How do you use the EntityModel class in Spring HATEOAS?

Table of Contents

• Introduction
  • 1. What is **EntityModel**?
  • 2. Creating an **EntityModel** with Links
    • Example: Using EntityModel to Return a Product Resource
  • 3. Enriching DTOs with **EntityModel**
    • Example: EntityModel with DTO
  • 4. Best Practices with **EntityModel**
  • 5. Returning Collections with **EntityModel**
    • Example: CollectionModel with EntityModel
• 6. Conclusion

Introduction

In Spring HATEOAS, the EntityModel class is used to represent a single resource in a REST API and enrich it with hypermedia links. These links provide the client with additional actions or related resources, making the API more discoverable and self-descriptive. The EntityModel is part of the HATEOAS (Hypermedia as the Engine of Application State) principle, which suggests that REST APIs should guide clients to the next possible actions via hypermedia links.

The EntityModel class is typically used to wrap domain objects (like entities or DTOs) and return them with embedded links. This enables clients to navigate the API dynamically based on the resources they retrieve.

1. What is **EntityModel**?

The EntityModel class is a specialized subclass of **RepresentationModel** in Spring HATEOAS. It is designed to represent individual resources with additional metadata in the form of hypermedia links.

• Purpose: It holds a resource (such as a domain object or DTO) and provides methods to add hypermedia links to the resource.
• Use Case: When you want to return a single resource (for example, a user or product) along with links to related actions (e.g., self-link, update link).

2. Creating an **EntityModel** with Links

To use EntityModel, you need to wrap the resource object (either an entity or DTO) and add hypermedia links that point to related resources or actions.

Example: Using EntityModel to Return a Product Resource

Let's say you have a simple Product entity, and you want to return it with HATEOAS links (e.g., self-link and an update link).

In this example:

• The getProductById method returns a Product wrapped in an EntityModel.
• Hypermedia Links: The self link allows the client to reference the resource itself (the current product), and the update link points to an endpoint for updating the product.

3. Enriching DTOs with **EntityModel**

Often, it's good practice to use DTOs (Data Transfer Objects) instead of entities when exposing data via a REST API. You can wrap a DTO in an EntityModel to return it with links.

Example: EntityModel with DTO

Here:

• We convert the Product entity to a ProductDTO and wrap it in an EntityModel.
• The EntityModel now contains the product data along with links for self-reference and update actions.

4. Best Practices with **EntityModel**

• Self-links: Always add a self-link (withSelfRel()) to help clients identify the resource’s own URL.
• Meaningful relations: Use meaningful relation names like update, delete, or list to describe the purpose of a link, making the API more understandable.
• Separation of concerns: Use DTOs to separate your internal domain models from the API's response structure. This makes it easier to evolve the internal data models without affecting the API.
• Hypermedia-driven client interaction: By returning resources with links, you make the API more discoverable. Clients can follow links to perform actions like updating or deleting resources, without needing prior knowledge of the API structure.

5. Returning Collections with **EntityModel**

While EntityModel is typically used for individual resources, you can also combine it with CollectionModel to return collections of resources with hypermedia links.

Example: CollectionModel with EntityModel

In this example:

• A collection of Product entities is returned wrapped in EntityModel objects, and the entire collection is wrapped in a CollectionModel.
• The CollectionModel contains a self-link to allow clients to access the collection.

6. Conclusion

The **EntityModel** class in Spring HATEOAS is a key component for building HATEOAS-compliant APIs. It allows you to wrap individual resources (domain objects or DTOs) and enrich them with hypermedia links, making the API more self-descriptive and easier for clients to interact with.

By using EntityModel, developers can provide resources with meaningful, discoverable links (such as self-links, update links, and others), making the client experience more dynamic and reducing the need for hardcoded knowledge of the API structure. Whether you're working with entities or DTOs, EntityModel helps you build more maintainable and flexible REST APIs.