REST API in Java - How to support PATCH with Hibernate Validator

When writing a REST-API in Java, the PATCH method requests that a "set of changes" described in the request entity be applied to the resource identified by the Request-URI. (see: https://datatracker.ietf.org/doc/html/rfc5789)

Given following resource:

{  productId: 7,  name: "container",  contactPerson: "peter",  price: 12}

With PATCH i should be able to set the "contactPerson" to NULL while not touching the other fields, like this:

{  productId: 7,  contactPerson: null}

Within the Java Ecosystem how can i implement this in a good way, considering that the most relevant libraries here need to have full support for it:

• Jackson (JSON Deserialization)
• Hibernate Validator (Bean Validation)
• Microprofile OpenAPI (API-Definition)

Java Optional

Among others i tried out Java-Optional, and while it initially seemed to work it is the wrong tool for this, as an Optional should not contain a NULL-Value, but it should also be possible to set a value to NULL explicitly within a PATCH (instead of omitting it).

• but i did get very far with using Optional. The only problem was that Hibernate Validator did not supported it in the way i needed it.

For Example, the following looked good initially (here for the name-field which is required = false and nullable = false):

@Schema(required = false, nullable = false)Optional<@NotNull @Size(max = 255) String> name = Optional.empty();

Hibernate validator considered the @NotNull Annotation to be violated when the name field was missing in the request, which i found strange because the annotation is within the Optional-Generic-Declaration (not directly on the Optional)

JsonNullable

Jackson provides a JsonNullable annotation. While this is the right tool for the task and is seemingly supported for openapi, it does not seem to be supported for hibernate validator, at least i could not find something about it.

Validation

Mostly the problem seems to be during validation of Bean Validation Annotations, which are important to have for an API.

• When using annotations like @Size(255), @Email etc. there is the requirement that...
  • ...those validations must be validated by the Hibernate Validator when the field is given/set in the PATCH
  • ...those validations must not be validated when the field is omitted (missing) in the PATCH.

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Is the REST-PATCH covered by those major libraries of the Java Ecosystem, and if yes, what is the recommended way to support it?