Post and put links

[ununbium]

Hi,

There doen't appear to be a documented method of manipulating links between resources using the full link. For example updating a link to another resources by performing a "put". Examples here;

http://stackoverflow.com/questions/26288986/how-to-put-and-post-links-with-spring-hateoas

Is this actually possible? Either way I think it would help people if it featured in the documentation somewhere (stating either how it's done or that spring-hateoas does not offer features for this)

Thanks,
Andy

[odrotbohm]

To be honest I don't quite get what you're asking for. It all depends on how you implement your Spring MVC controllers and build up links you put into responses in the first place. You basically need to invert this logic to map the resource back to whatever they identify in the backend. Usually that's done through URI templates in the @RequestMapping annotation.

[ununbium]

Hi Oliver,

Thanks for the quick response, I'll try to clairfy with an example; say I have a relationship between two resources e.g. an employee has a manager. In Spring Data Rest I could do a put (content type uri-list) to http://127.0.0.1:8080/employee/12/manager with the content http://127.0.0.1:8080/managers/2

In Spring Hateoas is there any infrastructure to help implementing similar functonality? I would assume that this would look something like resolving the string "http://127.0.0.1:8080/managers/2" to something more internal to the applicaiton, e.g. an internal primary key, entity or controller reference, etc.

Thanks,
Andy

[odrotbohm]

Yes, see the reference documentation.

No, to a REST client, the URI is the ID, hence the name. Only the server is allowed to be aware of the structure of the URI.

[ununbium]

Hi Oliver,

Thanks for the link but you appear to be linking the Spring Data Rest documentation - this is not the technology I'm trying to use. I understand that in Rest the URI is the resource, indeed that is the crux of my question. Sorry for the repetition of the question (I don't really have a way to move forward with this yet), but just to rephrase to be a little clearer;

What (if any) support/features are there in Spring Hateoas (where Spring Hateoas is on the server) for links (full URIs) being put or posted to controllers exposed through @RequestMapping (specifically parsing the URI back to meaningful server side representation in the put/post request mapping)?

Thanks,
Andy

p.s. just to note some potential confusion - when you say "no, to a REST client, the URI..." - I'm trying to build the server side, the client is angularJS and not part of my issue.

[odrotbohm]

So do I understand you correctly that you'd like to declare a controller method as follows:

@RequestMapping(value = "…", method = RequestMethod.PUT)
void someMethod(@RequestBody List<URI> uris) { … }

That could be achieved implemented a dedicated HttpMessageConverter.

[ununbium]

Hi Oliver,

Yep that is what I'm trying to do, although I'm on the next step to that; I have a URI list in my request handler (that all works fine) - how do I then make the URI meaningful to the server in terms of the resource classes/ids? Currently I parse out the id from the URI with a regex. That just feels wrong - it feels like there should be some way of passing the URI to Spring Hateoas and it returns an id and mapping controller (and/or resource object class).

Thanks,
Andy

[kevinconaway]

@olivergierke I believe I have the same question.

According to #66, the ID for a ResourceSupport DTO is the full URI, say http://localhost:8080/api/user/1234

Lets say I have a service where I want to add the user to a group, I might send a POST request to

/api/groups/:groupName/members

With an entity body:

{
    "user": "http://localhost:8080/api/user/1234"
}

Is there support for de-referencing that URI on the server side? What is the recommended approach?

[chrylis]

This is the main difficulty I've always had with the Spring HATEOAS approach: Sure, fine that you can POST to a link. But how can I retrieve the target of a link in a controller method to operate on it? This aspect seems to have been completely dropped on the floor, even though it should be manageable to invert the general approach of ControllerLinkBuilder and return information retrieved from the handler mapping that matches the provided link:

ControllerLinkParser.parseLink(resource.getLink('user'))
// returns an entity Class and a map of path variables?

[kevinconaway]

@chrylis FWIW below is the code I'm using to solve this problem. I'd to see a better way but it suits my purposes

In the above example, I referenced a user with the URI: http://localhost:8080/api/user/1234.

This maps to:

class UserController {
    UserResource getUser(@PathVariable("id") String id)
}

I have the following helper class:

import org.springframework.hateoas.core.AnnotationMappingDiscoverer;
import org.springframework.hateoas.core.DummyInvocationUtils;
import org.springframework.hateoas.core.MappingDiscoverer;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.util.UriTemplate;

import java.util.Map;
import java.util.function.Consumer;

public class UriTemplateResolver {

    private static final MappingDiscoverer DISCOVERER = new AnnotationMappingDiscoverer(RequestMapping.class);

    private final Object invocationValue;

    private UriTemplateResolver(Object invocationValue) {
        this.invocationValue = invocationValue;
    }

    public static <T> UriTemplateResolver template(Class<T> controller, Consumer<T> consumer) {
        T invocationValue = DummyInvocationUtils.methodOn(controller);
        consumer.accept(invocationValue);

        return new UriTemplateResolver(invocationValue);
    }

    public <T> Map<String, String> resolveVariables(String uri) {
        DummyInvocationUtils.LastInvocationAware invocations = (DummyInvocationUtils.LastInvocationAware) invocationValue;
        DummyInvocationUtils.MethodInvocation invocation = invocations.getLastInvocation();

        String mapping = DISCOVERER.getMapping(invocation.getTargetType(), invocation.getMethod());
        UriTemplate template = new UriTemplate(mapping);

        return template.match(uri);
    }

}

I can then use it to get the user ID from the above URI:

String getUserId(String uri) {
    Map<String, String> values = UriTemplateResolver
            .template(UserController.class, controller -> controller.getUser(null))
            .resolveVariables(uri);
    if (!values.containsKey("id")) {
        throw new IllegalArgumentException("Could not find id in URI: " + uri);
    }

    return values.get("id");
}

[chrylis]

@kevinconaway Interesting approach. Unfortunately, it appears to depend on Java 8 syntax (not just APIs), and thus would have problems with Groovy; am I wrong about that?

[kevinconaway]

I'm unfamiliar with how Java8 interacts with Groovy. I suspect you could rewrite the Consumer and the lambda that is passed to it to work with groovy closures, right?

[chrylis]

Theoretically, but I am not sure what the semantics for that controller -> controller.getUser(null) are; in particular, is it returning a method handle for the controller class (essentially, UserController::getUser)? There's not a Groovy syntax for obtaining method handles yet.