ParamConverter for Optional<T> doesn't support custom types

[joschi]

The ParamConverter implementation for the Java Optional<T> container which was introduced in Jersey 2.34 (and Jersey 3.0.1) doesn't support handling custom types.

With Optional<T> being a container similar to Collection<T> (and its derivates), I would expect it to support custom types for which a ParamConverterProvider and ParamConverter exist.

Unfortunately this is not the case. org.glassfish.jersey.internal.inject.ParamConverters.OptionalProvider only supports the other types defined in org.glassfish.jersey.internal.inject.ParamConverters.AggregatedProvider but not any other type of parameters for which a ParamConverterProvider and ParamConverter have been registered.

Here's an example (using the Jersey testing framework) with a custom ParamConverterProvider and ParamConverter for java.time.Instant:

import org.glassfish.jersey.internal.inject.ExtractorException;
import org.glassfish.jersey.server.ResourceConfig;
import org.glassfish.jersey.test.JerseyTest;
import org.junit.Test;

import javax.ws.rs.*;
import javax.ws.rs.core.*;
import javax.ws.rs.ext.*;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.time.Instant;
import java.util.Optional;

import static org.junit.Assert.assertEquals;

public class CustomParamConverterOptionalTest extends JerseyTest {
    @Override
    protected Application configure() {
        return new ResourceConfig(TestResource.class)
                .register(InstantParamConverterProvider.class);
    }

    @Test
    public void testInstant() {
        String paramMissing = target("/instant")
                .request().get(String.class);
        assertEquals("null", paramMissing);

        String optionalParamMissing = target("/optional")
                .request().get(String.class);
        assertEquals("default", optionalParamMissing);

        String paramProvided = target("/instant").queryParam("i", "2021-05-18T22:49:00Z")
                .request().get(String.class);
        assertEquals("2021-05-18T22:49:00Z", paramProvided);

        String optionalParamProvided = target("/optional").queryParam("i", "2021-05-18T22:49:00Z")
                .request().get(String.class); // 💥 Fails with: javax.ws.rs.NotFoundException: HTTP 404 Not Found
        assertEquals("2021-05-18T22:49:00Z", optionalParamProvided);
    }

    @Produces(MediaType.TEXT_PLAIN)
    @Path("/")
    public static class TestResource {
        @GET
        @Path("/optional")
        public String optionalInt(@QueryParam("i") Optional<Instant> i) {
            if (i == null) {
                return "null";
            }
            return i.map(Instant::toString).orElse("default");
        }

        @GET
        @Path("/instant")
        public String optionalInt(@QueryParam("i") Instant i) {
            if (i == null) {
                return "null";
            }
            return i.toString();
        }
    }

    @Provider
    public static class InstantParamConverterProvider implements ParamConverterProvider {
        @Override
        public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType, Annotation[] annotations) {
            if (rawType.equals(Instant.class)) {
                return new ParamConverter<T>() {
                    @Override
                    public T fromString(String value) {
                        if (value == null) {
                            throw new IllegalArgumentException();
                        }
                        try {
                            return rawType.cast(Instant.parse(value));
                        } catch (Exception e) {
                            throw new ExtractorException(e);
                        }
                    }

                    @Override
                    public String toString(T value) {
                        if (value == null) {
                            throw new IllegalArgumentException();
                        }
                        return value.toString();
                    }
                };
            } else {
                return null;
            }
        }
    }
}

Refs #4651
Refs #4690
Refs #4790