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RESTにおけるHATEOASの原則

Hypermedia as the Engine of Application State（HATEOAS）を理解し、REST APIに適用します。

「RESTにおけるHATEOASの原則」はCoddyKit上の無料Spring Boot 4 Microservices & REST APIsレッスンです。 これはレッスン3/3です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはSpring Boot 4 Microservices & REST APIs学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Spring Boot 4 Microservices & REST APIsコースには全3レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

HATEOAS Unpacked

HATEOAS stands for Hypermedia as the Engine of Application State. It's a core constraint of RESTful architecture that suggests an API should guide clients through available actions using hypermedia links.

Think of it like a website: you navigate by clicking links, not by typing URLs from memory. HATEOAS brings this same idea to APIs.

Benefits of HATEOAS

HATEOAS makes your API more discoverable and evolvable. Instead of hardcoding URLs, clients discover available actions directly from the API's responses.

  • Client Independence: Clients don't need to know URL structures beforehand.
  • API Evolution: You can change URL paths without breaking clients, as long as the link relation names ("rel") remain consistent.
  • Self-Documentation: Responses inherently describe what actions can be taken next.

Hypermedia as State Engine

The 'Engine of Application State' means that the client transitions between application states (e.g., from viewing a user to viewing their orders) by selecting links within the hypermedia response.

The API response doesn't just return data; it returns data PLUS instructions (links) on what you can do with that data or where you can go next.

Introducing Spring HATEOAS

Implementing HATEOAS manually can be verbose. Thankfully, Spring Boot provides the Spring HATEOAS library to simplify adding links to your REST resources.

It offers classes like EntityModel, CollectionModel, and WebMvcLinkBuilder to make link creation intuitive and robust.

Your First Self-Link

Let's start by adding a 'self' link to a single user resource. This link points back to the resource itself, allowing clients to easily retrieve its current state.

We use EntityModel to wrap our data and WebMvcLinkBuilder to construct the link.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.hateoas.EntityModel;
import org.springframework.hateoas.server.mvc.WebMvcLinkBuilder;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.linkTo;
import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.methodOn;

@SpringBootApplication
public class SelfLinkApp {
    public static void main(String[] args) {
        SpringApplication.run(SelfLinkApp.class, args);
    }
}

class User {
    private Long id;
    private String name;
    public User(Long id, String name) {
        this.id = id; this.name = name;
    }
    public Long getId() { return id; }
    public String getName() { return name; }
    public void setId(Long id) { this.id = id; }
    public void setName(String name) { this.name = name; }
}

@RestController
class UserController {
    @GetMapping("/users/{id}")
    public EntityModel<User> getUser(@PathVariable Long id) {
        User user = new User(id, "Alice"); // Dummy user
        EntityModel<User> resource = EntityModel.of(user);

        // Add a "self" link
        resource.add(linkTo(methodOn(this.getClass()).getUser(id))
                        .withSelfRel());
        return resource;
    }
}

`EntityModel` & `Link` Deep Dive

When you use Spring HATEOAS, your API responses will typically return objects like EntityModel or CollectionModel.

  • EntityModel<T>: A wrapper for a single domain object (like our User), allowing you to add links to it.
  • Link: Represents a hyperlink, containing a URI and a relation name (e.g., 'self', 'orders').

The client then parses these links to navigate the API.

Linking to Related Resources

Beyond 'self' links, you can add links to related resources. For instance, a user resource might include a link to their orders.

We use withRel("relationName") to define the relationship between the current resource and the linked resource.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.hateoas.EntityModel;
import org.springframework.hateoas.server.mvc.WebMvcLinkBuilder;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.linkTo;
import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.methodOn;

@SpringBootApplication
public class RelatedLinkApp {
    public static void main(String[] args) {
        SpringApplication.run(RelatedLinkApp.class, args);
    }
}

class User { // Re-using User class
    private Long id; private String name;
    public User(Long id, String name) {
        this.id = id; this.name = name;
    }
    public Long getId() { return id; }
    public String getName() { return name; }
    public void setId(Long id) { this.id = id; }
    public void setName(String name) { this.name = name; }
}

@RestController
class UserController {
    @GetMapping("/users/{id}")
    public EntityModel<User> getUser(@PathVariable Long id) {
        User user = new User(id, "Bob");
        EntityModel<User> resource = EntityModel.of(user);

        resource.add(linkTo(methodOn(this.getClass()).getUser(id))
                        .withSelfRel());
        
        // Add link to user's orders
        resource.add(linkTo(methodOn(this.getClass()).getUserOrders(id))
                        .withRel("orders")); // Define relation
        return resource;
    }

    @GetMapping("/users/{id}/orders")
    public String getUserOrders(@PathVariable Long id) {
        return "Orders for user " + id; // Dummy endpoint
    }
}

`WebMvcLinkBuilder` Magic

WebMvcLinkBuilder is crucial for HATEOAS in Spring. It allows you to create links by directly referencing your controller methods, rather than hardcoding URL strings.

This makes your links type-safe and automatically updates them if you refactor your controller's path or method names, making your API more robust.

HATEOAS for Collections

When returning a list of items, you should use CollectionModel. Each item in the collection can have its own self-link, and the collection itself can have a self-link (e.g., to the list endpoint).

This allows clients to navigate to individual items from a list, or to refresh the entire collection.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.hateoas.CollectionModel;
import org.springframework.hateoas.EntityModel;
import org.springframework.hateoas.server.mvc.WebMvcLinkBuilder;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

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

import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.linkTo;
import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.methodOn;

@SpringBootApplication
public class CollectionHateoasApp {
    public static void main(String[] args) {
        SpringApplication.run(CollectionHateoasApp.class, args);
    }
}

class User { // Re-using User class
    private Long id; private String name;
    public User(Long id, String name) {
        this.id = id; this.name = name;
    }
    public Long getId() { return id; }
    public String getName() { return name; }
    public void setId(Long id) { this.id = id; }
    public void setName(String name) { this.name = name; }
}

@RestController
class UserController {
    private List<User> users = Arrays.asList(
        new User(1L, "Charlie"),
        new User(2L, "David")
    );

    @GetMapping("/users")
    public CollectionModel<EntityModel<User>> getAllUsers() {
        List<EntityModel<User>> userResources = users.stream()
            .map(user -> EntityModel.of(user,
                linkTo(methodOn(this.getClass()).getUser(user.getId()))
                    .withSelfRel()))
            .collect(Collectors.toList());

        return CollectionModel.of(userResources,
            linkTo(methodOn(this.getClass()).getAllUsers())
                .withSelfRel()); // Link for the collection
    }

    // Need a getUser method for the link builder to reference
    @GetMapping("/users/{id}")
    public EntityModel<User> getUser(@PathVariable Long id) {
        return EntityModel.of(new User(id, "Dummy"),
            linkTo(methodOn(this.getClass()).getUser(id)).withSelfRel());
    }
}

HATEOAS Quick Check

Which of the following are key benefits of applying HATEOAS principles to a REST API?

HATEOAS Recap & Next

Great job! You've learned about HATEOAS and how it makes REST APIs more discoverable and evolvable by embedding hypermedia links in responses.

  • HATEOAS: Hypermedia as the Engine of Application State.
  • Spring HATEOAS: Provides tools like EntityModel, CollectionModel, and WebMvcLinkBuilder.
  • Links: Guide clients through API interactions, reducing hardcoding.

By applying HATEOAS, you create truly RESTful APIs that are more robust and easier for clients to consume over time.

よくある質問

「RESTにおけるHATEOASの原則」レッスンは無料ですか？

はい。「RESTにおけるHATEOASの原則」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Spring Boot 4 Microservices & REST APIsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Spring Boot 4 Microservices & REST APIsコースには全3レッスンが含まれています。

「RESTにおけるHATEOASの原則」で何を学びますか？

Hypermedia as the Engine of Application State（HATEOAS）を理解し、REST APIに適用します。 ブラウザで直接実行するハンズオンコードでSpring Boot 4 Microservices & REST APIsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Spring Boot 4 Microservices & REST APIsを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのSpring Boot 4 Microservices & REST APIsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン3/3です。

「RESTにおけるHATEOASの原則」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

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