Modified Fields Tracking for Setters

[funkrusher]

Describe the feature

An annotation that automatically makes the tracking of "modified fields" possible.
A "modified field" is a field of which the setter-function has been called at least once in the lifetime of the object-instance.

MyPojo {

    private Map<String, Object> modifiedFields = new HashMap<>();

    private String test;
    ....

    public MyPojo setTest(String test) {
        this.test = test;
        this.modifiedFields.put("test", test);
        return this;
    }
    ....
    public Map<String, Object> getModifiedFields() {
        return this.modifiedFields;
    }
}

Given that Pojos are often created during Deserialization (given data from a (rest)-endpoint, or from an import-file) / when external systems call the java-backend (for example with a REST-PATCH method call).

To support REST-PATCH it is necessary to be able to detected which of the incoming data-fields have really been given by the caller to decide (via openapi definition) if those fields are required=true/false. If they are not not required they may also be missing from the request, but if they are required, they must either provide a value or NULL as a value (which is also a valid value)

The modified-fields would help to be able to check in the code if a field has been set by the caller. For example

    if (pojo.getModifiedFields().contains("test") {
        update.set("test", pojo.getTest());
    }

Describe the target audience

• Java Programmers wanting to implement REST PATCH Api (PATCH needs the ability of modified fields tracking)
• Java Programmers wanting to create dynamic imports that only process data from fields in import-files (JSON, XML, ...) that have really been given

Additional context
An annotation on the class-level would be helpful. For example:

@SettersWithTracking
MyPojo {
    ...
}

If an interest is there to implement this feature, i would also think about helping implementing it in lombok / discussing it further (this is only a very raw concept)