I have an implementation using Tomcat v10.0.x and org.glassfish.jersey.* in version 3.0.5 to provide a web service. Initially, I had problems returning response Objects as MediaType.APPLICATION_JSON whenever they hat Attributes/Getters that are Optional.

Once I added jackson-datatype-jdk8 2.13.3 based on this question, everything started to work fine.

I am confused, though. I did not register Jdk8Module because I am not actively creating a Client instance. As far as I believe to understand the inner workings of Jersey as JAX-RS implementation, the @POST annotations I am using are responsible for handling Client objects. It might also be Tomcat. On the web there is a big gap between simple howtos with little explanation, and in-depth documentation that is hard to understand.

Where is Jdk8Module registered, and how, and why, or not at all? Simply speaking, why does my code work?

CodePudding user response：

Short answer: META-INF/services.

Longer answer: Jackson defines Module class which is used as a service (defined in META-INF/services) loaded by a standard ServiceLoader mechanism. Jackson defines a static method loading all Modules and the Jackson jackson-datatype-java8 module defines the proper service.

Jersey just uses the Jackson service-finding routine to register the module.