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Spring HATEOAS - Library to support implementing representations for hyper-text driven REST web services.
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Spring Hateoas

This project provides some APIs to ease creating REST representations that follow the HATEOAS principle when working with Spring and especially Spring MVC. The core problem it tries to address is link creation and representation assembly.

JAXB / JSON integration

As representations for REST web services are usually rendered in either XML or JSON the natural choice of technology to achieve this is either JAXB, JSON or both in combination. To follow HATEOAS principles you need to incorporate links into those representation. Spring HATEOAS provides a set of useful types to ease working with those.

Links

The Link value object follows the Atom link definition and consists of a rel and an href attribute. It contains a few constants for well known reels such as self, next etc. The XML representation will render in the Atom namespace.

Link link = new Link("http://localhost:8080/something");
assertThat(link.getHref(), is("http://localhost:8080/something"));
assertThat(link.getRel(), is(Link.SELF));

Link link = new Link("http://localhost:8080/something", "my-rel");
assertThat(link.getHref(), is("http://localhost:8080/something"));
assertThat(link.getRel(), is("my-rel"));

Resources

As pretty much every representation of a resource will contain some links (at least the self one) we provide a base class to actually inherit from when designing representation classes.

class PersonResource extends ResourceSupport {

  String firstname;
  String lastname;
}

Inheriting from ResourceSupport will allow adding links easily:

PersonResource resource = new PersonResource();
resource.firstname = "Dave";
resource.lastname = "Matthews";
resource.add(new Link("http://myhost/people"));

This would render as follows in JSON:

{ firstname : "Dave",
  lastname : "Matthews",
  _links : [ { rel : "self", href : "http://myhost/people" } ] }

… or slightly more verbose in XML …

<person xmlns:atom="http://www.w3.org/2005/Atom">
  <firstname>Dave</firstname>
  <lastname>Matthews</lastname>
  <links>
    <atom:link rel="self" href="http://myhost/people" />
  </links>
</person>

You can also easily access links contained in that resource:

Link selfLink = new Link("http://myhost/people");
assertThat(resource.getId(), is(selfLink);
assertThat(resource.getLink(Link.SELF), is(selfLink));

Link builder

Now we've got the domain vocabulary in place, but the main challenge remains: how to create the actual URIs to be wrapped into Links in a less fragile way. Right now we'd have to duplicate URI strings all over the place which is brittle and unmaintainable.

Assume you have your Spring MVC controllers implemented as follows:

@Controller
@RequestMapping("/people")
class PersonController {

  @RequestMapping(method = RequestMethod.GET)
  public HttpEntity<PersonResource> showAll() { … }

  @RequestMapping(value = "/{person}", method = RequestMethod.GET)
  public HttpEntity<PersonResource> show(@PathVariable Long person) { … }
}

We see two conventions here. There's a collection resource exposed through the controller class' @RequestMapping annotation with individual elements of that collections exposed as direct sub resource. The collection resource might be exposed at a simple URI (as just shown) or more complex ones like /people/{id}/addresses. Let's say you would like to actually link to the collection resource of all people. Following the approach from up above would cause two problems:

  1. To create an absolute URI you'd need to lookup the protocol, hostname, port, servlet base etc. This is cumbersome and requires ugly manual string concatenation code.
  2. You probably don't want to concatenate the /people on top of your base URI because you'd have to maintain the information in multiple places then. Change the mapping, change all the clients pointing to it.

Spring Hateoas now provides a ControllerLinkBuilder that allows to create links by pointing to controller classes:

import static org.sfw.hateoas.mvc.ControllerLinkBuilder.*;

Link link = linkTo(PersonController.class).withRel("people");
assertThat(link.getRel(), is("people"));
assertThat(link.getHref(), endsWith("/people"));

The ControllerLinkBuilder uses Spring's ServletUriComponentsBuilder under the hood to obtain the basic URI information from the current request. Assuming your application runs at http://localhost:8080/your-app This will be exactly the URI you're constructing additional parts on top. The builder now inspects the given controller class for its root mapping and thus end up with http://localhost:8080/your-app/people. You can also easily build more nested links as well:

Person person = new Person(1L, "Dave", "Matthews");
//                 /person                 /     1
Link link = linkTo(PersonController.class).slash(person.getId()).withSelfRel();
assertThat(link.getRel(), is(Link.SELF));
assertThat(link.getHref(), endsWith("/people/1"));

If your domain class implements the Identifiable interface the slash(…) method will rather invoke getId() on the given object instead of toString(). Thus the just shown link creation can be abbreviated to:

class Person implements Identifiable<Long> { 
  public Long getId() { … } 
}

Link link = linkTo(PersonController.class).slash(person).withSelfRel();

The builder also allows creating URI instances to build up e.g. response header values:

HttpHeaders headers = new HttpHeaders();
headers.setLocation(linkTo(PersonController.class).slash(person).toUri());
return new ResponseEntity<PersonResource>(headers, HttpStatus.CREATED);

Resource assembler

As the mapping from an entity to a resource type will have to be used in multiple places it makes sense to create a dedicated class responsible for doing so. The conversion will of course contain very custom steps but also a few boilerplate ones:

  1. Instantiation of the resource class
  2. Adding a link with rel self pointing to the resource that gets rendered.

Spring Hateoas now provides a ResourceAssemblerSupport base class that helps reducing the amount of code needed to be written:

class PersonResourceAssembler extends ResourceAssemblerSupport<Person, PersonResource> {

  public PersonResourceAssembler() {
    super(PersonController.class, PersonResource.class);
  }

  @Override
  public PersonResource toResource(Person person) {

    PersonResource resource = createResource(person);
    // … do further mapping
    return resource;
  }
}

Setting the class up like this gives you the following benefits: there are a hand full of createResource(…) methods that will allow you to create an instance of the resource and have it a Link with a rel of self added to it. The href of that link is determined by the configured controllers request mapping plus the id of the Identifiable (e.g. /people/1 in our case). The resource type gets instantiated by reflection and expects a no-arg constructor. Simply override instantiateResource(…) in case you'd like to use a dedicated constructor or avoid the reflection performance overhead.

The assembler can then be used to either assemble a single resource or an Iterable of them:

Person person = new Person(…);
Iterable<Person> people = Collections.singletonList(person);

PersonResourceAssembler assembler = new PersonResourceAssembler();
PersonResource resource = assembler.toResource(person);
List<PersonResource> resources = assembler.toResource(people);
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