#1354 - Change in implementation strategy for Maps in EntityModel.

We now use a custom serializer for EntityModel.getContent() as we need that to still return the Map content for programmatic clients. The serializer skips maps entirely and programmatically unwraps all other content.

A simple Converter (@JsonSerialize(convert = …)) to filter Maps unfortunately didn't do the trick as that causes a a delegating serializer to be registered for the property and @JsonUnwrapped stops working as it's designed to work with bean serializers only.

Adapted test cases accordingly.

Original pull request: #1353.

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

@@ -15,20 +15,24 @@
  */
 package org.springframework.hateoas;
 
+import java.io.IOException;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 
-import com.fasterxml.jackson.annotation.JsonIgnore;
-import com.fasterxml.jackson.annotation.JsonProperty;
 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.JsonUnwrapped;
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
+import com.fasterxml.jackson.databind.util.NameTransformer;
 
 /**
  * A simple {@link EntityModel} wrapping a domain object and adding links to it.
@@ -117,10 +121,11 @@ public class EntityModel<T> extends RepresentationModel<EntityModel<T>> {
 	 *
 	 * @return the content
 	 */
-	@JsonUnwrapped
 	@Nullable
+	@JsonUnwrapped
+	@JsonSerialize(using = MapSuppressingUnwrappingSerializer.class)
 	public T getContent() {
-		return !Map.class.isInstance(content) ? content : null;
+		return content;
 	}
 
 	// Hacks to allow deserialization into an EntityModel<Map<String, Object>>
@@ -128,7 +133,7 @@ public class EntityModel<T> extends RepresentationModel<EntityModel<T>> {
 	@Nullable
 	@JsonAnyGetter
 	@SuppressWarnings("unchecked")
-	public Map<String, Object> getMapContent() {
+	private Map<String, Object> getMapContent() {
 		return Map.class.isInstance(content) ? (Map<String, Object>) content : null;
 	}
 
@@ -190,4 +195,36 @@ public class EntityModel<T> extends RepresentationModel<EntityModel<T>> {
 		result += content == null ? 0 : 17 * content.hashCode();
 		return result;
 	}
+
+	private static class MapSuppressingUnwrappingSerializer extends StdSerializer<Object> {
+
+		public MapSuppressingUnwrappingSerializer() {
+			super(Object.class);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see com.fasterxml.jackson.databind.ser.std.StdSerializer#serialize(java.lang.Object, com.fasterxml.jackson.core.JsonGenerator, com.fasterxml.jackson.databind.SerializerProvider)
+		 */
+		@Override
+		public void serialize(Object value, JsonGenerator gen, SerializerProvider provider) throws IOException {
+
+			if (value == null || Map.class.isInstance(value)) {
+				return;
+			}
+
+			provider.findValueSerializer(value.getClass()) //
+					.unwrappingSerializer(NameTransformer.NOP) //
+					.serialize(value, gen, provider);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see com.fasterxml.jackson.databind.JsonSerializer#isUnwrappingSerializer()
+		 */
+		@Override
+		public boolean isUnwrappingSerializer() {
+			return true;
+		}
+	}
 }

src/test/java/org/springframework/hateoas/mediatype/hal/Jackson2HalIntegrationTest.java

@@ -26,7 +26,6 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
-import java.util.TreeMap;
 import java.util.stream.Stream;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -60,8 +59,10 @@ import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.PropertyNamingStrategy;
 import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.databind.type.TypeFactory;
+import com.jayway.jsonpath.Configuration;
 import com.jayway.jsonpath.DocumentContext;
 import com.jayway.jsonpath.JsonPath;
+import com.jayway.jsonpath.Option;
 
 /**
  * Integration tests for Jackson 2 HAL integration.
@@ -565,7 +566,7 @@ class Jackson2HalIntegrationTest {
 				.forEach(it -> assertThat(it).containsKey("someSample"));
 	}
 
-	@Test // #1157
+	@Test // #1157, #1352
 	void rendersMapContentCorrectly() throws Exception {
 
 		Map<String, Object> map = new HashMap<>();
@@ -574,10 +575,12 @@ class Jackson2HalIntegrationTest {
 
 		EntityModel<?> model = EntityModel.of(map, Link.of("foo", IanaLinkRelations.SELF));
 
-		DocumentContext context = JsonPath.parse(mapper.writeValueAsString(model));
+		DocumentContext context = JsonPath.parse(mapper.writeValueAsString(model),
+				Configuration.defaultConfiguration().addOptions(Option.SUPPRESS_EXCEPTIONS));
 
 		assertThat(context.read("$.key", String.class)).isEqualTo("value");
 		assertThat(context.read("$.anotherKey", String.class)).isEqualTo("anotherValue");
+		assertThat(context.read("$.content", Object.class)).isNull();
 	}
 
 	@Test // #1157
@@ -592,8 +595,7 @@ class Jackson2HalIntegrationTest {
 
 		EntityModel<Map<String, Object>> result = mapper.readValue(source, modelType);
 
-		assertThat(result.getContent()).isNull();
-		assertThat(result.getMapContent()).containsEntry("key", "value");
+		assertThat(result.getContent()).containsEntry("key", "value");
 	}
 
 	@Test // #1157
@@ -608,22 +610,6 @@ class Jackson2HalIntegrationTest {
 		assertThat(result.getContent().name).isEqualTo("Dave");
 	}
 
-	@Test // #1352
-	void rendersMapWithoutDuplicateEntries() throws JsonProcessingException {
-
-		Map<String, String> map = new TreeMap<>();
-		map.put("key", "value");
-		map.put("key2", "value2");
-
-		EntityModel<Map<String, String>> entityModel = EntityModel.of(map);
-		entityModel.add(Link.of("http://example.com"));
-
-		String serialized = mapper.writeValueAsString(entityModel);
-
-		assertThat(serialized)
-				.isEqualTo("{\"_links\":{\"self\":{\"href\":\"http://example.com\"}},\"key\":\"value\",\"key2\":\"value2\"}");
-	}
-
 	@Relation(collectionRelation = "someSample")
 	static class SomeSample {
 		@JsonProperty String name;