jackson-databind

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Describe the bug

Sometimes you care about the serialized json always beeing an object (not the string null even though it's valid JSON). I understand that in some cases, when passing a null value, you can't derive any sensible root name, so "null" is a good default (maybe you should be able to opt in to get an exception instead ? or use a generic name "data" or "result" but I guess you chose not to have a "default" root name?). But in some cases the root name is known, so the serializing should be possible ?

// both return "null" but could return "{\"Pojo\":null}" ?
// are there other cases when the root name is know ?
mapper.writer().withRootName("Pojo").writeValueAsString(null)
mapper.writerFor(Pojo.class).with(SerializationFeature.WRAP_ROOT_VALUE).writeValueAsString(null)

// for now these throw exception but should work if the serialization is allowed ?
mapper.readerFor(Pojo.class).with(DeserializationFeature.UNWRAP_ROOT_VALUE).readValue("{\"Pojo\":null}")
mapper.reader().withRootName("Pojo").readValue("{\"Pojo\":null}", Pojo.class)
//   Exception com.fasterxml.jackson.databind.exc.MismatchedInputException:
//     Cannot deserialize value of type `Pojo` from Null value (token `JsonToken.VALUE_NULL`)

A workaround is to wrap the null in another object that is never null but it's less expressive ?

// is there a better way to do this ?
mapper.writeValueAsString(mapper.createObjectNode().putPOJO("Pojo", null));
// correctly returns "{\"Pojo\":null}"
mapper.treeToValue(mapper.readTree("{\"Pojo\":null}").get("Pojo"), Pojo.class)
// correctly returns null

Closely related, rootname or WRAP_ROOT_VALUE already works to guarantee that you get an object for almost all cases (numbers, strings, lists, ...), just not null ? Hence changing this would make it more useful ?

Obviously in real world use cases this code is working on something that is sometimes null sometimes nonnull.

Version Information

2.15.2