#1158 - Register custom serializers to handle Map based EntityModel instances.

We now register a custom serializer to massage EntityModel instances into dedicated types that – in case of a Map being the content of the entity model – wrap the model into a type that applies the necessary Jackson tweaks to properly unwrap a Map.

src/main/java/org/springframework/hateoas/mediatype/hal/Jackson2HalModule.java

@@ -40,12 +40,14 @@ import org.springframework.lang.NonNull;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
+import com.fasterxml.jackson.annotation.JsonAnyGetter;
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonInclude.Include;
 import com.fasterxml.jackson.annotation.JsonUnwrapped;
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.core.JsonParseException;
 import com.fasterxml.jackson.core.JsonParser;
+import com.fasterxml.jackson.core.JsonStreamContext;
 import com.fasterxml.jackson.core.JsonToken;
 import com.fasterxml.jackson.core.Version;
 import com.fasterxml.jackson.databind.*;
@@ -63,6 +65,7 @@ import com.fasterxml.jackson.databind.ser.ContainerSerializer;
 import com.fasterxml.jackson.databind.ser.ContextualSerializer;
 import com.fasterxml.jackson.databind.ser.std.MapSerializer;
 import com.fasterxml.jackson.databind.ser.std.StdScalarSerializer;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
 import com.fasterxml.jackson.databind.type.TypeFactory;
 
 /**
@@ -85,6 +88,8 @@ public class Jackson2HalModule extends SimpleModule {
 		setMixInAnnotation(Link.class, LinkMixin.class);
 		setMixInAnnotation(RepresentationModel.class, RepresentationModelMixin.class);
 		setMixInAnnotation(CollectionModel.class, CollectionModelMixin.class);
+
+		addSerializer(EntityModel.class, new EntityModelSerializer());
 	}
 
 	/**
@@ -99,6 +104,75 @@ public class Jackson2HalModule extends SimpleModule {
 		return LinkMixin.class.equals(mapper.findMixInClassFor(Link.class));
 	}
 
+	/**
+	 * Custom serializer for {@link EntityModel} to make sure we get {@link Map} instances properly unwrapped. The
+	 * serializer wraps the model instance into a dedicated classes that apply the proper Jackson configuration that's
+	 * needed to achieve this and serializes those.
+	 *
+	 * @author Oliver Drotbohm
+	 * @see https://github.com/FasterXML/jackson-databind/issues/171
+	 */
+	@SuppressWarnings("rawtypes")
+	static class EntityModelSerializer extends StdSerializer<EntityModel> {
+
+		private static final long serialVersionUID = -5933309398043585183L;
+
+		public EntityModelSerializer() {
+			super(EntityModel.class, false);
+		}
+
+		/*
+		 * (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
+		@SuppressWarnings("null")
+		public void serialize(EntityModel value, JsonGenerator gen, SerializerProvider provider) throws IOException {
+
+			Object content = value.getContent();
+
+			RepresentationModel<?> wrapped = Map.class.isInstance(content) //
+					? new MapModel(value) //
+					: new NonMapModel(value);
+
+			provider.defaultSerializeValue(wrapped, gen);
+		}
+
+		static class MapModel extends RepresentationModel<MapModel> {
+
+			private final @Nullable Map<?, ?> map;
+
+			public MapModel(EntityModel<?> model) {
+
+				super(model.getLinks().toList());
+
+				this.map = (Map<?, ?>) model.getContent();
+			}
+
+			@Nullable
+			@JsonAnyGetter
+			public Map<?, ?> getContent() {
+				return map;
+			}
+		}
+
+		static class NonMapModel extends RepresentationModel<NonMapModel> {
+
+			private final EntityModel<?> model;
+
+			public NonMapModel(EntityModel<?> model) {
+				super(model.getLinks().toList());
+				this.model = model;
+			}
+
+			@Nullable
+			@JsonUnwrapped
+			public Object getContent() {
+				return model.getContent();
+			}
+		}
+	}
+
 	/**
 	 * Custom {@link JsonSerializer} to render Link instances in HAL compatible JSON.
 	 *

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

@@ -22,8 +22,10 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
+import java.util.Map;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -43,6 +45,8 @@ import org.springframework.hateoas.server.core.EmbeddedWrappers;
 import org.springframework.lang.Nullable;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.jayway.jsonpath.DocumentContext;
+import com.jayway.jsonpath.JsonPath;
 
 /**
  * Integration tests for Jackson 2 HAL integration.
@@ -88,8 +92,8 @@ public class Jackson2HalIntegrationTest extends AbstractJackson2MarshallingInteg
 				DefaultLinkRelationProvider.INSTANCE);
 
 		mapper.registerModule(new Jackson2HalModule());
-		mapper.setHandlerInstantiator(
-				new HalHandlerInstantiator(provider, CurieProvider.NONE, MessageResolver.DEFAULTS_ONLY, new HalConfiguration()));
+		mapper.setHandlerInstantiator(new HalHandlerInstantiator(provider, CurieProvider.NONE,
+				MessageResolver.DEFAULTS_ONLY, new HalConfiguration()));
 	}
 
 	/**
@@ -480,8 +484,9 @@ public class Jackson2HalIntegrationTest extends AbstractJackson2MarshallingInteg
 
 		AnnotationLinkRelationProvider provider = new AnnotationLinkRelationProvider();
 
-		mapper.setHandlerInstantiator(new HalHandlerInstantiator(provider, CurieProvider.NONE, MessageResolver.DEFAULTS_ONLY,
-				new HalConfiguration().withRenderSingleLinksFor("foo", RenderSingleLinks.AS_ARRAY)));
+		mapper
+				.setHandlerInstantiator(new HalHandlerInstantiator(provider, CurieProvider.NONE, MessageResolver.DEFAULTS_ONLY,
+						new HalConfiguration().withRenderSingleLinksFor("foo", RenderSingleLinks.AS_ARRAY)));
 
 		RepresentationModel<?> resource = new RepresentationModel<>();
 		resource.add(new Link("/some-href", "foo"));
@@ -511,6 +516,21 @@ public class Jackson2HalIntegrationTest extends AbstractJackson2MarshallingInteg
 		}).doesNotThrowAnyException();
 	}
 
+	@Test // #1157
+	void rendersMapContentCorrectly() throws Exception {
+
+		Map<String, Object> map = new HashMap<>();
+		map.put("key", "value");
+		map.put("anotherKey", "anotherValue");
+
+		EntityModel<?> model = new EntityModel<>(map, new Link("foo", IanaLinkRelations.SELF));
+
+		DocumentContext context = JsonPath.parse(mapper.writeValueAsString(model));
+
+		assertThat(context.read("$.key", String.class)).isEqualTo("value");
+		assertThat(context.read("$.anotherKey", String.class)).isEqualTo("anotherValue");
+	}
+
 	private void verifyResolvedTitle(String resourceBundleKey) throws Exception {
 
 		LocaleContextHolder.setLocale(Locale.US);