没有类型变量的实例U存在,以便Foo符合CompletionStage <U>

Question

我已经在这几个小时,但似乎无法解开这个.该错误与此段代码有关:

(这个问题底部的完整代码.我正在使用屏幕截图直观地显示问题.)

错误本身是:

没有类型变量的实例U存在,以便GetUsersForAdAccountResponse符合CompletionStage

在thenComposes 链的最开始,我擦除return并使用IntelliJ的"引入局部变量..."功能来查看整个链(直到并包括第1118行)返回的类型:

结果是

final CompletionStage<U> uCompletionStage = ...

但是你可以看到包含方法的返回类型是

public CompletionStage<GetUsersForAdAccountResponse> ...

什么阻止编译器推断GetUsersForAdAccountResponse？(再次,通常这里有一个return.)

我还尝试在每个过程中引入局部变量thenCompose,它们看起来都是正确的.每个产生一个CompletionStage<Foo>,下thenCompose一个Foo产生一个lambda期望并产生一个CompletionStage<Bar>,等等.(在一次代码重组中,我确实看到了嵌套,CompletionStage<CompletionStage<Foo>>但我认为这是我自己重写的一件神器.)

---

我不知道它是否会有所帮助,但这是整个方法:

  @Override
  public CompletionStage<GetUsersForAdAccountResponse> getUsersForAdAccount(
      RequestContext context, GetUsersForAdAccountRequest request) {

    Uuid adAccountId = request.getAdAccountId();

    return verifyAuthorization(context,
        PortcullisTemplates.Action.GET_USERS_FOR_AD_ACCOUNT.getName(),
        portcullisTemplates.topOrganizationResource())
        .thenCompose(auditLogPrincipal -> jdbiExecutor.executeInTransaction(handler -> {

              // We purposely safeguard the account lookup as well behind Portcullis.
              AdAccountDao adAccountDao = handler.attach(AdAccountDao.class);
              if (adAccountDao.getAdAccountById(adAccountId) == null) {
                throw new ValidationException(SERVICE_NAME,
                    "Ad account not found: " + UuidUtils.toString(adAccountId));
              }

              AdAccountRoleUserMappingDao roleDao = handler.attach(AdAccountRoleUserMappingDao.class);

              List<String> roleNames = request.getRoleNamesList();
              return roleNames.isEmpty() ?
                     roleDao.getAdAccountRoleUserMappingsByAdAccount(adAccountId) :
                     roleDao.getAdAccountRoleUserMappingsByAdAccountAndRoles(adAccountId, roleNames);

        })).thenCompose(adAccountRoleUserMappings -> jdbiExecutor.execute(UserDao.class, userDao -> {
          return userDao
              .getUsersBy]UserIds(
                  adAccountRoleUserMappings.stream()
                      .map(AdAccountRoleUserMapping::userId)
                      .collect(Collectors.toList())
              ).stream()
                  .collect(Collectors.toMap(
                      User::userId,
                      user -> new EncryptedFieldsBuilder()
                          .firstName(user.encryptedFirstName())
                          .lastName(user.encryptedLastName())
                          .email(user.encryptedEmail())
                          .build()
              ));
        }).thenCompose(
            userEncryptedFields -> padlockService.decryptUserAccounts(userEncryptedFields)
        ).thenCompose(decryptedUsers -> GetUsersForAdAccountResponse.newBuilder()
            .addAllUserWithRole(
                adAccountRoleUserMappings.stream()
                    .filter(mapping -> decryptedUsers.containsKey(mapping.userId()))
                    .map(mapping -> UserWithRole.newBuilder()
                        .setAccount(decryptedUsers.get(mapping.userId()))
                        .setRoleName(mapping.roleName())
                        .build())
                    .collect(Collectors.toSet())
            ).build()
        ));
  }

Answer 1

thenCompose链中的最后一个应该是thenApply. 传递给它的函数正在返回一个GetUsersForAdAccountResponse不是CompletionStage。

thenCompose是flatMap的CompletableFuture。它需要一个返回 a 的函数并将CompletionStage结果展平。