UserListWriter.java

package svanimpe.reminders.json;

import java.io.IOException;
import java.io.OutputStream;
import java.lang.annotation.Annotation;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import javax.json.Json;
import javax.json.JsonArrayBuilder;
import javax.json.JsonObjectBuilder;
import javax.json.JsonWriter;
import javax.ws.rs.Produces;
import javax.ws.rs.WebApplicationException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.MultivaluedMap;
import javax.ws.rs.ext.MessageBodyWriter;
import javax.ws.rs.ext.Provider;
import svanimpe.reminders.domain.User;

@Provider
@Produces(MediaType.APPLICATION_JSON)
public class UserListWriter implements MessageBodyWriter<List<User>>
{
    @Override
    public boolean isWriteable(Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType)
    {
        if (!List.class.isAssignableFrom(type)) {
            return false;
        }

        if (genericType instanceof ParameterizedType) {
            Type[] arguments = ((ParameterizedType) genericType).getActualTypeArguments();
            return arguments.length == 1 && arguments[0].equals(User.class);
        } else {
            return false;
        }
    }

    @Override
    public long getSize(List<User> users, Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType)
    {
        return -1;
    }

    @Override
    public void writeTo(List<User> users, Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType, MultivaluedMap<String, Object> httpHeaders, OutputStream entityStream) throws IOException, WebApplicationException
    {
        JsonArrayBuilder jsonList = Json.createArrayBuilder();
        
        for (User user : users) {        
            JsonObjectBuilder jsonUser = Json.createObjectBuilder();
            jsonUser.add("username", user.getUsername());
            if (user.getFullName() != null) {
                jsonUser.add("fullName", user.getFullName());
            }
            jsonList.add(jsonUser);
        }

        try (JsonWriter writer = Json.createWriter(entityStream)) {
            writer.writeArray(jsonList.build());
        }
    }
}