结合DDD和Spring Boot实现基于REST API的并发控制 -DZone Java

@Entity @Table(name = "book")
class BookEntity {
  //... 
  @Version
  private long version;
  //...
} 

public class Version {
  private final long value;
  private Version(long value) {
    this.value = value;
  }
  public static Version from(long value) {
    return new Version(value);
  }
  public long asLong() {
    return value;
  }
} 

public interface Book { 
  //...
  Version version()
}

public class StaleStateIdentified extends RuntimeException {
  private StaleStateIdentified(UUID id) {     
    super(String.format("Aggregate of id %s is stale", id));
  }
  public static StaleStateIdentified forAggregateWith(UUID id) {     
    return new StaleStateIdentified(id);
  }
}

@Component
class JpaBasedBookRepository implements BookRepository {
    private final JpaBookRepository jpaBookRepository;
    //constructor + other methods
    @Override
    public void save(Book book) {
        try {
            BookEntity entity = BookEntity.from(book);
            jpaBookRepository.save(entity);
        } catch (OptimisticLockingFailureException ex) {
            throw StaleStateIdentified.forAggregateWith(book.id().getValue());
        }
    }
}
interface JpaBookRepository extends Repository<BookEntity, UUID> {
    void save(BookEntity bookEntity);
}

409 (Conflict)状态代码表示请求无法
完成，因为与目标的当前状态发生冲突
的资源。此代码用于用户可能
能够解决冲突并重新提交请求的情况。服务器
应该产生一个有效载荷，该载荷包括足够的信息供
用户识别冲突的根源。

@Test
public void shouldSignalConflict() throws Exception {
  //given
  AvailableBook availableBook = availableBookInTheSystem();
  //and
  BookView book = api.viewBookWith(availableBook.id());
  //and
  AvailableBook updatedBook = bookWasModifiedInTheMeantime(bookIdFrom(book.getId()));
  //when Bruce places book on hold
  PatronId bruce = somePatronId();
  ResultActions bruceResult =  api.sendPlaceOnHoldCommandFor(book.getId(), bruce,
        book.getVersion());
  //then
  bruceResult
      .andExpect(status().isConflict())
      .andExpect(jsonPath("$.id").value(updatedBook.id().asString()))
      .andExpect(jsonPath("$.title").value(updatedBook.title().asString()))
      .andExpect(jsonPath("$.isbn").value(updatedBook.isbn().asString()))
      .andExpect(jsonPath("$.author").value(updatedBook.author().asString()))
      .andExpect(jsonPath("$.status").value("AVAILABLE"))
      .andExpect(jsonPath("$.version").value(not(updatedBook.version().asLong())));
}

@RestController
@RequestMapping("/books")
class BookHoldingController {
  private final PlacingOnHold placingOnHold;
  BookHoldingController(PlacingOnHold placingOnHold) {
    this.placingOnHold = placingOnHold;
  }
  @PatchMapping("/{bookId}")
  ResponseEntity updateBookStatus(@PathVariable("bookId") UUID bookId,
                                  @RequestBody UpdateBookStatus command) {
    if (PLACED_ON_HOLD.equals(command.getStatus())) {
        PlaceOnHoldCommand placeOnHoldCommand =
            new PlaceOnHoldCommand(BookId.of(bookId), command.patronId(), command.version());
        Result result = placingOnHold.handle(placeOnHoldCommand);
        return buildResponseFrom(result);
    } else {
        return ResponseEntity.ok().build(); //we do not care about it now
    }
  }
  private ResponseEntity buildResponseFrom(Result result) {
    if (result instanceof BookPlacedOnHold) {
        return ResponseEntity.noContent().build();
    } else if (result instanceof BookNotFound) {
        return ResponseEntity.notFound().build();
    } else if (result instanceof BookConflictIdentified) {
        return ResponseEntity.status(HttpStatus.CONFLICT)
                .body(((BookConflictIdentified) result)
                        .currentState()
                        .map(BookView::from)
                        .orElse(null));
    } else {
        return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).build();
    }
  }
}

条件请求是HTTP请求[RFC7231]，其中包括一个或多个标头字段，这些字段指示在将方法语义应用于目标资源之前要测试的前提条件。