(properties, "Properties must not be null!");
+
+		return new ExtendedProblem<Map<String, Object>>(type, title, status, detail, instance, properties);
 	}
 
+	@Nullable
+	@JsonProperty("status")
 	@JsonInclude(Include.NON_NULL)
-	public URI getType() {
-		return this.type;
+	Integer getStatusAsInteger() {
+		return status != null ? status.value() : null;
 	}
 
-	@JsonInclude(Include.NON_NULL)
-	public String getTitle() {
-		return this.title;
-	}
+	@Value
+	@Getter(onMethod = @__(@JsonIgnore))
+	@EqualsAndHashCode(callSuper = true)
+	@NoArgsConstructor(force = true, access = AccessLevel.PRIVATE)
+	public static class ExtendedProblem<T> extends Problem {
 
-	@JsonInclude(Include.NON_NULL)
-	public Integer getStatus() {
-		if (status != null) {
-			return status.value();
+		private @NonFinal T extendedProperties;
+
+		ExtendedProblem(@Nullable URI type, @Nullable String title, @Nullable HttpStatus status, @Nullable String detail,
+				@Nullable URI instance, @Nullable T properties) {
+
+			super(type, title, status, detail, instance);
+
+			this.extendedProperties = properties;
 		}
 
-		return null;
-	}
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.hateoas.mediatype.problem.Problem#withType(java.net.URI)
+		 */
+		@Override
+		public ExtendedProblem<T> withType(@Nullable URI type) {
+			return new ExtendedProblem<>(type, getTitle(), getStatus(), getDetail(), getInstance(), extendedProperties);
+		}
 
-	@JsonInclude(Include.NON_NULL)
-	public String getDetail() {
-		return detail;
-	}
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.hateoas.mediatype.problem.Problem#withTitle(java.lang.String)
+		 */
+		@Override
+		public ExtendedProblem<T> withTitle(@Nullable String title) {
+			return new ExtendedProblem<>(getType(), title, getStatus(), getDetail(), getInstance(), extendedProperties);
+		}
 
-	@JsonInclude(Include.NON_NULL)
-	public URI getInstance() {
-		return instance;
-	}
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.hateoas.mediatype.problem.Problem#withDetail(java.lang.String)
+		 */
+		@Override
+		public ExtendedProblem<T> withDetail(@Nullable String detail) {
+			return new ExtendedProblem<>(getType(), getTitle(), getStatus(), detail, getInstance(), extendedProperties);
+		}
 
-	@Override
-	public boolean equals(Object o) {
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.hateoas.mediatype.problem.Problem#withInstance(java.net.URI)
+		 */
+		@Override
+		public ExtendedProblem<T> withInstance(@Nullable URI instance) {
+			return new ExtendedProblem<>(getType(), getTitle(), getStatus(), getDetail(), instance, extendedProperties);
+		}
 
-		if (this == o)
-			return true;
-		if (o == null || getClass() != o.getClass())
-			return false;
-		Problem problem = (Problem) o;
-		return Objects.equals(type, problem.type) && //
-				Objects.equals(title, problem.title) && //
-				status == problem.status && //
-				Objects.equals(detail, problem.detail) && //
-				Objects.equals(instance, problem.instance); //
-	}
+		/**
+		 * Returns the additional properties.
+		 *
+		 * @return
+		 */
+		@JsonIgnore
+		public T getProperties() {
+			return extendedProperties;
+		}
 
-	@Override
-	public int hashCode() {
-		return Objects.hash(type, title, status, detail, instance);
-	}
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.hateoas.mediatype.problem.Problem#withProperties(java.lang.Object)
+		 */
+		@Override
+		public <S> ExtendedProblem<S> withProperties(S payload) {
+			return super.withProperties(payload);
+		}
 
-	@Override
-	public String toString() {
+		// Payload type based serialization
 
-		return "Problem{" + //
-				"type=" + type + //
-				", title='" + title + '\'' + //
-				", status=" + status + //
-				", detail='" + detail + '\'' + //
-				", instance=" + instance + //
-				'}';
+		@Nullable
+		@JsonUnwrapped
+		T getExtendedProperties() {
+			return Map.class.isInstance(extendedProperties) ? null : extendedProperties;
+		}
+
+		// Map based serialization
+
+		@Nullable
+		@JsonAnyGetter
+		@SuppressWarnings("unchecked")
+		Map<String, Object> getPropertiesAsMap() {
+			return Map.class.isInstance(extendedProperties) ? (Map<String, Object>) extendedProperties : null;
+		}
+
+		// Map based deserialization
+
+		@JsonAnySetter
+		void setPropertiesAsMap(String key, Object value) {
+			getOrInitAsMap().put(key, value);
+		}
+
+		@SuppressWarnings("unchecked")
+		private Map<String, Object> getOrInitAsMap() {
+
+			if (this.extendedProperties == null) {
+				this.extendedProperties = (T) new LinkedHashMap<>();
+			}
+
+			return (Map<String, Object>) this.extendedProperties;
+		}
 	}
 }