#1157 - 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

@@ -45,12 +45,14 @@ 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.*;
@@ -69,6 +71,7 @@ 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;
 
 /**
@@ -91,6 +94,8 @@ 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());
 	}
 
 	/**
@@ -105,6 +110,75 @@ 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.
 	 *