JSON

Write the document with placeholders where the values go. The compiler parses it while it builds, and you get a class whose constructor takes those values and whose toJson() renders the document — each value encoded properly, so nothing can break out of its position.

Writing one

package examples.simplejson;

import org.jmod.dsl.json.JsonObject;

public external Person extends JsonObject<JsonConf> {
{
  "name": #[name]<String>,
  "age": #[age]<int>,
  "tags": #[tags]<String[]>
}
}

String body = new Person("Ada", 36, tags).toJson();
JsonNode tree = new Person("Ada", 36, tags).toJsonNode();

Checking against a schema

Give the configuration a JSON Schema — JSONMOD_SCHEMA_AWARE and JSONMOD_SCHEMA_URI — and the document is validated twice: at build time against the types you declared, and again at toJson() against the actual values. Drafts 4 through 2020-12 are supported.

P.jmod: JSON Schema validation failed: $.age: string found, integer expected

P.jmod: JSON Schema validation failed: $: required property 'age' not found

The build-time pass proves the shape is right for the declared types; the runtime pass catches what only the values can violate — a negative age under minimum: 0, a string that fails a pattern — and throws rather than emitting a document that does not conform.

Worth knowing

Values can be booleans, numbers, strings or arrays of those; objects are not encoded reflectively. Placeholders go where a JSON value goes — not inside a string, and not as a key — and the structure is fixed when you compile, so there is no way to add or omit a field per instance.