Building REST services with Spring

REST has quickly become the de facto standard for building web services on the web because REST services are easy to build and easy to consume.

A much larger discussion can be had about how REST fits in the world of microservices. However, for this tutorial, we look only at building RESTful services.

Why REST? REST embraces the precepts of the web, including its architecture, benefits, and everything else. This is no surprise, given that its author (Roy Fielding) was involved in probably a dozen specs which govern how the web operates.

What benefits? The web and its core protocol, HTTP, provide a stack of features:

Suitable actions (GET, POST, PUT, DELETE, and others)
Caching
Redirection and forwarding
Security (encryption and authentication)

These are all critical factors when building resilient services. However, that is not all. The web is built out of lots of tiny specs. This architecture lets it easily evolve without getting bogged down in "standards wars".

Developers can draw upon third-party toolkits that implement these diverse specs and instantly have both client and server technology at their fingertips.

By building on top of HTTP, REST APIs provide the means to build:

Backwards compatible APIs
Evolvable APIs
Scaleable services
Securable services
A spectrum of stateless to stateful services

Note that REST, however ubiquitous, is not a standard per se but an approach, a style, a set of constraints on your architecture that can help you build web-scale systems. This tutorial uses the Spring portfolio to build a RESTful service while takin advantage of the stackless features of REST.

Getting Started

To get started, you need:

A favorite text editor or IDE, such as:
- IntelliJ IDEA
- VSCode
Java 17 or later

As we work through this tutorial, we use Spring Boot. Go to Spring Initializr and add the following dependencies to a project:

Spring Web
Spring Data JPA
H2 Database

Change the Name to "Payroll" and then choose Generate Project. A .zip file downloads. Unzip it. Inside, you should find a simple, Maven-based project that includes a pom.xml build file. (Note: You can use Gradle. The examples in this tutorial will be Maven-based.)

To complete the tutorial, you could start a new project from scratch or you could look at the solution repository in GitHub.

If you choose to create your own blank project, this tutorial walks you through building your application sequentially. You do not need multiple modules.

Rather than providing a single, final solution, the completed GitHub repository uses modules to separate the solution into four parts. The modules in the GitHub solution repository build on one another, with the links module containing the final solution. The modules map to the following headers:

The Story So Far (nonrest)
Spring HATEOAS (rest)
Simplifying Link Creation (evolution)
Building links into your REST API (links)

The Story so Far

This tutorial starts by building up the code in the nonrest module.

We start off with the simplest thing we can construct. In fact, to make it as simple as possible, we can even leave out the concepts of REST. (Later on, we add REST, to understand the difference.)

Big picture: We are going to create a simple payroll service that manages the employees of a company. We store employee objects in a (H2 in-memory) database, and access them (through something called JPA). Then we wrap that with something that allows access over the internet (called the Spring MVC layer).

The following code defines an Employee in our system.

nonrest/src/main/java/payroll/Employee.java

package payroll;

import java.util.Objects;

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;


@Entity
class Employee {

  private @Id
  @GeneratedValue Long id;
  private String name;
  private String role;

  Employee() {}

  Employee(String name, String role) {

    this.name = name;
    this.role = role;
  }

  public Long getId() {
    return this.id;
  }

  public String getName() {
    return this.name;
  }

  public String getRole() {
    return this.role;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public void setName(String name) {
    this.name = name;
  }

  public void setRole(String role) {
    this.role = role;
  }

  @Override
  public boolean equals(Object o) {

    if (this == o)
      return true;
    if (!(o instanceof Employee))
      return false;
    Employee employee = (Employee) o;
    return Objects.equals(this.id, employee.id) && Objects.equals(this.name, employee.name)
        && Objects.equals(this.role, employee.role);
  }

  @Override
  public int hashCode() {
    return Objects.hash(this.id, this.name, this.role);
  }

  @Override
  public String toString() {
    return "Employee{" + "id=" + this.id + ", name='" + this.name + '\'' + ", role='" + this.role + '\'' + '}';
  }
}

Despite being small, this Java class contains much:

@Entity is a JPA annotation to make this object ready for storage in a JPA-based data store.
id, name, and role are attributes of our Employee domain object. id is marked with more JPA annotations to indicate that it is the primary key and is automatically populated by the JPA provider.
A custom constructor is created when we need to create a new instance but do not yet have an id.

With this domain object definition, we can now turn to Spring Data JPA to handle the tedious database interactions.

Spring Data JPA repositories are interfaces with methods that support creating, reading, updating, and deleting records against a back end data store. Some repositories also support data paging and sorting, where appropriate. Spring Data synthesizes implementations based on conventions found in the naming of the methods in the interface.

There are multiple repository implementations besides JPA. You can use Spring Data MongoDB, Spring Data Cassandra, and others. This tutorial sticks with JPA.

Spring makes accessing data easy. By declaring the following EmployeeRepository interface, we can automatically:

Create new employees
Update existing employees
Delete employees
Find employees (one, all, or search by simple or complex properties)

nonrest/src/main/java/payroll/EmployeeRepository.java

package payroll;

import org.springframework.data.jpa.repository.JpaRepository;

interface EmployeeRepository extends JpaRepository<Employee, Long> {

}

To get all this free functionality, all we have to do is declare an interface that extends Spring Data JPA’s JpaRepository, specifying the domain type as Employee and the id type as Long.

Spring Data’s repository solution makes it possible to sidestep data store specifics and, instead, solve a majority of problems by using domain-specific terminology.

Believe it or not, this is enough to launch an application! A Spring Boot application is, at a minimum, a public static void main entry-point and the @SpringBootApplication annotation. This tells Spring Boot to help out wherever possible.

nonrest/src/main/java/payroll/PayrollApplication.java

package payroll;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class PayrollApplication {

  public static void main(String... args) {
    SpringApplication.run(PayrollApplication.class, args);
  }
}

@SpringBootApplication is a meta-annotation that pulls in component scanning, auto-configuration, and property support. We do not dive into the details of Spring Boot in this tutorial. However, in essence, it starts a servlet container and serves up our service.

An application with no data is not very interesting, so we preload that it has data. The following class gets loaded automatically by Spring:

nonrest/src/main/java/payroll/LoadDatabase.java

package payroll;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
class LoadDatabase {

  private static final Logger log = LoggerFactory.getLogger(LoadDatabase.class);

  @Bean
  CommandLineRunner initDatabase(EmployeeRepository repository) {

    return args -> {
      log.info("Preloading " + repository.save(new Employee("Bilbo Baggins", "burglar")));
      log.info("Preloading " + repository.save(new Employee("Frodo Baggins", "thief")));
    };
  }
}

What happens when it gets loaded?

Spring Boot runs ALL CommandLineRunner beans once the application context is loaded.
This runner requests a copy of the EmployeeRepository you just created.
The runner creates two entities and stores them.

Right-click and Run PayRollApplication, and you get:

Fragment of console output showing preloading of data

...
20yy-08-09 11:36:26.169  INFO 74611 --- [main] payroll.LoadDatabase : Preloading Employee(id=1, name=Bilbo Baggins, role=burglar)
20yy-08-09 11:36:26.174  INFO 74611 --- [main] payroll.LoadDatabase : Preloading Employee(id=2, name=Frodo Baggins, role=thief)
...

This is not the whole log, but only the key bits of preloading data.

HTTP is the Platform

To wrap your repository with a web layer, you must turn to Spring MVC. Thanks to Spring Boot, you need add only a little code. Instead, we can focus on actions:

nonrest/src/main/java/payroll/EmployeeController.java

package payroll;

import java.util.List;

import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.PutMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
class EmployeeController {

  private final EmployeeRepository repository;

  EmployeeController(EmployeeRepository repository) {
    this.repository = repository;
  }


  // Aggregate root
  // tag::get-aggregate-root[]
  @GetMapping("/employees")
  List<Employee> all() {
    return repository.findAll();
  }
  // end::get-aggregate-root[]

  @PostMapping("/employees")
  Employee newEmployee(@RequestBody Employee newEmployee) {
    return repository.save(newEmployee);
  }

  // Single item
  
  @GetMapping("/employees/{id}")
  Employee one(@PathVariable Long id) {
    
    return repository.findById(id)
      .orElseThrow(() -> new EmployeeNotFoundException(id));
  }

  @PutMapping("/employees/{id}")
  Employee replaceEmployee(@RequestBody Employee newEmployee, @PathVariable Long id) {
    
    return repository.findById(id)
      .map(employee -> {
        employee.setName(newEmployee.getName());
        employee.setRole(newEmployee.getRole());
        return repository.save(employee);
      })
      .orElseGet(() -> {
        return repository.save(newEmployee);
      });
  }

  @DeleteMapping("/employees/{id}")
  void deleteEmployee(@PathVariable Long id) {
    repository.deleteById(id);
  }
}

@RestController indicates that the data returned by each method is written straight into the response body instead of rendering a template.
An EmployeeRepository is injected by constructor into the controller.
We have routes for each operation (@GetMapping, @PostMapping, @PutMapping and @DeleteMapping, corresponding to HTTP GET, POST, PUT, and DELETE calls). (We recommend reading each method and understanding what they do.)
EmployeeNotFoundException is an exception used to indicate when an employee is looked up but not found.

nonrest/src/main/java/payroll/EmployeeNotFoundException.java

package payroll;

class EmployeeNotFoundException extends RuntimeException {

  EmployeeNotFoundException(Long id) {
    super("Could not find employee " + id);
  }
}

When an EmployeeNotFoundException is thrown, this extra tidbit of Spring MVC configuration is used to render an HTTP 404 error:

nonrest/src/main/java/payroll/EmployeeNotFoundAdvice.java

package payroll;

import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.bind.annotation.RestControllerAdvice;

@RestControllerAdvice
class EmployeeNotFoundAdvice {

  @ExceptionHandler(EmployeeNotFoundException.class)
  @ResponseStatus(HttpStatus.NOT_FOUND)
  String employeeNotFoundHandler(EmployeeNotFoundException ex) {
    return ex.getMessage();
  }
}

@RestControllerAdvice signals that this advice is rendered straight into the response body.
@ExceptionHandler configures the advice to only respond when an EmployeeNotFoundException is thrown.
@ResponseStatus says to issue an HttpStatus.NOT_FOUND — that is, an HTTP 404 error.
The body of the advice generates the content. In this case, it gives the message of the exception.

To launch the application, you can right-click the public static void main in PayRollApplication and select Run from your IDE.

Alternatively, Spring Initializr creates a Maven wrapper, so you can run the following command:

$ ./mvnw clean spring-boot:run

Alternatively, you can use your installed Maven version, as follows:

$ mvn clean spring-boot:run

When the app starts, you can immediately interrogate it, as follows:

$ curl -v localhost:8080/employees

Doing so yields the following:

Details

*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> GET /employees HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200
< Content-Type: application/json;charset=UTF-8
< Transfer-Encoding: chunked
< Date: Thu, 09 Aug 20yy 17:58:00 GMT
<
* Connection #0 to host localhost left intact
[{"id":1,"name":"Bilbo Baggins","role":"burglar"},{"id":2,"name":"Frodo Baggins","role":"thief"}]

You can see the pre-loaded data in a compacted format.

Now try to query a user that doesn’t exist, as follows:

$ curl -v localhost:8080/employees/99

When you do so, you get the following output:

Details

*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> GET /employees/99 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 404
< Content-Type: text/plain;charset=UTF-8
< Content-Length: 26
< Date: Thu, 09 Aug 20yy 18:00:56 GMT
<
* Connection #0 to host localhost left intact
Could not find employee 99

This message nicely shows an HTTP 404 error with the custom message: Could not find employee 99.

It is not hard to show the currently coded interactions.

If you use Windows command prompt to issue cURL commands, the following command probably does not work properly. You must either pick a terminal that support single-quoted arguments, or use double quotation marks and then escape the quotation marks inside the JSON.

To create a new Employee record, use the following command in a terminal (the $ at the beginning signifies that what follows it is a terminal command):

$ curl -X POST localhost:8080/employees -H 'Content-type:application/json' -d '{"name": "Samwise Gamgee", "role": "gardener"}'

Then it stores the newly created employee and sends it back to us:

{"id":3,"name":"Samwise Gamgee","role":"gardener"}

You can update the user. For example, you can change the role:

$ curl -X PUT localhost:8080/employees/3 -H 'Content-type:application/json' -d '{"name": "Samwise Gamgee", "role": "ring bearer"}'

Now we can see the change reflected in the output:

{"id":3,"name":"Samwise Gamgee","role":"ring bearer"}

The way you construct your service can have significant impacts. In this situation, we said update, but replace is a better description. For example, if the name was NOT provided, it would instead get nulled out.

Finally, you can delete users, as follows:

$ curl -X DELETE localhost:8080/employees/3

# Now if we look again, it's gone
$ curl localhost:8080/employees/3
Could not find employee 3

This is all well and good, but do we have a RESTful service yet? (The answer is no.)

What’s missing?

What Makes a Service RESTful?

So far, you have a web-based service that handles the core operations that involve employee data. However, that is not enough to make things "RESTful".

Pretty URLs, such as`/employees/3`, aren’t REST.
Merely using GET, POST, and so on is not REST.
Having all the CRUD operations laid out is not REST.

In fact, what we have built so far is better described as RPC (Remote Procedure Call), because there is no way to know how to interact with this service. If you published this today, you wouldd also have to write a document or host a developer’s portal somewhere with all the details.

This statement of Roy Fielding’s may further lend a clue to the difference between REST and RPC:

I am getting frustrated by the number of people calling any HTTP-based interface a REST API. Today’s example is the SocialSite REST API. That is RPC. It screams RPC. There is so much coupling on display that it should be given an X rating.

What needs to be done to make the REST architectural style clear on the notion that hypertext is a constraint? In other words, if the engine of application state (and hence the API) is not being driven by hypertext, then it cannot be RESTful and cannot be a REST API. Period. Is there some broken manual somewhere that needs to be fixed?

— Roy Fielding
https://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven

The side effect of nnot including hypermedia in our representations is that clients must hard-code URIs to navigate the API. This leads to the same brittle nature that predated the rise of e-commerce on the web. It signifies that our JSON output needs a little help.

Spring HATEOAS

Now we can introduce Spring HATEOAS, a Spring project aimed at helping you write hypermedia-driven outputs. To upgrade your service to being RESTful, add the following to your build:

If you are following along in the solution repository, the next section switches to the rest module.

Adding Spring HATEOAS to dependencies section of pom.xml

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

This tiny library gives us the constructs that define a RESTful service and then render it in an acceptable format for client consumption.

A critical ingredient to any RESTful service is adding links to relevant operations. To make your controller more RESTful, add links like the following to the existing one method in EmployeeController:

Getting a single item resource

@GetMapping("/employees/{id}")
EntityModel<Employee> one(@PathVariable Long id) {

  Employee employee = repository.findById(id) //
      .orElseThrow(() -> new EmployeeNotFoundException(id));

  return EntityModel.of(employee, //
      linkTo(methodOn(EmployeeController.class).one(id)).withSelfRel(),
      linkTo(methodOn(EmployeeController.class).all()).withRel("employees"));
}

You also need to include new imports:

Details

import org.springframework.hateoas.EntityModel;
import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.*;

This tutorial is based on Spring MVC and uses the static helper methods from WebMvcLinkBuilder to build these links. If you are using Spring WebFlux in your project, you must instead use WebFluxLinkBuilder.

This is very similar to what we had before, but a few things have changed:

The return type of the method has changed from Employee to EntityModel<Employee>. EntityModel<T> is a generic container from Spring HATEOAS that includes not only the data but a collection of links.
linkTo(methodOn(EmployeeController.class).one(id)).withSelfRel() asks that Spring HATEOAS build a link to the one method of EmployeeController and flag it as a self link.
linkTo(methodOn(EmployeeController.class).all()).withRel("employees") asks Spring HATEOAS to build a link to the aggregate root, all(), and call it "employees".

What do we mean by "build a link?" One of Spring HATEOAS’s core types is Link. It includes a URI and a rel (relation). Links are what empower the web. Before the World Wide Web, other document systems would render information or links, but it was the linking of documents WITH this kind of relationship metadata that stitched the web together.

Roy Fielding encourages building APIs with the same techniques that made the web successful, and links are one of them.

If you restart the application and query the employee record of Bilbo, you get a slightly different response than earlier:

Curling prettier

When your curl output gets more complex it can become hard to read. Use this or other tips to prettify the json returned by curl:

# The indicated part pipes the output to json_pp and asks it to make your JSON pretty. (Or use whatever tool you like!)
#                                  v------------------v
curl -v localhost:8080/employees/1 | json_pp

RESTful representation of a single employee

{
  "id": 1,
  "name": "Bilbo Baggins",
  "role": "burglar",
  "_links": {
    "self": {
      "href": "http://localhost:8080/employees/1"
    },
    "employees": {
      "href": "http://localhost:8080/employees"
    }
  }
}

This decompressed output shows not only the data elements you saw earlier (id, name, and role) but also a _links entry that contains two URIs. This entire document is formatted using HAL.

HAL is a lightweight mediatype that allows encoding not only data but also hypermedia controls, alerting consumers to other parts of the API to which they can navigate. In this case, there is a "self" link (kind of like a this statement in code) along with a link back to the aggregate root.

To make the aggregate root also be more RESTful, you want to include top level links while also including any RESTful components within.

So we modify the following (located in the nonrest module of the completed code):

Getting an aggregate root

@GetMapping("/employees")
List<Employee> all() {
  return repository.findAll();
}

We want the following (located in the rest module of the completed code):

Getting an aggregate root resource

@GetMapping("/employees")
CollectionModel<EntityModel<Employee>> all() {

  List<EntityModel<Employee>> employees = repository.findAll().stream()
      .map(employee -> EntityModel.of(employee,
          linkTo(methodOn(EmployeeController.class).one(employee.getId())).withSelfRel(),
          linkTo(methodOn(EmployeeController.class).all()).withRel("employees")))
      .collect(Collectors.toList());

  return CollectionModel.of(employees, linkTo(methodOn(EmployeeController.class).all()).withSelfRel());
}

That method, which used to be merely repository.findAll(), is "all grown up."" Not to worry. Now we can unpack it.

CollectionModel<> is another Spring HATEOAS container. It is aimed at encapsulating collections of resources instead of a single resource entity, such as EntityModel<> from earlier. CollectionModel<>, too, lets you include links.

Do not let that first statement slip by. What does "encapsulating collections" mean? Collections of employees?

Not quite.

Since we are talking REST, it should encapsulate collections of employee resources.

That is why you fetch all the employees but then transform them into a list of EntityModel<Employee> objects. (Thanks Java Streams!)

If you restart the application and fetch the aggregate root, you can see what it looks like now:

curl -v localhost:8080/employees | json_pp

RESTful representation of a collection of employee resources

{
  "_embedded": {
    "employeeList": [
      {
        "id": 1,
        "name": "Bilbo Baggins",
        "role": "burglar",
        "_links": {
          "self": {
            "href": "http://localhost:8080/employees/1"
          },
          "employees": {
            "href": "http://localhost:8080/employees"
          }
        }
      },
      {
        "id": 2,
        "name": "Frodo Baggins",
        "role": "thief",
        "_links": {
          "self": {
            "href": "http://localhost:8080/employees/2"
          },
          "employees": {
            "href": "http://localhost:8080/employees"
          }
        }
      }
    ]
  },
  "_links": {
    "self": {
      "href": "http://localhost:8080/employees"
    }
  }
}

For this aggregate root, which serves up a collection of employee resources, there is a top-level "self" link. The "collection" is listed underneath the "_embedded" section. This is how HAL represents collections.

Each individual member of the collection has their information as well as related links.

What is the point of adding all these links? It makes it possible to evolve REST services over time. Existing links can be maintained while new links can be added in the future. Newer clients may take advantage of the new links, while legacy clients can sustain themselves on the old links. This is especially helpful if services get relocated and moved around. As long as the link structure is maintained, clients can still find and interact with things.

Simplifying Link Creation

If you are following along in the solution repository, the next section switches to the evolution module.

In the code earlier, did you notice the repetition in single employee link creation? The code to provide a single link to an employee, as well as to create an "employees" link to the aggregate root, was shown twice. If that raised a concern, good! There’s a solution.

You need to define a function that converts Employee objects to EntityModel<Employee> objects. While you could easily code this method yourself, Spring HATEOAS’s RepresentationModelAssembler interface does the work for you. Create a new class EmployeeModelAssembler:

evolution/src/main/java/payroll/EmployeeModelAssembler.java

package payroll;

import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.*;

import org.springframework.hateoas.EntityModel;
import org.springframework.hateoas.server.RepresentationModelAssembler;
import org.springframework.stereotype.Component;

@Component
class EmployeeModelAssembler implements RepresentationModelAssembler<Employee, EntityModel<Employee>> {

  @Override
  public EntityModel<Employee> toModel(Employee employee) {

    return EntityModel.of(employee, //
        linkTo(methodOn(EmployeeController.class).one(employee.getId())).withSelfRel(),
        linkTo(methodOn(EmployeeController.class).all()).withRel("employees"));
  }
}

This simple interface has one method: toModel(). It is based on converting a non-model object (Employee) into a model-based object (EntityModel<Employee>).

All the code you saw earlier in the controller can be moved into this class. Also, by applying Spring Framework’s @Component annotation, the assembler is automatically created when the app starts.

Spring HATEOAS’s abstract base class for all models is RepresentationModel. However, for simplicity, we recommend using EntityModel<T> as your mechanism to easily wrap all POJOs as models.

To leverage this assembler, you have only to alter the EmployeeController by injecting the assembler in the constructor:

Injecting EmployeeModelAssembler into the controller

@RestController
class EmployeeController {

  private final EmployeeRepository repository;

  private final EmployeeModelAssembler assembler;

  EmployeeController(EmployeeRepository repository, EmployeeModelAssembler assembler) {

    this.repository = repository;
    this.assembler = assembler;
  }

  ...

}

From here, you can use that assembler in the single-item employee method one that already exists in EmployeeController:

Getting single item resource using the assembler

	@GetMapping("/employees/{id}")
	EntityModel<Employee> one(@PathVariable Long id) {

		Employee employee = repository.findById(id) //
				.orElseThrow(() -> new EmployeeNotFoundException(id));

		return assembler.toModel(employee);
	}

This code is almost the same, except that, instead of creating the EntityModel<Employee> instance here, you delegate it to the assembler. Maybe that is not impressive.

Applying the same thing in the aggregate root controller method is more impressive. This change is also to the EmployeeController class:

Getting aggregate root resource using the assembler

@GetMapping("/employees")
CollectionModel<EntityModel<Employee>> all() {

  List<EntityModel<Employee>> employees = repository.findAll().stream() //
      .map(assembler::toModel) //
      .collect(Collectors.toList());

  return CollectionModel.of(employees, linkTo(methodOn(EmployeeController.class).all()).withSelfRel());
}

The code is, again, almost the same. However, you get to replace all that EntityModel<Employee> creation logic with map(assembler::toModel). Thanks to Java method references, it is super easy to plug in and simplify your controller.

A key design goal of Spring HATEOAS is to make it easier to do The Right Thing™. In this scenario, that means adding hypermedia to your service without hard coding a thing.

At this stage, you have created a Spring MVC REST controller that actually produces hypermedia-powered content. Clients that do not speak HAL can ignore the extra bits while consuming the pure data. Clients that do speak HAL can navigate your empowered API.

But that is not the only thing needed to build a truly RESTful service with Spring.

Evolving REST APIs

With one additional library and a few lines of extra code, you have added hypermedia to your application. But that is not the only thing needed to make your service RESTful. An important facet of REST is the fact that it is neither a technology stack nor a single standard.

REST is a collection of architectural constraints that, when adopted, make your application much more resilient. A key factor of resilience is that when you make upgrades to your services, your clients do not suffer downtime.

In the "olden" days, upgrades were notorious for breaking clients. In other words, an upgrade to the server required an update to the client. In this day and age, hours or even minutes of downtime spent doing an upgrade can cost millions in lost revenue.

Some companies require that you present management with a plan to minimize downtime. In the past, you could get away with upgrading at 2:00 a.m. on a Sunday when load was at a minimum. But in today’s Internet-based e-commerce with international customers in other time zones, such strategies are not as effective.

SOAP-based services and CORBA-based services were incredibly brittle. It was hard to roll out a server that could support both old and new clients. With REST-based practices, it is much easier, especially using the Spring stack.

Supporting Changes to the API

Imagine this design problem: You have rolled out a system with this Employee-based record. The system is a major hit. You have sold your system to countless enterprises. Suddenly, the need for an employee’s name to be split into firstName and lastName arises.

Uh oh. You did not think of that.

Before you open up the Employee class and replace the single field name with firstName and lastName, stop and think. Does that break any clients? How long will it take to upgrade them? Do you even control all the clients accessing your services?

Downtime = lost money. Is management ready for that?

There is an old strategy that precedes REST by years.

Never delete a column in a database.

— Unknown

You can always add columns (fields) to a database table. But do not take one away. The principle in RESTful services is the same.

Add new fields to your JSON representations, but do not take any away. Like this:

JSON that supports multiple clients

{
  "id": 1,
  "firstName": "Bilbo",
  "lastName": "Baggins",
  "role": "burglar",
  "name": "Bilbo Baggins",
  "_links": {
    "self": {
      "href": "http://localhost:8080/employees/1"
    },
    "employees": {
      "href": "http://localhost:8080/employees"
    }
  }
}

This format shows firstName, lastName, and name. While it sports duplication of information, the purpose is to support both old and new clients. That means you can upgrade the server without requiring clients to upgrade at the same time. This is good move that should reduce downtime.

Not only should you show this information in both the "old way" and the "new way", but you should also process incoming data both ways.

Employee record that handles both "old" and "new" clients

package payroll;

import java.util.Objects;

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;

@Entity
class Employee {

  private @Id @GeneratedValue Long id;
  private String firstName;
  private String lastName;
  private String role;

  Employee() {}

  Employee(String firstName, String lastName, String role) {

    this.firstName = firstName;
    this.lastName = lastName;
    this.role = role;
  }

  public String getName() {
    return this.firstName + " " + this.lastName;
  }

  public void setName(String name) {
    String[] parts = name.split(" ");
    this.firstName = parts[0];
    this.lastName = parts[1];
  }

  public Long getId() {
    return this.id;
  }

  public String getFirstName() {
    return this.firstName;
  }

  public String getLastName() {
    return this.lastName;
  }

  public String getRole() {
    return this.role;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public void setFirstName(String firstName) {
    this.firstName = firstName;
  }

  public void setLastName(String lastName) {
    this.lastName = lastName;
  }

  public void setRole(String role) {
    this.role = role;
  }

  @Override
  public boolean equals(Object o) {

    if (this == o)
      return true;
    if (!(o instanceof Employee))
      return false;
    Employee employee = (Employee) o;
    return Objects.equals(this.id, employee.id) && Objects.equals(this.firstName, employee.firstName)
        && Objects.equals(this.lastName, employee.lastName) && Objects.equals(this.role, employee.role);
  }

  @Override
  public int hashCode() {
    return Objects.hash(this.id, this.firstName, this.lastName, this.role);
  }

  @Override
  public String toString() {
    return "Employee{" + "id=" + this.id + ", firstName='" + this.firstName + '\'' + ", lastName='" + this.lastName
        + '\'' + ", role='" + this.role + '\'' + '}';
  }
}

This class is similar to the previous version of Employee, with a few changes:

Field name has been replaced by firstName and lastName.
A "virtual" getter for the old name property, getName(), is defined. It uses the firstName and lastName fields to produce a value.
A "virtual" setter for the old name property, setName(), is also defined. It parses an incoming string and stores it into the proper fields.

Of course, not change to your API is as simple as splitting a string or merging two strings. But itis surely not impossible to come up with a set of transforms for most scenarios, right?

Do not forget to change how you preload your database (in LoadDatabase) to use this new constructor.

log.info("Preloading " + repository.save(new Employee("Bilbo", "Baggins", "burglar")));
log.info("Preloading " + repository.save(new Employee("Frodo", "Baggins", "thief")));

Proper Responses

Another step in the right direction involves ensuring that each of your REST methods returns a proper response. Update the POST method (newEmployee) in the EmployeeController:

POST that handles "old" and "new" client requests

@PostMapping("/employees")
ResponseEntity<?> newEmployee(@RequestBody Employee newEmployee) {

  EntityModel<Employee> entityModel = assembler.toModel(repository.save(newEmployee));

  return ResponseEntity //
      .created(entityModel.getRequiredLink(IanaLinkRelations.SELF).toUri()) //
      .body(entityModel);
}

You also need to add the imports:

Details

import org.springframework.hateoas.IanaLinkRelations;
import org.springframework.http.ResponseEntity;

The new Employee object is saved, as before. However, the resulting object is wrapped in the EmployeeModelAssembler.
Spring MVC’s ResponseEntity is used to create an HTTP 201 Created status message. This type of response typically includes a Location response header, and we use the URI derived from the model’s self-related link.
Additionally, the model-based version of the saved object is returned.

With these tweaks in place, you can use the same endpoint to create a new employee resource and use the legacy name field:

$ curl -v -X POST localhost:8080/employees -H 'Content-Type:application/json' -d '{"name": "Samwise Gamgee", "role": "gardener"}' | json_pp

The output is as follows:

Details

> POST /employees HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
> Content-Type:application/json
> Content-Length: 46
>
< Location: http://localhost:8080/employees/3
< Content-Type: application/hal+json;charset=UTF-8
< Transfer-Encoding: chunked
< Date: Fri, 10 Aug 20yy 19:44:43 GMT
<
{
  "id": 3,
  "firstName": "Samwise",
  "lastName": "Gamgee",
  "role": "gardener",
  "name": "Samwise Gamgee",
  "_links": {
    "self": {
      "href": "http://localhost:8080/employees/3"
    },
    "employees": {
      "href": "http://localhost:8080/employees"
    }
  }
}

This not only has the resulting object rendered in HAL (both name as well as firstName and lastName), but also the Location header populated with http://localhost:8080/employees/3. A hypermedia-powered client could opt to "surf" to this new resource and proceed to interact with it.

The PUT controller method (replaceEmployee) in EmployeeController needs similar tweaks:

Handling a PUT for different clients

@PutMapping("/employees/{id}")
ResponseEntity<?> replaceEmployee(@RequestBody Employee newEmployee, @PathVariable Long id) {

  Employee updatedEmployee = repository.findById(id) //
      .map(employee -> {
        employee.setName(newEmployee.getName());
        employee.setRole(newEmployee.getRole());
        return repository.save(employee);
      }) //
      .orElseGet(() -> {
        return repository.save(newEmployee);
      });

  EntityModel<Employee> entityModel = assembler.toModel(updatedEmployee);

  return ResponseEntity //
      .created(entityModel.getRequiredLink(IanaLinkRelations.SELF).toUri()) //
      .body(entityModel);
}

The Employee object built by the save() operation is then wrapped in the EmployeeModelAssembler to create an EntityModel<Employee> object. Using the getRequiredLink() method, you can retrieve the Link created by the EmployeeModelAssembler with a SELF rel. This method returns a Link, which must be turned into a URI with the toUri method.

Since we want a more detailed HTTP response code than 200 OK, we use Spring MVC’s ResponseEntity wrapper. It has a handy static method (created()) where we can plug in the resource’s URI. It is debatable whether HTTP 201 Created carries the right semantics, since we do not necessarily "create" a new resource. However, it comes pre-loaded with a Location response header, so we use it. Restart your application, run the following command, and observe the results:

$ curl -v -X PUT localhost:8080/employees/3 -H 'Content-Type:application/json' -d '{"name": "Samwise Gamgee", "role": "ring bearer"}' | json_pp

Details

* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> PUT /employees/3 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
> Content-Type:application/json
> Content-Length: 49
>
< HTTP/1.1 201
< Location: http://localhost:8080/employees/3
< Content-Type: application/hal+json;charset=UTF-8
< Transfer-Encoding: chunked
< Date: Fri, 10 Aug 20yy 19:52:56 GMT
{
	"id": 3,
	"firstName": "Samwise",
	"lastName": "Gamgee",
	"role": "ring bearer",
	"name": "Samwise Gamgee",
	"_links": {
		"self": {
			"href": "http://localhost:8080/employees/3"
		},
		"employees": {
			"href": "http://localhost:8080/employees"
		}
	}
}

That employee resource has now been updated and the location URI has been sent back. Finally, update the DELETE operation (deleteEmployee) in EmployeeController:

Handling DELETE requests

@DeleteMapping("/employees/{id}")
ResponseEntity<?> deleteEmployee(@PathVariable Long id) {

  repository.deleteById(id);

  return ResponseEntity.noContent().build();
}

This returns an HTTP 204 No Content response. Restart your application, run the following command, and observe the results:

$ curl -v -X DELETE localhost:8080/employees/1

Details

* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> DELETE /employees/1 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 204
< Date: Fri, 10 Aug 20yy 21:30:26 GMT

Making changes to the fields in the Employee class requires coordination with your database team, so that they can properly migrate existing content into the new columns.

You are now ready for an upgrade that does not disturb existing clients while newer clients can take advantage of the enhancements.

By the way, are you worried about sending too much information over the wire? In some systems where every byte counts, evolution of APIs may need to take a backseat. However, you should not pursue such premature optimization until you measure the impact of your changes.

Building links into your REST API

If you are following along in the solution repository, the next section switches to the links module.

So far, you have built an evolvable API with bare bones links. To grow your API and better serve your clients, you need to embrace the concept of Hypermedia as the Engine of Application State.

What does that mean? This section explores it in detail.

Business logic inevitably builds up rules that involve processes. The risk of such systems is we often carry such server-side logic into clients and build up strong coupling. REST is about breaking down such connections and minimizing such coupling.

To show how to cope with state changes without triggering breaking changes in clients, imagine adding a system that fulfills orders.

As a first step, define a new Order record:

links/src/main/java/payroll/Order.java

package payroll;

import java.util.Objects;

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;
import jakarta.persistence.Table;

@Entity
@Table(name = "CUSTOMER_ORDER")
class Order {

  private @Id @GeneratedValue Long id;

  private String description;
  private Status status;

  Order() {}

  Order(String description, Status status) {

    this.description = description;
    this.status = status;
  }

  public Long getId() {
    return this.id;
  }

  public String getDescription() {
    return this.description;
  }

  public Status getStatus() {
    return this.status;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public void setDescription(String description) {
    this.description = description;
  }

  public void setStatus(Status status) {
    this.status = status;
  }

  @Override
  public boolean equals(Object o) {

    if (this == o)
      return true;
    if (!(o instanceof Order))
      return false;
    Order order = (Order) o;
    return Objects.equals(this.id, order.id) && Objects.equals(this.description, order.description)
        && this.status == order.status;
  }

  @Override
  public int hashCode() {
    return Objects.hash(this.id, this.description, this.status);
  }

  @Override
  public String toString() {
    return "Order{" + "id=" + this.id + ", description='" + this.description + '\'' + ", status=" + this.status + '}';
  }
}

The class requires a JPA @Table annotation that changes the table’s name to CUSTOMER_ORDER because ORDER is not a valid name for table.
It includes a description field as well as a status field.

Orders must go through a certain series of state transitions from the time a customer submits an order and it is either fulfilled or cancelled. This can be captured as a Java enum called Status:

links/src/main/java/payroll/Status.java

package payroll;

enum Status {

  IN_PROGRESS, //
  COMPLETED, //
  CANCELLED
}

This enum captures the various states an Order can occupy. For this tutorial, we keep it simple.

To support interacting with orders in the database, you must define a corresponding Spring Data repository called OrderRepository:

Spring Data JPA’s JpaRepository base interface

interface OrderRepository extends JpaRepository<Order, Long> {
}

We also need to create a new exception class called OrderNotFoundException:

Details

package payroll;

class OrderNotFoundException extends RuntimeException {

  OrderNotFoundException(Long id) {
    super("Could not find order " + id);
  }
}

With this in place, you can now define a basic OrderController with the required imports:

Import Statements

import java.util.List;
import java.util.stream.Collectors;

import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.*;

import org.springframework.hateoas.CollectionModel;
import org.springframework.hateoas.EntityModel;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

links/src/main/java/payroll/OrderController.java

@RestController
class OrderController {

  private final OrderRepository orderRepository;
  private final OrderModelAssembler assembler;

  OrderController(OrderRepository orderRepository, OrderModelAssembler assembler) {

    this.orderRepository = orderRepository;
    this.assembler = assembler;
  }

  @GetMapping("/orders")
  CollectionModel<EntityModel<Order>> all() {

    List<EntityModel<Order>> orders = orderRepository.findAll().stream() //
        .map(assembler::toModel) //
        .collect(Collectors.toList());

    return CollectionModel.of(orders, //
        linkTo(methodOn(OrderController.class).all()).withSelfRel());
  }

  @GetMapping("/orders/{id}")
  EntityModel<Order> one(@PathVariable Long id) {

    Order order = orderRepository.findById(id) //
        .orElseThrow(() -> new OrderNotFoundException(id));

    return assembler.toModel(order);
  }

  @PostMapping("/orders")
  ResponseEntity<EntityModel<Order>> newOrder(@RequestBody Order order) {

    order.setStatus(Status.IN_PROGRESS);
    Order newOrder = orderRepository.save(order);

    return ResponseEntity //
        .created(linkTo(methodOn(OrderController.class).one(newOrder.getId())).toUri()) //
        .body(assembler.toModel(newOrder));
  }
}

It contains the same REST controller setup as the controllers you have built so far.
It injects both an OrderRepository and a (not yet built) OrderModelAssembler.
The first two Spring MVC routes handle the aggregate root as well as a single item Order resource request.
The third Spring MVC route handles creating new orders, by starting them in the IN_PROGRESS state.
All the controller methods return one of Spring HATEOAS’s RepresentationModel subclasses to properly render hypermedia (or a wrapper around such a type).

Before building the OrderModelAssembler, we should discuss what needs to happen. You are modeling the flow of states between Status.IN_PROGRESS, Status.COMPLETED, and Status.CANCELLED. A natural thing when serving up such data to clients is to let the clients make the decision about what they can do, based on this payload.

But that would be wrong.

What happens when you introduce a new state in this flow? The placement of various buttons on the UI would probably be erroneous.

What if you changed the name of each state, perhaps while coding international support and showing locale-specific text for each state? That would most likely break all the clients.

Enter HATEOAS or Hypermedia as the Engine of Application State. Instead of clients parsing the payload, give them links to signal valid actions. Decouple state-based actions from the payload of data. In other words, when CANCEL and COMPLETE are valid actions, you should dynamically add them to the list of links. Clients need to show users the corresponding buttons only when the links exist.

This decouples clients from having to know when such actions are valid, reducing the risk of the server and its clients getting out of sync on the logic of state transitions.

Having already embraced the concept of Spring HATEOAS RepresentationModelAssembler components, the OrderModelAssembler is the perfect place to capture the logic for this business rule:

links/src/main/java/payroll/OrderModelAssembler.java

package payroll;

import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.*;

import org.springframework.hateoas.EntityModel;
import org.springframework.hateoas.server.RepresentationModelAssembler;
import org.springframework.stereotype.Component;

@Component
class OrderModelAssembler implements RepresentationModelAssembler<Order, EntityModel<Order>> {

  @Override
  public EntityModel<Order> toModel(Order order) {

    // Unconditional links to single-item resource and aggregate root

    EntityModel<Order> orderModel = EntityModel.of(order,
        linkTo(methodOn(OrderController.class).one(order.getId())).withSelfRel(),
        linkTo(methodOn(OrderController.class).all()).withRel("orders"));

    // Conditional links based on state of the order

    if (order.getStatus() == Status.IN_PROGRESS) {
      orderModel.add(linkTo(methodOn(OrderController.class).cancel(order.getId())).withRel("cancel"));
      orderModel.add(linkTo(methodOn(OrderController.class).complete(order.getId())).withRel("complete"));
    }

    return orderModel;
  }
}

This resource assembler always includes the self link to the single-item resource as well as a link back to the aggregate root. However, it also includes two conditional links to OrderController.cancel(id) as well as OrderController.complete(id) (not yet defined). These links are shown only when the order’s status is Status.IN_PROGRESS.

If clients can adopt HAL and the ability to read links instead of simply reading the data of plain old JSON, they can trade in the need for domain knowledge about the order system. This naturally reduces coupling between client and server. It also opens the door to tuning the flow of order fulfillment without breaking clients in the process.

To round out order fulfillment, add the following to the OrderController for the cancel operation:

Creating a "cancel" operation in the OrderController

@DeleteMapping("/orders/{id}/cancel")
ResponseEntity<?> cancel(@PathVariable Long id) {

  Order order = orderRepository.findById(id) //
      .orElseThrow(() -> new OrderNotFoundException(id));

  if (order.getStatus() == Status.IN_PROGRESS) {
    order.setStatus(Status.CANCELLED);
    return ResponseEntity.ok(assembler.toModel(orderRepository.save(order)));
  }

  return ResponseEntity //
      .status(HttpStatus.METHOD_NOT_ALLOWED) //
      .header(HttpHeaders.CONTENT_TYPE, MediaTypes.HTTP_PROBLEM_DETAILS_JSON_VALUE) //
      .body(Problem.create() //
          .withTitle("Method not allowed") //
          .withDetail("You can't cancel an order that is in the " + order.getStatus() + " status"));
}

It checks the Order status before letting it be cancelled. If it is not a valid state, it returns an RFC-7807 Problem, a hypermedia-supporting error container. If the transition is indeed valid, it transitions the Order to CANCELLED.

Now we need to add this to the OrderController as well for order completion:

Creating a "complete" operation in the OrderController

@PutMapping("/orders/{id}/complete")
ResponseEntity<?> complete(@PathVariable Long id) {

  Order order = orderRepository.findById(id) //
      .orElseThrow(() -> new OrderNotFoundException(id));

  if (order.getStatus() == Status.IN_PROGRESS) {
    order.setStatus(Status.COMPLETED);
    return ResponseEntity.ok(assembler.toModel(orderRepository.save(order)));
  }

  return ResponseEntity //
      .status(HttpStatus.METHOD_NOT_ALLOWED) //
      .header(HttpHeaders.CONTENT_TYPE, MediaTypes.HTTP_PROBLEM_DETAILS_JSON_VALUE) //
      .body(Problem.create() //
          .withTitle("Method not allowed") //
          .withDetail("You can't complete an order that is in the " + order.getStatus() + " status"));
}

This implements similar logic to prevent an Order status from being completed unless in the proper state.

Let’s update LoadDatabase to pre-load some Order objectss along with the Employee objects it was loading before.

Updating the database pre-loader

package payroll;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
class LoadDatabase {

  private static final Logger log = LoggerFactory.getLogger(LoadDatabase.class);

  @Bean
  CommandLineRunner initDatabase(EmployeeRepository employeeRepository, OrderRepository orderRepository) {

    return args -> {
      employeeRepository.save(new Employee("Bilbo", "Baggins", "burglar"));
      employeeRepository.save(new Employee("Frodo", "Baggins", "thief"));

      employeeRepository.findAll().forEach(employee -> log.info("Preloaded " + employee));

      
      orderRepository.save(new Order("MacBook Pro", Status.COMPLETED));
      orderRepository.save(new Order("iPhone", Status.IN_PROGRESS));

      orderRepository.findAll().forEach(order -> {
        log.info("Preloaded " + order);
      });
      
    };
  }
}

Now you can test. Restart your application to make sure you are running the latest code changes. To use the newly minted order service, you can perform a few operations:

$ curl -v http://localhost:8080/orders | json_pp

Details

{
  "_embedded": {
    "orderList": [
      {
        "id": 3,
        "description": "MacBook Pro",
        "status": "COMPLETED",
        "_links": {
          "self": {
            "href": "http://localhost:8080/orders/3"
          },
          "orders": {
            "href": "http://localhost:8080/orders"
          }
        }
      },
      {
        "id": 4,
        "description": "iPhone",
        "status": "IN_PROGRESS",
        "_links": {
          "self": {
            "href": "http://localhost:8080/orders/4"
          },
          "orders": {
            "href": "http://localhost:8080/orders"
          },
          "cancel": {
            "href": "http://localhost:8080/orders/4/cancel"
          },
          "complete": {
            "href": "http://localhost:8080/orders/4/complete"
          }
        }
      }
    ]
  },
  "_links": {
    "self": {
      "href": "http://localhost:8080/orders"
    }
  }
}

This HAL document immediately shows different links for each order, based upon its present state.

The first order, being COMPLETED, only has the navigational links. The state transition links are not shown.
The second order, being IN_PROGRESS, additionally has the cancel link as well as the complete link.

Now try cancelling an order:

$ curl -v -X DELETE http://localhost:8080/orders/4/cancel | json_pp

You may need to replace the number 4 in the preceding URL, based on the specific IDs in your database. That information can be found from the previous /orders call.

Details

> DELETE /orders/4/cancel HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200
< Content-Type: application/hal+json;charset=UTF-8
< Transfer-Encoding: chunked
< Date: Mon, 27 Aug 20yy 15:02:10 GMT
<
{
  "id": 4,
  "description": "iPhone",
  "status": "CANCELLED",
  "_links": {
    "self": {
      "href": "http://localhost:8080/orders/4"
    },
    "orders": {
      "href": "http://localhost:8080/orders"
    }
  }
}

This response shows an HTTP 200 status code, indicating that it was successful. The response HAL document shows that order in its new state (CANCELLED). Also, the state-altering links gone.

Now try the same operation again:

$ curl -v -X DELETE http://localhost:8080/orders/4/cancel | json_pp

Details

* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> DELETE /orders/4/cancel HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 405
< Content-Type: application/problem+json
< Transfer-Encoding: chunked
< Date: Mon, 27 Aug 20yy 15:03:24 GMT
<
{
  "title": "Method not allowed",
  "detail": "You can't cancel an order that is in the CANCELLED status"
}

You can see an HTTP 405 Method Not Allowed response. DELETE has become an invalid operation. The Problem response object clearly indicates that you are not allowed to "cancel" an order already in the "CANCELLED" status.

Additionally, trying to complete the same order also fails:

$ curl -v -X PUT localhost:8080/orders/4/complete | json_pp

Details

* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> PUT /orders/4/complete HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 405
< Content-Type: application/problem+json
< Transfer-Encoding: chunked
< Date: Mon, 27 Aug 20yy 15:05:40 GMT
<
{
  "title": "Method not allowed",
  "detail": "You can't complete an order that is in the CANCELLED status"
}

With all this in place, your order fulfillment service is capable of conditionally showing what operations are available. It also guards against invalid operations.

By using the protocol of hypermedia and links, clients can be made sturdier and be less likely to break simply because of a change in the data. Spring HATEOAS eases building the hypermedia you need to serve to your clients.

Summary

Throughout this tutorial, you have engaged in various tactics to build REST APIs. As it turns out, REST is not just about pretty URIs and returning JSON instead of XML.

Instead, the following tactics help make your services less likely to break existing clients you may or may not control:

Do not remove old fields. Instead, support them.
Use rel-based links so clients need not hard code URIs.
Retain old links as long as possible. Even if you have to change the URI, keep the rels so that older clients have a path to the newer features.
Use links, not payload data, to instruct clients when various state-driving operations are available.

It may appear to be a bit of effort to build up RepresentationModelAssembler implementations for each resource type and to use these components in all of your controllers. However, this extra bit of server-side setup (made easy thanks to Spring HATEOAS) can ensure the clients you control (and more importantly, those you do not control) can upgrade with ease as you evolve your API.

This concludes our tutorial on how to build RESTful services using Spring. Each section of this tutorial is managed as a separate subproject in a single github repo:

nonrest — Simple Spring MVC app with no hypermedia
rest — Spring MVC + Spring HATEOAS app with HAL representations of each resource
evolution — REST app where a field is evolved but old data is retained for backward compatibility
links — REST app where conditional links are used to signal valid state changes to clients

To view more examples of using Spring HATEOAS, see https://github.com/spring-projects/spring-hateoas-examples.

To do some more exploring, check out the following video by Spring teammate Oliver Drotbohm:

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