Spring Boot has a specific starter for HATEOAS called spring-boot-starter-hateoas. We need to add it to the pom.xml file.

The following code snippet shows the dependency block:

    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-hateoas</artifactId>
    </dependency>

One of the important dependencies that's included as part of spring-boot-starter-hateoas is spring-hateoas, which provides the HATEOAS features, is as follows:

    <dependency>
      <groupId>org.springframework.hateoas</groupId>
      <artifactId>spring-hateoas</artifactId>
    </dependency>

Let's enhance the retrieveTodo resource (/users/{name}/todos/{id}) to return a link to retrieve all todos (/users/{name}/todos) in the response:

    @GetMapping(path = "/users/{name}/todos/{id}")
    public Resource<Todo> retrieveTodo(
    @PathVariable String name, @PathVariable int id) {

         Todo todo = todoService.retrieveTodo(id);
      
         if (todo == null) {
              throw new TodoNotFoundException("Todo Not Found");
         }

         Resource<Todo> todoResource = new Resource<Todo>(todo);

         ControllerLinkBuilder linkTo =  
                             linkTo(methodOn(this.getClass()).retrieveTodos(name));

         todoResource.add(linkTo.withRel("parent"));
 
         return todoResource;

    }

Some important points to note are as follows:

• ControllerLinkBuilder linkTo = linkTo(methodOn(this.getClass()).retrieveTodos(name)): We want to get a link to the retrieveTodos method on the current class.
• linkTo.withRel("parent"): The relationship with the current resource is the parent.

The following snippet shows the response when a GET request is sent to http://localhost:8080/users/Jack/todos/1:

   {
     "id": 1,
     "user": "Jack",
     "desc": "Learn Spring MVC",
     "targetDate": 1484038262110,
     "done": false,
     "_links": {
               "parent": {
                      "href": "http://localhost:8080/users/Jack/todos"
               }
        }
   }

The _links section will contain all the links. Currently, we have one link with the relation parent and href as http://localhost:8080/users/Jack/todos.

HATEOAS is not something that is commonly used in most resources today; however, it has the potential to be really useful in reducing the coupling between the service provider and the consumer.