#786 - Implement vendor neutral error handling with RFC-7807.

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

@@ -76,8 +76,7 @@ public class MediaTypes {
 	 * Public constant media type for {@code application/vnd.amundsen-uber+json}.
 	 */
 	public static final MediaType UBER_JSON = MediaType.parseMediaType(UBER_JSON_VALUE);
-
-
+
 	/**
 	 * A String equivalent of {@link MediaTypes#VND_ERROR_JSON}.
 	 */
@@ -87,4 +86,14 @@ public class MediaTypes {
 	 * Public constant media type for {@code application/vnd.error+json}.
 	 */
 	public static final MediaType VND_ERROR_JSON = MediaType.valueOf(VND_ERROR_JSON_VALUE);
+
+	/**
+	 * A String equivalent of {@link MediaTypes#PROBLEM_JSON_VALUE}.
+	 */
+	public static final String PROBLEM_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);
 }

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

@@ -0,0 +1,161 @@
+/*
+ * Copyright 2019 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.net.URI;
+import java.util.Objects;
+
+import org.springframework.http.HttpStatus;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonInclude.Include;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+/**
+ * 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
+ */
+public class Problem<T extends Problem<? extends T>> {
+
+	private URI type;
+	private String title;
+	private HttpStatus status;
+	private String detail;
+	private URI instance;
+
+	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;
+	}
+
+	@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.
+	 *
+	 * @see https://tools.ietf.org/html/rfc7807#section-4.2
+	 */
+	public Problem(HttpStatus httpStatus) {
+		this(URI.create("about:blank"), httpStatus.getReasonPhrase(), httpStatus, null, null);
+	}
+
+	@SuppressWarnings("unchecked")
+	public T withType(URI type) {
+		this.type = type;
+		return (T) this;
+	}
+
+	@SuppressWarnings("unchecked")
+	public T withTitle(String title) {
+		this.title = title;
+		return (T) this;
+	}
+
+	@SuppressWarnings("unchecked")
+	public T withStatus(HttpStatus status) {
+		this.status = status;
+		return (T) this;
+	}
+
+	@SuppressWarnings("unchecked")
+	public T withDetail(String detail) {
+		this.detail = detail;
+		return (T) this;
+	}
+
+	@SuppressWarnings("unchecked")
+	public T withInstance(URI instance) {
+		this.instance = instance;
+		return (T) this;
+	}
+
+	@JsonInclude(Include.NON_NULL)
+	public URI getType() {
+		return this.type;
+	}
+
+	@JsonInclude(Include.NON_NULL)
+	public String getTitle() {
+		return this.title;
+	}
+
+	@JsonInclude(Include.NON_NULL)
+	public Integer getStatus() {
+		if (status != null) {
+			return status.value();
+		}
+
+		return null;
+	}
+
+	@JsonInclude(Include.NON_NULL)
+	public String getDetail() {
+		return detail;
+	}
+
+	@JsonInclude(Include.NON_NULL)
+	public URI getInstance() {
+		return instance;
+	}
+
+	@Override
+	public boolean equals(Object o) {
+
+		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); //
+	}
+
+	@Override
+	public int hashCode() {
+		return Objects.hash(type, title, status, detail, instance);
+	}
+
+	@Override
+	public String toString() {
+
+		return "Problem{" + //
+				"type=" + type + //
+				", title='" + title + '\'' + //
+				", status=" + status + //
+				", detail='" + detail + '\'' + //
+				", instance=" + instance + //
+				'}';
+	}
+}

src/main/java/org/springframework/hateoas/mediatype/problem/package-info.java

@@ -0,0 +1,5 @@
+/**
+ * Value objects to build Problem representations.
+ */
+@org.springframework.lang.NonNullApi
+package org.springframework.hateoas.mediatype.problem;

src/test/java/org/springframework/hateoas/mediatype/hal/forms/HalFormsWebFluxIntegrationTest.java

@@ -15,10 +15,13 @@
  */
 package org.springframework.hateoas.mediatype.hal.forms;
 
+import static org.assertj.core.api.Assertions.*;
 import static org.hamcrest.CoreMatchers.*;
 import static org.hamcrest.collection.IsCollectionWithSize.*;
 import static org.springframework.hateoas.support.JsonPathUtils.*;
 
+import java.net.URI;
+
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -31,9 +34,11 @@
 import org.springframework.hateoas.config.EnableHypermediaSupport;
 import org.springframework.hateoas.config.EnableHypermediaSupport.HypermediaType;
 import org.springframework.hateoas.config.WebClientConfigurer;
+import org.springframework.hateoas.mediatype.problem.Problem;
 import org.springframework.hateoas.support.MappingUtils;
 import org.springframework.hateoas.support.WebFluxEmployeeController;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
 import org.springframework.test.context.ContextConfiguration;
 import org.springframework.test.context.junit.jupiter.SpringExtension;
 import org.springframework.test.context.web.WebAppConfiguration;
@@ -128,6 +133,23 @@ void createNewEmployee() throws Exception {
 				.expectHeader().valueEquals(HttpHeaders.LOCATION, "http://localhost/employees/2");
 	}
 
+	@Test
+	void problemReturningControllerMethod() {
+
+		Problem<?> problem = this.testClient.get().uri("http://localhost/employees/problem").accept(MediaTypes.PROBLEM_JSON) //
+				.exchange() //
+				.expectStatus().isBadRequest() //
+				.expectHeader().contentType(MediaTypes.PROBLEM_JSON) //
+				.expectBody(Problem.class) //
+				.returnResult().getResponseBody();
+
+		assertThat(problem).isNotNull();
+		assertThat(problem.getType()).isEqualTo(URI.create("http://example.com/problem"));
+		assertThat(problem.getTitle()).isEqualTo("Employee-based problem");
+		assertThat(problem.getStatus()).isEqualTo(HttpStatus.BAD_REQUEST.value());
+		assertThat(problem.getDetail()).isEqualTo("This is a test case");
+	}
+
 	@Configuration
 	@EnableWebFlux
 	@EnableHypermediaSupport(type = { HypermediaType.HAL_FORMS })