#1694 - Avoid serializing empty beans in EntityModel.

The unwrapping nature of EntityModel.getContent() does not work well with value that would result in an empty object being rendered. We now skip the rendering if failing for empty beans is disabled.

Author: odrotbohm

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

@@ -33,9 +33,11 @@
 import com.fasterxml.jackson.databind.BeanProperty;
 import com.fasterxml.jackson.databind.JsonMappingException;
 import com.fasterxml.jackson.databind.JsonSerializer;
+import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.databind.SerializerProvider;
 import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 import com.fasterxml.jackson.databind.ser.ContextualSerializer;
+import com.fasterxml.jackson.databind.ser.impl.UnknownSerializer;
 import com.fasterxml.jackson.databind.ser.std.JsonValueSerializer;
 import com.fasterxml.jackson.databind.ser.std.StdSerializer;
 import com.fasterxml.jackson.databind.util.NameTransformer;
@@ -235,6 +237,11 @@ public void serialize(@Nullable Object value, @Nullable JsonGenerator gen, @NonN
 
 			JsonSerializer<Object> serializer = provider.findValueSerializer(value.getClass());
 
+			if (UnknownSerializer.class.isInstance(serializer)
+					&& !provider.isEnabled(SerializationFeature.FAIL_ON_EMPTY_BEANS)) {
+				return;
+			}
+
 			if (JsonValueSerializer.class.isInstance(serializer)) {
 				throw new IllegalStateException(
 						"@JsonValue rendered classes can not be directly nested in EntityModel as they do not produce a document key!");

src/test/java/org/springframework/hateoas/EntityModelIntegrationTest.java

@@ -21,6 +21,9 @@
 
 import com.fasterxml.jackson.annotation.JsonAutoDetect;
 import com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility;
+import com.fasterxml.jackson.databind.JsonMappingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.SerializationFeature;
 
 /**
  * Integration tests for {@link EntityModel}.
@@ -59,6 +62,22 @@ void readsResourceSupportCorrectly() throws Exception {
 		assertThat(result.getContent().lastname).isEqualTo("Matthews");
 	}
 
+	@Test // #1686
+	void doesNotFailOnSerializingEmptyBean() {
+
+		ObjectMapper mapper = MappingTestUtils.defaultObjectMapper();
+
+		// Fail if we're supposed to
+		assertThatExceptionOfType(JsonMappingException.class) //
+				.isThrownBy(() -> mapper.enable(SerializationFeature.FAIL_ON_EMPTY_BEANS)
+						.writeValueAsString(EntityModel.of(new Empty())));
+
+		// Ignore empty bean if we're supposed to
+		assertThatNoException() //
+				.isThrownBy(() -> mapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS)
+						.writeValueAsString(EntityModel.of(new Empty())));
+	}
+
 	static class PersonModel extends EntityModel<Person> {
 
 		public PersonModel(Person person) {
@@ -74,4 +93,6 @@ static class Person {
 		String firstname;
 		String lastname;
 	}
+
+	static class Empty {}
 }