Map content is not getting serialized properly

[uber-cool]

spring-hateoas version: 1.2.0-SNAPSHOT
When the content is a Map in EntityModel, it is not getting converted in proper JSON. The content is getting duplicated.

Test case to describe this behavior:

        @Test
	public void testSerializationOfMap() throws JsonProcessingException {
		final Map map = new HashMap();
		map.put("key", "value");
		final String serialized = new ObjectMapper().writerWithDefaultPrettyPrinter().writeValueAsString(EntityModel.of(map));

		System.out.println("Incorrect representation, key value are duplicating at root level as well as under content");
		System.out.println(serialized);

		class KeyValue {
			public KeyValue(String value) {
				key = value;
			}
			public String key;
		}

		KeyValue keyValue = new KeyValue("value");
		final String serializedKeyValue = new ObjectMapper().writerWithDefaultPrettyPrinter().writeValueAsString(EntityModel.of(keyValue));
		System.out.println("Correct representation when using custom class KeyValue");
		System.out.println(serializedKeyValue);
	}

[odrotbohm]

This test contains the invalid assumption that a bare ObjectMapper was supposed to create a valid representation which it is not. You need to register the Jackson module and HandlerInstantiator for the media type you want this to be rendered for.

[gregturn]

Actually, I coded this test case inside Jackson2HalIntegrationTest...

@Test
void foo() 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("{\"content\":{\"key\":\"value\",\"key2\":\"value2\"},\"_links\":{\"self\":{\"href\":\"http://example.com\"}},\"key\":\"value\",\"key2\":\"value2\"}");

	SomeSample sample = new SomeSample();
	sample.name = "foobar";
	EntityModel<SomeSample> entityModel2 = EntityModel.of(sample);
	entityModel2.add(Link.of("http://example.com"));

	assertThat(mapper.writeValueAsString(entityModel2)).isEqualTo("{\"name\":\"foobar\",\"_links\":{\"self\":{\"href\":\"http://example.com\"}}}");
}

This does confirm that wrapping a Map with EntityModel appears to duplicate data (while a POJO has no issues).

[gregturn]

Looks like the problem is here, inside EntityModel:

/**
 * Returns the underlying entity.
 *
 * @return the content
 */
@JsonUnwrapped
@Nullable
public T getContent() {
		return content;
}

// Hacks to allow deserialization into an EntityModel<Map<String, Object>>

@Nullable
@JsonAnyGetter
@SuppressWarnings("unchecked")
private Map<String, Object> getMapContent() {
	return Map.class.isInstance(content) ? (Map<String, Object>) content : null;
}

Only one of these two methods should ever return data.

[gregturn]

This seems to do the trick:

@JsonUnwrapped
@Nullable
public T getContent() {
	return Map.class.isInstance(content) ? null : content;
}

@Test
void foo() 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\"}");
}

[gregturn]

Looks like we have at least one unit test that only verifies the Map property appears at the root level, but does NOT demonstrate the duplication. Working...

[uber-cool]

This test contains the invalid assumption that a bare ObjectMapper was supposed to create a valid representation which it is not. You need to register the Jackson module and HandlerInstantiator for the media type you want this to be rendered for.

@odrotbohm
I am pointing that serialization works differently when content is Map and when it is a POJO. I added two cases the output should be same but it is not.

@gregturn has got the problem and the fix he suggested seems working. I think this has to be fixed in hateoas.

[odrotbohm]

Yes, you're correct.Jackson2HalIntegrationTest.rendersMapContentCorrectly() indeed only checks for the map to be serialized, not the content being omitted.