JSpecify null validation when mapping to non-null fields in MappingMongoConverter

[nealeu]

Since Spring Boot 4 et al, adopted JSpecify, we've been trying to null-safe our code. However, at the boundaries, nulls sneak in, and in MongoDB projections in-particular, we can end up with a null value in a non-null field JSpecify @NullMarked class or record.

Could the mapping infrastructure help us out here, rather than having a big debug session on our hands, by asserting nullness (configurable as assert x != null; (for those that want to use -ea in dev/CI/QA but not prod) or Objects.requireNonNull(x)?

[nealeu]

Looking a bit further, would this be something done in commons, at e.g. BeanWrapper.setProperty() and have PersistentProperty have nullness information which soon enough will be including the https://openjdk.org/jeps/8303099 changes to java.lang.reflect.Type ?

[mp911de]

What do you expect from the mapping layer?

MongoDB documents may not contain a field or contain a null value. Also, when limiting the field projection, a field can be excluded leaving the mapping layer with a null value. Property population skips setting properties for properties that do not have a value in the result Document. Any explicitly set nulls in the result document are considered having a value.

With constructor creation, we cannot skip properties as we must have a value to invoke the constructor.

[nealeu]

Hi Mark,
I'd expect the mapping layer, as part of determining what converters it needs, to have determined that the target requires non-null, so would instead of passing null to a field that is @NonNull, it would throw an exception.

As far as I understand it, this is what the JVM will be doing when the Null-Restricted and Nullable Types JEP is implemented.