Jackson does not support deserializing new Java 9 unmodifiable collections

Describe the bug Java 9 introduced new unmodifiable collection APIs. For instance, List.of.

Today, Jackson has a special deserializer for Collections.unmodifiableList but not for these new classes: https://github.com/FasterXML/jackson-databind/blob/a64e1bfd7ced17c2e19e73da1a00f3e679011fec/src/main/java/com/fasterxml/jackson/databind/deser/impl/JavaUtilCollectionsDeserializers.java#L81

Version information 2.11.2

To Reproduce

@Test
public void testUnmodifiableList() throws JsonMappingException, JsonProcessingException {
	final ObjectMapper mapper = new ObjectMapper();
	mapper.activateDefaultTypingAsProperty(
		new NoCheckSubTypeValidator(),
		ObjectMapper.DefaultTyping.NON_FINAL,
		"@class"
	);

	final List<String> list = Collections.unmodifiableList(Collections.singletonList("a"));
	final String actualJson = mapper.writeValueAsString(list);
	System.out.println(actualJson);
	final List output = mapper.readValue(actualJson, List.class);
	System.out.println(output);
}

@Test
public void testJava9UmodifiableList() throws JsonMappingException, JsonProcessingException {
	final ObjectMapper mapper = new ObjectMapper();
	mapper.activateDefaultTypingAsProperty(
		new NoCheckSubTypeValidator(),
		ObjectMapper.DefaultTyping.NON_FINAL,
		"@class"
	);

	final List<String> list = List.of("a");
	final String actualJson = mapper.writeValueAsString(list);
	System.out.println(actualJson);
	final List output = mapper.readValue(actualJson, List.class);
	System.out.println(output);
}

@Test
public void testJava9ListWrapped() throws JsonMappingException, JsonProcessingException {
	final ObjectMapper mapper = new ObjectMapper();
	mapper.activateDefaultTypingAsProperty(
		new NoCheckSubTypeValidator(),
		ObjectMapper.DefaultTyping.NON_FINAL,
		"@class"
	);

	final List<String> list = Collections.unmodifiableList(List.of("a"));
	final String actualJson = mapper.writeValueAsString(list);
	System.out.println(actualJson);
	final List output = mapper.readValue(actualJson, List.class);
	System.out.println(output);
}

testJava9UmodifiableList fails. The other 2 pass.

Expected behavior I would think the new classes are added as additional aliases as candidates for special deserialization.

Unfortunately, I think this is tricky for two reasons:

1. These are Java 9 classes and Jackson supports a lower minimum Java version. I’m not sure if there’s a way to add them without breaking backwards compatibility or introducing a new Java 9 add-on library, like there exists for Java 8.
2. There are multiple classes. For List.of, there seem to be 3 different ones depending on the number of arguments:

• Empty list if 0 arguments.
• List12 if 1 or 2 arguments.
• ListN if more than 2 arguments. Source: https://hg.openjdk.java.net/jdk/jdk11/file/1ddf9a99e4ad/src/java.base/share/classes/java/util/List.java#l1031

[cowtowncoder]

Ok, had a chance to look a bit into this. I agree with notes on 2 main challenges, but there is also third immediate problem wrt tests: type for List is actually final which is what sort of breaks the test since:

1. During serialization, actual type – which is final – will result in no type information being added, BUT
2. During deserialization, requested type – List – is NON-FINAL and as such no type information is excepted

so test would fail even if special handling was added.

For now I think I can at least add failing test(s), under src/test-jdk14/ to allow reproducing the issue, in case someone has time to look deeper into possible solutions.

[Sam-Kruglov]

Hi, 2.13.0 doesn’t seem to work with Set.of() and I don’t see any tests for it, there’re only for List.of() and Map.of().