`DeserializationContext.readTreeAsValue()` handles null nodes differently from `ObjectMapper.treeToValue()`

[FWest98]

Search before asking

• I searched in the issues and found nothing similar.

Describe the bug

The ObjectMapper functions handle the case where the root node is the null node by specifically calling getNullValue of the root deserializer. The equivalent DeserializationContext functions do not. I would expect the two functions to behave similarly, or the differences should be documented.

See below for a reproducer and use case.

Version Information

2.18.1

Reproduction

My use case is that I have a custom deserializer that reads the defaultValue metadata field that is populated by the JsonProperty annotation. I implement a custom getAbsentValue function that will handle the default values. However, I also want to allow "null" as a default value.

override fun getAbsentValue(ctxt: DeserializationContext): Property<*> {
    val defaultString = property?.metadata?.defaultValue // This is the string "null"

    if(defaultString != null) {
        val tree = ctxt.parser.codec.factory.createParser(defaultString).readValueAsTree<JsonNode>()
        val thisWorks = (ctxt.parser.codec as ObjectMapper).treeToValue<Any>(tree, valueType)
        val doesNotWork = ctxt.readTreeAsValue<Any>(tree, valueType)

        // Further handling
}

The workaround is to cast the codec to an ObjectMapper, but that does not seem very nice and I would prefer not to do it, since there are no guarantees of the cast being allowed.

[cowtowncoder]

Ok, I'll need to think of how to write a test case. I think you are right since although this:

    public <T> T readTreeAsValue(JsonNode n, Class<T> targetType) throws IOException
    {
        if (n == null) {
            return null;
        }
        try (TreeTraversingParser p = _treeAsTokens(n)) {
            return readValue(p, targetType);
        }
    }

at first looks correct, I forgot that JsonDeserializers are not expected to actually deal with JsonToken.VALUE_NULL themselves; it's "parent deserializer" that should handle that -- or in case of root value, DeserializationContext.

But getting access to a DeserializationContext from test, outside handler callbacks is bit trickier.

[cowtowncoder]

@FWest98 I fail to reproduce the issue against 2.18 branch (for 2.18.3), see #4940. So could use help here. It looks like getNullValue() would be called as expected from limited test I added.

(I did add a small fix to handle "missing node" case tho so that's useful).

[JooHyukKim]

@FWest98 Yeah would be helpful if we ca get the actual working reproduction.

[FWest98]

It seems like the BooleanDeserializer handles null tokens explicitly, without getNullValue being called. In my usecase, things went wrong with Duration, but I can reproduce it with an arbitrary bean as follows (adapted from your tests):

// [databind#4934]
@Test
public void testTreeAsValueFromNulls() throws Exception
{
    final JsonNodeFactory nodeF = MAPPER.getNodeFactory();
    try (JsonParser p = MAPPER.createParser("abc")) {
        DeserializationContext ctxt = MAPPER.readerFor(String.class).createDeserializationContext(p);

        assertNull(MAPPER.treeToValue(nodeF.nullNode(), Bean.class)); // works
        assertNull(ctxt.readTreeAsValue(nodeF.nullNode(), Bean.class)); // fails, throws exception
    }
}

[cowtowncoder]

Interesting. Thank you @FWest98.

[cowtowncoder]

I can reproduce the issue and know what is needed. But fix is likely bit riskier than what should go in a patch so will target 2.19, instead of 2.18 like I initially planned.