With the same mindset of giving developers as much productivity as possible, Java EE includes standards to transparently map POJOs to JSON or XML. The example you saw with JAX-RS implicitly used JSON-B to map our User types to JSON objects and arrays, respectively.
This again uses the principle of convention over configuration. If nothing is else specified JSON-B assumes to map the POJO properties directly as JSON object key-value pairs. The user's id was present in the JSON output as well.
The same holds true for the Java Architecture for XML Binding (JAXB) and its XML binding, which was included in Java EE much earlier than JSON-B. Both standards support a declarative configuration approach using annotations that ...
