How do you create hypermedia links with Spring HATEOAS?

Table of Contents

Introduction

In a HATEOAS (Hypermedia as the Engine of Application State)-driven REST API, hypermedia links are used to provide dynamic navigation between related resources. These links allow the client to explore the API's capabilities without having to hard-code the URLs or know the server’s structure in advance. By using Spring HATEOAS, developers can easily add hypermedia links to their API responses, making the application more flexible and discoverable.

This article will show you how to create and manage hypermedia links using Spring HATEOAS in a Spring Boot application.

What is HATEOAS?

HATEOAS is one of the key principles of RESTful architecture. It allows the client to dynamically navigate the application state by providing links to related resources. Instead of hardcoding endpoint URLs, the server includes links in its responses, guiding the client to the next possible actions or resources.

For instance, when retrieving a User resource, the response might include links to the User's orders, User's profile update, or other related resources. These links make the application more flexible, as clients do not need to be aware of all the available operations beforehand.

Spring HATEOAS provides an easy way to create hypermedia links in your REST API responses. The Link class and EntityModel (or CollectionModel) are the core components used for adding hypermedia links.

1. Add Spring HATEOAS Dependency

First, you need to add the Spring HATEOAS dependency to your pom.xml (Maven) or build.gradle (Gradle) file.

For Maven, include the following dependency:

For Gradle:

The Link class in Spring HATEOAS represents a hyperlink to another resource. You can create links dynamically using **WebMvcLinkBuilder** to generate URLs for your controllers.

Here’s how you would create a link to a Product resource:

In this example:

  • The linkTo method generates a URL based on the controller method.
  • The withSelfRel method adds a rel attribute to the link, which indicates that the link is a reference to the current resource (self).

3. Use **EntityModel** or **CollectionModel** to Return Resources

EntityModel is used to wrap a single resource and add links to it. For collections of resources, you can use CollectionModel to wrap multiple resources.

In this example, we create a collection of products, and each product is wrapped with an EntityModel containing a self link. The collection itself is wrapped in CollectionModel, and we also add a link to the collection itself.

You can create custom links based on your application’s requirements. For example, when fetching a User resource, you might want to add links to update the user profile or delete the user.

In this case, we add custom links (update and delete) to the User resource, which guide the client on what operations can be performed on the resource.

5. Returning Hypermedia-Driven Responses

When you return a resource wrapped in EntityModel (for a single resource) or CollectionModel (for a collection of resources), Spring HATEOAS automatically serializes the hypermedia links and the resource data to JSON.

For example, a response for a Product might look like:

And for a list of products:

Here’s a real-world example of using Spring HATEOAS for an Order resource in an e-commerce system.

Order Resource Class

Controller Method for Order

Conclusion

Creating hypermedia links with Spring HATEOAS is a simple yet powerful way to add dynamic navigation to your REST APIs. By wrapping resources with EntityModel and adding relevant links, your API becomes more discoverable and flexible for clients. Clients can now navigate the application state and interact with resources using the links provided, adhering to the HATEOAS principle and making your application more adaptable to future changes.

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