JGen | ️ Generate a text-based journal from a template file | Plugin library

I have a completely flat (POGO/POJO) object which I need to serialize into a nested JSON structure. Preferably using Jackson annotations and/or a custom Serializer

I'm only interested in serializing from object to JSON, deserializing is not needed.

I would like to turn this class:

I know there are several questions of the same topic but I don't think they quite cover my problem in a Spring Boot 2 application.

I have a model which uses LocalTimeDate. Getting this via rest api works fine, date is of the format "date":"2019-12-17T08:50:00"

I have created Serializer so that the json output is customised as there are reference fields that I do not want expanded.

The relevant code in the custom serialzer is

I want to add a wrapper which named is determined at runtime, because it depends of the class name (I could use @JsonRootName but I don't want to because I would have to use it on every sub class, which is not efficient).

I suppose I should use @JsonSerialize to override the default serializer, but I want that just to create the wrapper; I don't want to serialize the object fields myself (also I am in an abstract class, so I don't even know the fields of the sub class!). I don't care about them, I just care about the wrapper! So I would like the default serializer to handle those fields for me, or something like that.

I have the following JSON:

I have a class where employeeDetails is being read from a data store as json string.

I have two classes Athlete and Injury, the last one contains Athlete object, when the serialization happens I get the following JSON representation back: {"id":X,"kindOfInjury":"...","muscle":"...","side":"...","outOfTrainig":Y,"injuryDate":"2018-Jun-02","athlete":{"id":X,"firstName":"...","lastName":"...","age":X,"email":"..."}}

I don't want to get all the information about Athlete - just an id value, like "athleteId":1, instead of getting the entire object representation.

So, I have found that I need to apply my custom Serializer which implements StdSerializer on Injury class. So this is what I got so far:

I'm trying to serialize through jackson some fields, which can be null, with a custom serializer. Reading online it seems that jackson should write a "null" string, and if I want a different output I should follow this example from its official github

The problem is that I get a NullPointerException instead of a "null" string. This occurs both with JsonGenerator.writeNumberField, both with JsonGenerator.writeStringField.
To write an empty string should I manage the exception, or is there something I'm missing?

This is the stacktrace:

Here is the class that I want to serialize.

I need to serialize the following class: