JSON
Writers and readers
Json.obj() builds a tree inline, which gets repetitive once the same record is serialized in three handlers.
JsonWriter<T> and JsonReader<T> name that mapping so it can be reused.
Both have one method, so both are lambdas:
static final JsonWriter<Deck> DECK = deck -> Json.obj()
.put("id", deck.id())
.put("name", deck.name());
static final JsonWriter<List<Deck>> DECKS = JsonWriter.list(DECK);
record NewDeck(String name) { }
static final JsonReader<NewDeck> NEW_DECK =
json -> new NewDeck(json.asObject().getString("name"));app.get("/api/decks", req -> WebResponse.json(deckService.decks(), DECKS));
app.post("/api/decks", req -> {
Deck deck = deckService.create(req.bodyJson(NEW_DECK).name()); // no key -> 400
return WebResponse.json(deck, DECK).status(HttpStatus.CREATED);
});Still no reflection: the mapping is code you wrote, so a field rename changes the wire format only if you edit it.
Reading values
getString throws IllegalArgumentException on a missing key or a value of the wrong type, and req.bodyJson(reader) turns that into a 400: a handler gets a whole value or none, the same contract as pathParamLong.
For a key that is allowed to be absent, optString/optLong/optDouble/optBoolean answer a default instead, for a missing key and for an explicit JSON null alike.
A parsed array reads with for-each: for (Json.JsonValue v : json.asArray()).