The constructor middleware (introduced in JsonMapper version 2.14.0) uses reflection to register a custom factory to the factory registry which can utilise the class constructor. This enables the use of custom constructors without having to manually write factories. This feature can be combined with the readonly properties introduced with PHP 8.1.

class User
    public function __construct(
        public readonly $name,
    ) {}

$cache = new NullCache();
$factoryRegistry = new FactoryRegistry();
$mapper = JsonMapperBuilder::new()
    ->withPropertyMapper(new PropertyMapper($factoryRegistry))

$mapper->mapToClassFromString('{ "name": "John Doe" }', User::class);

echo $object->name; // "John Doe"