【初心者向け】Spring BootのRestTemplateを使ったREST API通信の全知識

@Configuration
public class RestTemplateConfig {
    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

User user = restTemplate.getForObject(url, User.class);
// ↓ ここは、レスポンスが返ってきてから実行される
System.out.println(user.getName());

https://jsonplaceholder.typicode.com/users/1

{
  "id": 1,
  "name": "Leanne Graham",
  "email": "leanne@example.com"
}

public class User {
    private int id;
    private String name;
    private String email;

    // getter/setterを忘れずに（Lombokでも可）
    public int getId() { return id; }
    public void setId(int id) { this.id = id; }

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

    public String getEmail() { return email; }
    public void setEmail(String email) { this.email = email; }
}

import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;
import org.springframework.beans.factory.annotation.Autowired;

@Service
public class ApiService {

    @Autowired
    private RestTemplate restTemplate;

    public User fetchUserById(int id) {
        String url = "https://jsonplaceholder.typicode.com/users/" + id;
        return restTemplate.getForObject(url, User.class);
    }
}

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class UserController {

    private final ApiService apiService;

    public UserController(ApiService apiService) {
        this.apiService = apiService;
    }

    @GetMapping("/fetch-user/{id}")
    public User getUser(@PathVariable int id) {
        return apiService.fetchUserById(id);
    }
}

GET http://localhost:8080/fetch-user/1

{
  "id": 1,
  "name": "Leanne Graham",
  "email": "leanne@example.com"
}

String url = "https://jsonplaceholder.typicode.com/users/{id}";
User user = restTemplate.getForObject(url, User.class, 1);

try {
    User user = restTemplate.getForObject(url, User.class);
    return user;
} catch (HttpClientErrorException e) {
    System.out.println("クライアントエラー：" + e.getStatusCode());
} catch (ResourceAccessException e) {
    System.out.println("通信エラー：" + e.getMessage());
}

POST https://example.com/api/users

{
  "name": "Taro Yamada",
  "email": "taro@example.com"
}

public class User {
    private String name;
    private String email;

    // Getter/Setter
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }

    public String getEmail() { return email; }
    public void setEmail(String email) { this.email = email; }
}

public void createUser() {
    String url = "https://example.com/api/users";

    User user = new User();
    user.setName("Taro Yamada");
    user.setEmail("taro@example.com");

    // レスポンスとしてUserが返ってくる想定
    User createdUser = restTemplate.postForObject(url, user, User.class);

    System.out.println("登録成功！ID: " + createdUser.getId());
}

POST /api/users
Content-Type: application/json

{
  "name": "Taro Yamada",
  "email": "taro@example.com"
}

{
  "id": 123,
  "name": "Taro Yamada",
  "email": "taro@example.com"
}

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);

HttpEntity<User> entity = new HttpEntity<>(user, headers);

ResponseEntity<User> response = restTemplate.postForEntity(url, entity, User.class);

User created = response.getBody();

PUT https://example.com/api/users/1

public void updateUser(int id) {
    String url = "https://example.com/api/users/" + id;

    User user = new User();
    user.setName("Updated Name");
    user.setEmail("updated@example.com");

    restTemplate.put(url, user);
}

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);

HttpEntity<User> entity = new HttpEntity<>(user, headers);

restTemplate.exchange(url, HttpMethod.PUT, entity, Void.class);

DELETE https://example.com/api/users/1

public void deleteUser(int id) {
    String url = "https://example.com/api/users/" + id;
    restTemplate.delete(url);
}

try {
    restTemplate.put(url, user);  // or delete(url)
} catch (HttpClientErrorException e) {
    System.out.println("HTTPエラー: " + e.getStatusCode());
} catch (ResourceAccessException e) {
    System.out.println("通信エラー: " + e.getMessage());
}

GET /api/data
Authorization: Bearer xxxxxx-abcdef-12345

public User getUserWithAuth() {
    String url = "https://example.com/api/users/1";

    // 認証トークンの設定
    HttpHeaders headers = new HttpHeaders();
    headers.set("Authorization", "Bearer your-token-here");

    // ヘッダー込みのリクエストエンティティを作成
    HttpEntity<String> entity = new HttpEntity<>(headers);

    // exchangeを使ってGET実行
    ResponseEntity<User> response = restTemplate.exchange(
        url,
        HttpMethod.GET,
        entity,
        User.class
    );

    return response.getBody();
}

public void createUserWithAuth(User user) {
    String url = "https://example.com/api/users";

    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_JSON);
    headers.set("Authorization", "Bearer your-token-here");

    HttpEntity<User> entity = new HttpEntity<>(user, headers);

    ResponseEntity<User> response = restTemplate.exchange(
        url,
        HttpMethod.POST,
        entity,
        User.class
    );

    System.out.println("登録成功！ID: " + response.getBody().getId());
}

try {
    User user = restTemplate.getForObject(url, User.class);
    return user;
} catch (HttpClientErrorException e) {
    System.out.println("クライアントエラー：" + e.getStatusCode());
} catch (HttpServerErrorException e) {
    System.out.println("サーバーエラー：" + e.getStatusCode());
} catch (ResourceAccessException e) {
    System.out.println("接続エラー：" + e.getMessage());
}

catch (HttpClientErrorException e) {
    System.out.println("エラー内容: " + e.getResponseBodyAsString());
}

public User safeFetchUser(int id) {
    try {
        return restTemplate.getForObject("https://api.example.com/users/" + id, User.class);
    } catch (HttpClientErrorException.NotFound e) {
        System.out.println("ユーザーが見つかりませんでした");
        return null;
    } catch (Exception e) {
        System.out.println("予期せぬエラー: " + e.getMessage());
        return null;
    }
}

User user = restTemplate.getForObject(url1, User.class);
Post post = restTemplate.getForObject(url2, Post.class); // userを待つ

@Configuration
@EnableAsync
public class AsyncConfig {
}

@Service
public class ApiService {

    @Autowired
    private RestTemplate restTemplate;

    @Async
    public CompletableFuture<User> getUserAsync(int id) {
        String url = "https://example.com/api/users/" + id;
        User user = restTemplate.getForObject(url, User.class);
        return CompletableFuture.completedFuture(user);
    }

    @Async
    public CompletableFuture<Post> getPostAsync(int id) {
        String url = "https://example.com/api/posts/" + id;
        Post post = restTemplate.getForObject(url, Post.class);
        return CompletableFuture.completedFuture(post);
    }
}

@RestController
public class ApiController {

    @Autowired
    private ApiService apiService;

    @GetMapping("/parallel")
    public String callInParallel() throws Exception {
        CompletableFuture<User> userFuture = apiService.getUserAsync(1);
        CompletableFuture<Post> postFuture = apiService.getPostAsync(1);

        // 並列に実行されて、ここで全完了を待機
        CompletableFuture.allOf(userFuture, postFuture).join();

        User user = userFuture.get();
        Post post = postFuture.get();

        return "ユーザー: " + user.getName() + " / 投稿: " + post.getTitle();
    }
}

@Configuration
@EnableAsync
public class AsyncConfig {

    @Bean(name = "taskExecutor")
    public Executor taskExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(5);     // 最小スレッド数
        executor.setMaxPoolSize(10);     // 最大スレッド数
        executor.setQueueCapacity(25);   // キューの長さ
        executor.setThreadNamePrefix("ApiThread-");
        executor.initialize();
        return executor;
    }
}

@Async("taskExecutor")
public CompletableFuture<User> getUserAsync(int id) {
    ...
}

WebClient webClient = WebClient.create();

User user = webClient.get()
        .uri("https://example.com/api/users/1")
        .retrieve()
        .bodyToMono(User.class)
        .block(); // ブロッキングで待機（同期的に使う）

User user = new User("Taro", "taro@example.com");

User response = WebClient.create()
        .post()
        .uri("https://example.com/api/users")
        .header("Authorization", "Bearer token")
        .bodyValue(user)
        .retrieve()
        .bodyToMono(User.class)
        .block();