Improve to deserialize when not exists default constructor

Hi. Current jackson is fail to deserialize when not implement default constructor and none of the arguments match. (If enable FAIL_ON_UNKNOWN_PROPERTIES).

for example. this code is serialized to { "str": "content" }. but this value is not deserialize. because constructor argument name is not match str and someStr. so jackson can’t create object.

class SomeClass {
    private String str;
    
    public SomeClass(String someStr) {
        this.str = someStr;
    }

    public String getStr() {
        return str;
    }
}

The possible solution to this case is, Inject the value after forcing the object to create. (if enabled visibility field).

If using objenesis, it is possible force create to object. http://objenesis.org/

[cowtowncoder]

Ok good! Actually I also realized that there might be better way than using AnnotationIntrospector: when BeanDeserializerModifier is registered, its updateBuilder() gets built, passing BeanDeserializerBuilder. It has ValueInstantiator initialized, and modifier could check if it can actually create an instance or not (method canInstantiate() checks all possibilities, but there are other methods for more granular). If not, then it could create a new one, override default one. That will be used by otherwise standard BeanDeserializer.

I think this is doable, and you are probably right that others would also find it useful!

[cowtowncoder]

I would not add a dependency, for sure, but my concern is more whether it is good idea to even add code that does what Objenesis does. I know roughly how it is likely done, by-passing construction same way as JDK serialization does – and the idea of by-passing Object initialization is dangerous and possibly security risk. It is also likely to break with JDK 9 or later.

In fact, I think that a better way to do this would be to create a module that allows this functionality: that could easily depend on Objenesis (or use equivalent code). It could probably provide an AnnotationIntrospector that defines findValueInstantiator(), and construct ValueInstantiator that implements createUsingDefault(ctxt) (and canCreateUsingDefault()) as expected.

Module could then return null for cases where other instantiator options exist.

The main challenge might be that of recognizing cases where NOT to create ValueInstantiator however.