A PHP library to support implementing representations for HATEOAS REST web services.

• Installation
  • Working With Symfony2
• Usage
  • Introduction
  • Configuring Links
  • Embedding Resources
  • Dealing With Collections
  • The Expression Language
    • Context
    • Adding Your Own Context Variables
  • URL Generators
  • Helpers
    • LinkHelper
  • Serializers & Formats
    • The JsonHalSerializer
    • The XmlSerializer
    • The XmlHalSerializer
    • Adding New Serializers
  • The HateoasBuilder
    • XML Serializer
    • JSON Serializer
    • URL Generator
    • Expression Evaluator/Expression Language
    • Relation Provider
    • (JMS) Serializer Specific
    • Others
  • Configuring a Cache Directory
  • Configuring Metadata Locations
• Reference
  • XML
  • YAML
  • Annotations
    • @Relation
    • @Route
    • @Embed
    • @Exclusion
    • @RelationProvider
• Internals
  • Expression Functions

The recommended way to install Hateoas is through Composer. Require the willdurand/hateoas package into your composer.json file:

{
    "require": {
        "willdurand/hateoas": "2.0.*@dev"
    }
}

Otherwise, install the library and setup the autoloader yourself.

There is a bundle for that! Install the BazingaHateoasBundle, and enjoy!

Important: For those who use the 1.0 version, you can jump to this documentation page as the following documentation has been written for Hateoas 2.0 and above.

Hateoas leverages the Serializer library to provide a nice way to build HATEOAS REST web services. HATEOAS stands for Hypermedia as the Engine of Application State, and basically adds hypermedia links to your representations (ie. your API responses). HATEOAS is about the discoverability of actions on a resource.

For instance, let's say you have a User API which returns a representation of a single user as follow:

{
    "user": {
        "id": 123,
        "first_name": "John",
        "last_name": "Doe"
    }
}

In order to tell your API consumers how to retrieve the data for this specific user, you have to add your very first link to this representation, let's call it self as it is the URI for this particular user:

{
    "user": {
        "id": 123,
        "first_name": "John",
        "last_name": "Doe",
        "_links": {
            "self": { "href": "http://example.com/api/users/123" }
        }
    }
}

Let's dig into Hateoas now.

In Hateoas terminology, links are seen as relations added to resources. It is worth mentioning that relations also refer to embedded resources too, but this topic will be covered in the Embedding Resources section.

A link is a relation which is identified by a name and that owns a href link:

use JMS\Serializer\Annotation as Serializer;
use Hateoas\Configuration\Annotation as Hateoas;

/**
 * @Serializer\XmlRoot("user")
 *
 * @Hateoas\Relation("self", href = "expr('/api/users/' ~ object.getId())")
 */
class User
{
    /** @Serializer\XmlAttribute */
    private $id;
    private $firstName;
    private $lastName;

    public function getId() {}
}

In the example above, we configure a self relation that is a link because of the href parameter. Its value, which may look weird at first glance, will be extensively covered in The Expression Language section.

In this section, annotations are used to configure Hateoas. However, XML and YAML formats are also supported. If you wish, you can use plain PHP too.

Important: you must configure both Serializer and Hateoas the same way. E.g. if you use YAML for configuring Serializer, use YAML for configuring Hateoas.

The easiest way to give Hateoas a try is to use the HateoasBuilder. This builder has numerous methods to configure the Hateoas serializer, but we won't dig into it right now (see The HateoasBuilder). Everything works fine out of the box:

use Hateoas\HateoasBuilder;

$hateoas = HateoasBuilder::create()->build();

$user = new User(42, 'Adrien', 'Brault');
$json = $hateoas->serialize($user, 'json');
$xml  = $hateoas->serialize($user, 'xml');

The $hateoas object is an instance of JMS\Serializer\SerializerInterface, coming from the Serializer library. Hateoas does not come with its own serializer, it simply hooks into the JMS Serializer one.

By default, Hateoas uses the Hypertext Application Language (HAL) for JSON serialization:

{
    "id": 42,
    "first_name": "Adrien",
    "last_name": "Brault",
    "_links": {
        "self": {
            "href": "/api/users/42"
        }
    }
}

And, Atom Links are used by default for XML serialization:

<user id="42">
    <first_name><![CDATA[Adrien]]></first_name>
    <last_name><![CDATA[Brault]]></last_name>
    <link rel="self" href="/api/users/42"/>
</user>

It is worth mentioning that these formats are the default ones, not the only available ones. You can use different formats through different serializers, and even add your owns.

Now that you know how to add links, let's see how to add embedded resources.

Sometimes, it's more efficient to embed related resources rather than link to them, as it prevents clients from having to make extra round trips.

An embedded resource is a named relation that contains data, represented by the embed parameter.

use JMS\Serializer\Annotation as Serializer;
use Hateoas\Configuration\Annotation as Hateoas;

/**
 * ...
 *
 * @Hateoas\Relation(
 *     "manager",
 *     href = "expr('/api/users/' ~ object.getManager().getId())",
 *     embed = "expr(object.getManager())",
 *     exclusion = @Hateoas\Exclusion(excludeIf = "expr(object.getManager() === null)")
 * )
 */
class User
{
    ...

    /** @Serializer\Exclude */
    private $manager;
}

Note: You will need to exclude the manager property from the serialization, otherwise both the serializer and Hateoas will serialize it. You will also have to exclude the manager relation when the manager is null, because otherwise an error will occur when creating the href link (calling getId() on null).

$hateoas = HateoasBuilder::create()->build();

$user = new User(42, 'Adrien', 'Brault', new User(23, 'Will', 'Durand'));
$json = $hateoas->serialize($user, 'json');
$xml  = $hateoas->serialize($user, 'xml');

Serializing embed relations are also HAL compliant:

{
    "id": 42,
    "first_name": "Adrien",
    "last_name": "Brault",
    "_links": {
        "self": {
            "href": "/api/users/42"
        },
        "manager": {
            "href": "/api/users/23"
        },
    },
    "_embedded": {
        "manager": {
            "id": 23,
            "first_name": "Will",
            "last_name": "Durand",
            "_links": {
                "self": {
                    "href": "/api/users/23"
                }
            }
        }
    }
}

In XML, serializing embed relations will create new elements:

<user id="42">
    <first_name><![CDATA[Adrien]]></first_name>
    <last_name><![CDATA[Brault]]></last_name>
    <link rel="self" href="/api/users/42"/>
    <link rel="manager" href="/api/users/23"/>
    <manager rel="manager" id="23">
        <first_name><![CDATA[Will]]></first_name>
        <last_name><![CDATA[Durand]]></last_name>
        <link rel="self" href="/api/users/23"/>
    </manager>
</user>

The tag name of an embedded resource is inferred from the @XmlRoot annotation (xml_root_name in YAML, xml-root-name in XML) coming from the Serializer configuration.

The library provides the several classes in the Hateoas\Representation\* namespace to help you with common tasks. These are simple classes configured with the library's annotations.

The PaginatedCollection and SimpleCollection classes are probably the most interesting ones:

use Hateoas\Representation\PaginatedCollection;
use Hateoas\Representation\SimpleCollection;

$paginatedCollection = new PaginatedCollection(
    new SimpleCollection(
        array($user1, $user2, ...),
        'users', // embed rel
        'users' // xml element name
    ),
    'user_list', // route
    array(), // route parameters
    1, // page
    20, // limit
    4, // total pages
    'page', // page route parameter name, optional, defaults to 'page'
    'limit' // limit route parameter name, optional, defaults to 'limit'
);

$json = $hateoas->serialize($paginatedCollection, 'json');
$xml  = $hateoas->serialize($paginatedCollection, 'xml');

The SimpleCollection class allows you to dynamically configure the collection resources rel, and the xml root element name.

The PaginatedCollection is designed to add self, first, and when possible last, next, previous links.

The Hateoas library also provides a PagerfantaFactory to easily build PaginatedCollection from a Pagerfanta instance:

use Hateoas\Representation\Factory\PagerfantaFactory;

$pagerfantaFactory   = new PagerfantaFactory(); // you can pass the page and limit parameters name
$paginatedCollection = $pagerfantaFactory->create(
    $pager,
    'user_list',
    array() // route parameters
);

$json = $hateoas->serialize($paginatedCollection, 'json');
$xml  = $hateoas->serialize($paginatedCollection, 'xml');

You would get the following JSON content:

{
    "users": [
        { "id": 123 },
        { "id": 456 }
    ],
    "page": 1,
    "limit": 10,
    "pages": 1,
    "_links": {
        "self": {
            "href": "/api/users?page=1&limit=10"
        },
        "first": {
            "href": "/api/users?page=1&limit=10"
        },
        "last": {
            "href": "/api/users?page=1&limit=10"
        }
    }
}

And the following XML content:

<?xml version="1.0" encoding="UTF-8"?>
<collection page="1" limit="10" pages="1">
    <user id="123"></user>
    <user id="456"></user>
    <link rel="self" href="/api/users?page=1&amp;limit=10" />
    <link rel="first" href="/api/users?page=1&amp;limit=10" />
    <link rel="last" href="/api/users?page=1&amp;limit=10" />
</collection>

Hateoas relies on the powerful Symfony ExpressionLanguage component to retrieve values such as links, ids or objects to embed.

Basically, each time you can fill in a value, you can either pass an hardcoded value or an expression. In order to use the Expression Language, you have to use the expr() notation:

/**
 * @Hateoas\Relation("self", href = "expr('/api/users/' ~ object.getId())")
 */

You can learn more about the Expression Syntax by reading the official documentation: The Expression Syntax.

Natively, a special variable named object is available in each expression, and represents the current object:

expr(object.getId())

We call such a variable a context variable.

You can add your own context variables to the Expression Language context by adding them to the ExpressionEvaluator.

Using the HateoasBuilder, call the setExpressionContextVariable() method to add new context variables:

use Hateoas\HateoasBuilder;

$hateoas = HateoasBuilder::create()
    ->setExpressionContextVariable('foo', new Foo())
    ->build();

The foo variable is now available:

expr(foo !== null)

Since you can use the Expression Language to define the relations links (href key), you can do a lot by default. However if you are using a framework, chances are that you will want to use routes to build links.

You will first need to configure an UrlGenerator on the builder. You can either implement the Hateoas\UrlGenerator\UrlGeneratorInterface, or use the Hateoas\UrlGenerator\CallableUrlGenerator:

use Hateoas\UrlGenerator\CallableUrlGenerator;

$hateoas = HateoasBuilder::create()
    ->setUrlGenerator(
        null, // By default all links uses the generator configured with the null name
        new CallableUrlGenerator(function ($route, array $parameters, $absolute) use ($myFramework) {
            return $myFramework->generateTheUrl($route, $parameters, $absolute);
        })
    )
    ->build()
;

You will then be able to use the @Route annotation:

use Hateoas\Configuration\Annotation as Hateoas;

/**
 * @Hateoas\Relation(
 *      "self",
 *      href = @Hateoas\Route(
 *          "user_get",
 *          parameters = {
 *              "id" = "expr(object.getId())"
 *          }
 *      )
 * )
 */
class User

{
    "id": 42,
    "first_name": "Adrien",
    "last_name": "Brault",
    "_links": {
        "self": {
            "href": "/api/users/42"
        }
    }
}

Note that the library comes with a SymfonyUrlGenerator. For example to use it in Silex:

use Hateoas\UrlGenerator\SymfonyUrlGenerator;

$hateoas = HateoasBuilder::create()
    ->setUrlGenerator(null, new SymfonyUrlGenerator($app['url_generator']))
    ->build()
;

Hateoas provides a set of helpers to ease the process of building APIs.

The LinkHelper class provides a getLinkHref($object, $rel, $absolute = false) method that allows you to get the href value of any object, for any given relation name. Basically, it is able to generate an URI (either absolute or relative) from any link relation:

$user = new User(123, 'William', 'Durand');

$linkHelper->getLinkHref($user, 'self');
// /api/users/123

$linkHelper->getLinkHref($user, 'self', true);
// http://example.com/api/users/123

The feature above is also available in your expressions (cf. The Expression Language) through the link(object, rel, absolute) function:

/**
 * @Hateoas\Relation(
 *     "self",
 *     href = @Hateoas\Route("post_get", parameters = {"id" = "expr(object.getId())"})
 * )
 */
class Post {}

/**
 * @Hateoas\Relation(
 *     "self",
 *     href = @Hateoas\Route("user_get", parameters = {"id" = "expr(object.getId())"})
 * )
 * @Hateoas\Relation(
 *     "post",
 *     href = "expr(link(object.getPost(), 'self', true))"
 * )
 * @Hateoas\Relation(
 *     "relative",
 *     href = "expr(link(object.getRelativePost(), 'self'))"
 * )
 */
class User
{
    ...

    public function getPost()
    {
        return new Post(456);
    }

    public function getRelativePost()
    {
        return new Post(789);
    }
}

Pay attention to the href expressions for the post and relative relations, as well as their corresponding values in the following JSON content:

{
    "user": {
        "id": 123,
        "first_name": "William",
        "last_name": "Durand",
        "_links": {
            "self": { "href": "http://example.com/api/users/123" },
            "post": { "href": "http://example.com/api/posts/456" },
            "relative": { "href": "/api/posts/789" }
        }
    }
}

It is worth mentioning that you can force whether you want an absolute or relative URI by using the third argument in both the getLinkHref() method and the link function.

Important: by default, all URIs will be relative, even those which are defined as absolute in their configuration.

$linkHelper->getLinkHref($user, 'post');
// /api/posts/456

$linkHelper->getLinkHref($user, 'post', true);
// http://example.com/api/posts/456

$linkHelper->getLinkHref($user, 'relative');
// /api/posts/789

$linkHelper->getLinkHref($user, 'relative', true);
// http://example.com/api/posts/789

Hateoas provides a set of serializers. Each serializer allows you to generate either XML or JSON content following a specific format, such as HAL, or Atom Links for instance.

The JsonHalSerializer allows you to generate HAL compliant relations in JSON. It is the default JSON serializer in Hateoas.

HAL provides its linking capability with a convention which says that a resource object has a reserved property called _links. This property is an object that contains links. These links are key'ed by their link relation.

HAL also describes another convention which says that a resource may have another reserved property named _embedded. This property is similar to _links in that embedded resources are key'ed by relation name. The main difference is that rather than being links, the values are resource objects.

{
    "message": "Hello, World!",
    "_links": {
        "self": {
            "href": "/notes/0"
        }
    },
    "_embedded": {
        "associated_events": [
            {
                "name": "SymfonyCon",
                "date": "2013-12-12T00:00:00+0100"
            }
        ]
    }
}

#### The XmlSerializer

The XmlHalSerializer allows you to generate Atom Links into your XML documents. It is the default XML serializer.

<?xml version="1.0" encoding="UTF-8"?>
<note>
    <message><![CDATA[Hello, World!]]></message>
    <link rel="self" href="/notes/0" />
    <events rel="associated_events">
        <event>
            <name><![CDATA[SymfonyCon]]></name>
            <date><![CDATA[2013-12-12T00:00:00+0100]]></date>
        </event>
    </events>
</note>

The XmlHalSerializer allows you to generate HAL compliant relations in XML.

HAL in XML is similar to HAL in JSON in the sense that it describes link tags and resource tags.

Note: the self relation will actually become an attribute of the main resource instead of being a link tag. Other links will be generated as link tags.

<?xml version="1.0" encoding="UTF-8"?>
<note href="/notes/0">
    <message><![CDATA[Hello, World!]]></message>

    <resource rel="associated_events">
        <name><![CDATA[SymfonyCon]]></name>
        <date><![CDATA[2013-12-12T00:00:00+0100]]></date>
    </resource>
</note>

For JSON related formats, you must implement the JsonSerializerInterface interface, and for XML related formats, you must implement the XmlSerializerInterface. Both interfaces describe two methods to serialize links and embedded relations.

The HateoasBuilder class is used to easily configure Hateoas thanks to a powerful and fluent API.

use Hateoas\HateoasBuilder;

$hateoas = HateoasBuilder::create()
    ->setCacheDir('/path/to/cache/dir')
    ->setDebug($trueOrFalse)
    ->setDefaultXmlSerializer()
    ...
    ->build();

All the methods below return the current builder, so that you can chain them.

• setXmlSerializer(XmlSerializerInterface $xmlSerializer): sets the XML serializer to use. Default is: XmlSerializer;
• setDefaultXmlSerializer(): sets the default XML serializer (XmlSerializer).

• setJsonSerializer(JsonSerializerInterface $jsonSerializer): sets the JSON serializer to use. Default is: JsonHalSerializer;
• setDefaultJsonSerializer(): sets the default JSON serializer (JsonHalSerializer).

• setUrlGenerator($name = null, UrlGeneratorInterface $urlGenerator): adds a new named URL generator. If $name is null, the URL generator will be the default one.

• setExpressionContextVariable($name, $value): adds a new expression context variable;
• setExpressionLanguage(ExpressionLanguage $expressionLanguage).

• addRelationProviderResolver(RelationProviderResolverInterface $resolver): adds a new relation provider resolver.

• includeInterfaceMetadata($include): whether to include the metadata from the interfaces;
• setMetadataDirs(array $namespacePrefixToDirMap): sets a map of namespace prefixes to directories. This method overrides any previously defined directories;
• addMetadataDir($dir, $namespacePrefix = ''): adds a directory where the serializer will look for class metadata;
• addMetadataDirs(array $namespacePrefixToDirMap): adds a map of namespace prefixes to directories;
• replaceMetadataDir($dir, $namespacePrefix = ''): similar to addMetadataDir(), but overrides an existing entry.

Please read the official Serializer documentation for more details.

• setDebug($debug): enables or disables the debug mode;
• setCacheDir($dir): sets the cache directory.

Both the serializer and the Hateoas library collects several metadata about your objects from various sources such as YML, XML, or annotations. In order to make this process as efficient as possible, it is encourage to let them cache that information. For that, you can configure a cache directory:

$builder = \Hateoas\HateoasBuilder::create();

$hateoas = $builder
    ->setCacheDir($someWritableDir)
    ->build();

Hateoas supports several metadata sources. By default, it uses Doctrine annotations, but you may also store metadata in XML, or YAML files. For the latter, it is necessary to configure a metadata directory where those files are located:

$hateoas = \Hateoas\HateoasBuilder::create()
    ->addMetadataDir($someDir)
    ->build();

Hateoas would expect the metadata files to be named like the fully qualified class names where all \ are replaced with .. So, if you class would be named Vendor\Package\Foo the metadata file would need to be located at $someDir/Vendor.Package.Foo.(xml|yml).

<?xml version="1.0" encoding="UTF-8"?>
<serializer>
<class name="Acme\Demo\Representation\User" h:providers="Class::getRelations, getRelations" xmlns:h="https://github.com/willdurand/Hateoas">
        <h:relation rel="self">
            <h:href uri="http://acme.com/foo/1" />
        </h:relation>
        <h:relation rel="expr(object.getFriendsDynamicRel())">
            <h:href router="user_friends" generator="my_custom_generator">
                <h:parameter name="id" value="expr(object.getId())" />
                <h:parameter name="page" value="1" />
            </h:ref>
            <h:embed xml-element-name="users">
                <h:content>expr(object.getFriends())</h:content>
                <h:exclusion ... />
            </h:embed>
            <h:exclusion groups="Default, user_full" since-version="1.0" until-version="2.2" exclude-if="expr(object.getFriends() === null)" />
        </h:relation>
    </class>
</serializer>

See the hateoas.xsd file for more details.

Acme\Demo\Representation\User:
    relations:
        -   rel: self
            href: http://acme.com/foo/1
        -   rel: expr(object.getFriendsDynamicRel())
            href:
                route: user_friends
                parameters:
                    id: expr(object.getId())
                    page: 1
                generator: my_custom_generator
            embed:
                content: expr(object.getFriends())
                xmlElementName: users
                exclusion: ...
            exclusion:
                groups: [Default, user_full]
                since_version: 1.0
                until_version: 2.2
                exclude_if: expr(object.getFriends() === null)

    relation_providers: [ 'Class::getRelations', 'getRelations' ]

This annotation can be defined on a class.

use Hateoas\Configuration\Annotation as Hateoas;

/**
 * @Hateoas\Relation(
 *     name = "self",
 *     href = "http://hello",
 *     embed = "expr(object.getHello())",
 *     attributes = { "foo" = "bar" },
 *     exclusion = ...,
 * )
 */

use Hateoas\Configuration\Annotation as Hateoas;

/**
 * @Hateoas\Relation(
 *     name = "self",
 *     href = @Hateoas\Route(
 *         "user_get",
 *         parameters = { "id" = "expr(object.getId())" },
 *         absolute = true,
 *         generator = "my_custom_generator"
 *     )
 * )
 */

This annotation can be defined in the href property of the @Relation annotation.

use Hateoas\Configuration\Annotation as Hateoas;

/**
 * @Hateoas\Relation(
 *     name = "friends",
 *     embed = @Hateoas\Embed(
 *         "expr(object.getFriends())",
 *         exclusion = ...,
 *         xmlElementName = "users"
 *     )
 * )
 */

This annotation can be defined in the embed property of the @Relation annotation.

This annotation can be defined in the exclusion property of both the @Relation and @Embed annotations.

All values exception excludeIf as if it was defined on regular properties with the serializer.

excludeIf expects a boolean; and is helpful when an other expression would fail under some circumstances:

/**
 * @Hateoas\Relation(
 *     "manager",
 *     href = @Hateoas\Route(
 *         "user_get",
 *         parameters = { "id" = "expr(object.getManager().getId())" }
 *     ),
 *     exclusion = @Hateoas\Exclusion(excludeIf = "expr(null === object.getManager())")
 * )
 */
class User
{
    public function getId() {}

    /**
     * @return User|null
     */
    public function getManager() {}
}

This annotation can be defined on a class.

The can be:

• A method: addRelations
• A static method: Class::addRelations
• A Symfony2 service method: acme_foo.service:addRelations

use Hateoas\Configuration\Metadata\ClassMetadataInterface;
use Hateoas\Configuration as Hateoas;

class MyRelationProvider
{
    public function addRelations($object, ClassMetadataInterface $classMetadata)
    {
        // You need to return the relations
        // Adding the relations to the $classMetadata won't work
        return array(
            new Hateoas\Relation(
                'self',
                new Hateoas\Route(
                    'foo_get',
                    array('id' => 'expr(object.getId())')
                )
            )
        );
    }
}

This section refers to the Hateoas internals, providing documentation about hidden parts of this library, not always relevant for end users, but interesting for developers or people interested in learning how things work under the hood.

Expression Functions are custom functions used to extend the Expression Language as explained in chapter Extending the ExpressionLanguage, part of the Symfony documentation.

By now, Hateoas does not let users add their own custom functions. Only core functions are registered, such as the LinkExpressionFunction described in LinkHelper - The link Function.

The ExpressionFunctionInterface is designed to represent an expression function. Adding a new expression function is a matter of implementing this interface and registering it into the ExpressionEvaluator through the registerFunction() method.

See CONTRIBUTING file.

Install the Composer dev dependencies:

php composer.phar install --dev

Then, run the test suite using PHPUnit:

bin/phpunit

Hateoas is released under the MIT License. See the bundled LICENSE file for details.