Calling REST Services with RestClient and HTTP Interfaces

//StoreApp.java
package com.javabrahman.rest.client;

import java.net.URI;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.http.HttpStatus;
import org.springframework.http.HttpStatusCode;
import org.springframework.http.ProblemDetail;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ExceptionHandler;
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.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.RestControllerAdvice;
import org.springframework.web.client.HttpClientErrorException;
import org.springframework.web.client.ResourceAccessException;
import org.springframework.web.client.RestClient;
import org.springframework.web.service.annotation.GetExchange;
import org.springframework.web.service.annotation.HttpExchange;
import org.springframework.web.service.annotation.PostExchange;
import org.springframework.web.service.registry.ImportHttpServices;
import org.springframework.web.servlet.support.ServletUriComponentsBuilder;

record Stock(String isbn, int quantity, String warehouse) {
}

record Reservation(String isbn, int quantity, String reservationId) {
}

// ---- The "remote" inventory service ----
@RestController
@RequestMapping("/inventory")
class InventoryController {

  @GetMapping("/{isbn}")
  ResponseEntity<Stock> stock(@PathVariable String isbn) {
    return isbn.startsWith("978")
        ? ResponseEntity.ok(new Stock(isbn, 14, "Leeds"))
        : ResponseEntity.notFound().build();
  }

  @PostMapping("/{isbn}/reservations")
  ResponseEntity<Reservation> reserve(@PathVariable String isbn, @RequestParam int quantity) {
    Reservation r = new Reservation(isbn, quantity, "R-1001");
    URI location = ServletUriComponentsBuilder.fromCurrentRequestUri()   // the request URL without its query string
        .path("/{id}").buildAndExpand(r.reservationId()).toUri();
    return ResponseEntity.created(location).body(r);
  }

  @GetMapping("/slow/{isbn}")
  Stock slow(@PathVariable String isbn) throws InterruptedException {
    Thread.sleep(3000);   // a service having a bad day
    return new Stock(isbn, 14, "Leeds");
  }
}

// ---- The store, version 1: RestClient ----
class InventoryNotFoundException extends RuntimeException {
  InventoryNotFoundException(String isbn) {
    super("The inventory service has no record of ISBN " + isbn);
  }
}

@RestController
@RequestMapping("/api/store")
class StoreController {
  private final RestClient inventory;

  StoreController(RestClient.Builder builder, @Value("${inventory.url}") String inventoryUrl) {
    this.inventory = builder.baseUrl(inventoryUrl).build();
  }

  @GetMapping("/books/{isbn}/stock")
  String stock(@PathVariable String isbn) {
    Stock stock = inventory.get()
        .uri("/inventory/{isbn}", isbn)
        .retrieve()
        .onStatus(status -> status.value() == 404, (request, response) -> {
          throw new InventoryNotFoundException(isbn);
        })
        .body(Stock.class);
    return stock.quantity() + " copies in " + stock.warehouse();
  }

  @PostMapping("/books/{isbn}/reserve")
  String reserve(@PathVariable String isbn, @RequestParam int quantity) {
    ResponseEntity<Reservation> response = inventory.post()
        .uri(uriBuilder -> uriBuilder.path("/inventory/{isbn}/reservations").queryParam("quantity", quantity).build(isbn))
        .retrieve()
        .toEntity(Reservation.class);
    return "inventory answered " + response.getStatusCode().value() + ", reservation " + response.getBody().reservationId()
        + " at " + response.getHeaders().getLocation();
  }

  @GetMapping("/slow/{isbn}")
  Stock slow(@PathVariable String isbn) {
    return inventory.get().uri("/inventory/slow/{isbn}", isbn).retrieve().body(Stock.class);
  }
}

// ---- The store, version 2: an HTTP interface client ----
@HttpExchange("/inventory")
interface InventoryClient {
  @GetExchange("/{isbn}")
  Stock stock(@PathVariable String isbn);

  @PostExchange("/{isbn}/reservations")
  ResponseEntity<Reservation> reserve(@PathVariable String isbn, @RequestParam int quantity);
}

@RestController
@RequestMapping("/api/store/v2")
class StoreControllerV2 {
  private final InventoryClient inventory;

  StoreControllerV2(InventoryClient inventory) {
    this.inventory = inventory;
  }

  @GetMapping("/books/{isbn}/stock")
  String stock(@PathVariable String isbn) {
    Stock stock = inventory.stock(isbn);
    return stock.quantity() + " copies in " + stock.warehouse() + " (via the HTTP interface)";
  }

  @PostMapping("/books/{isbn}/reserve")
  String reserve(@PathVariable String isbn, @RequestParam int quantity) {
    ResponseEntity<Reservation> response = inventory.reserve(isbn, quantity);
    return "reservation " + response.getBody().reservationId() + " at " + response.getHeaders().getLocation();
  }
}

// ---- Turning remote failures into problems for our own clients ----
@RestControllerAdvice
class RemoteErrorHandler {
  @ExceptionHandler(InventoryNotFoundException.class)
  ProblemDetail notFound(InventoryNotFoundException e) {
    return ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, e.getMessage());
  }

  @ExceptionHandler(HttpClientErrorException.NotFound.class)
  ProblemDetail remoteNotFound(HttpClientErrorException.NotFound e) {
    return ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, "The inventory service answered 404");
  }

  @ExceptionHandler(ResourceAccessException.class)
  ProblemDetail unavailable(ResourceAccessException e) {
    return ProblemDetail.forStatusAndDetail(HttpStatusCode.valueOf(503),
        "The inventory service did not answer in time. Please try again later.");
  }
}

@SpringBootApplication
@ImportHttpServices(group = "inventory", types = InventoryClient.class)
public class StoreApp {
  public static void main(String[] args) {
    SpringApplication.run(StoreApp.class, args);
  }
}

inventory.url=http://localhost:8080
spring.http.clients.read-timeout=1s
spring.http.serviceclient.inventory.base-url=http://localhost:8080
spring.mvc.problemdetails.enabled=true