Why is Spring Data MongoDB unable to instantiate this nested type structure?

Question

My document structure is like:

{
    _id: \"A\",
    groups:[{
        groupId: \"someId\",
        groupName: \"someName\",
        params: {
            type1:         


        

Answer 1

Failed to instantiate ... using constructor public ... ReflectionEntityInstantiator

says it cannot create the objects using reflection.

Do you have getters and setters for all the fields in all your classes? Your code above does not have them for ParamData, Config and DataConfig.

Also, if you happen to have non-default constructors in any of your classes make sure you have an empty argument constructor, else reflection will fail.

Answer 2

This seems to be an issue with doubly nested inner classes and the synthetically generated constructors created by the compiler. I could reproduce that issue locally and see if we can provide a fix. In the meantime you have two options:

1. Turn the inner class into static ones as this will remove the synthetic constructors and instantiation will work correctly.
2. Nest the type declarations in the same way you nest the properties. I.e. move the ParamData class into the Group class, DataConfig into Config as that will cause the synthetic constructors created in a way they match instantiation order Spring Data currently relies on.

I'd suggest the former approach as it doesn't artificially bind the classes to instances of the outer class.