由于其他服务注入，服务无法找到依赖项-电脑我帮您

由于其他服务注入，服务无法找到依赖项

我有一个NestJs应用程序，在我对另外两个服务（PmsConnectionService-> UserService）进行依赖注入后，先前运行的服务会引发错误（ReviewService找不到DoctorLoaderService）。所有这些都来自单独的模块。

我的nestjs版本是5.1.0。PmsConnectionService是我新创建的模块的一部分。我不得不使用forwardRef连接到拥有DoctorLoaderService的模块。注意，在下面的代码中，PmsConnectionService注入了DoctorClinicService，后者正在注入DoctorLoaderService。当我在PmsConnectionService中注释掉DoctorClinicService时，错误消失了。但是我仍然不知道为什么错误首先发生。

服务

@Injectable()
export class PmsConnectionService {
    constructor(
        private clinicService: DoctorClinicService,
    ) {}

@Injectable()
export class UserService {
  constructor(
    private readonly connectionService: PmsConnectionService,
  ) { }

@Injectable()
export class DoctorLoaderService {
  constructor(
    @Inject(forwardRef(() => ReviewService))
    private readonly reviewService: ReviewService,
  ) {}

@Injectable()
export class ReviewService {
  constructor(
    private readonly doctorLoader: DoctorLoaderService,
    private readonly reviewManager: ReviewManagerService,
  ) { }

模块：

@Module({
    imports: [
        forwardRef(() => DoctorModule),
        TypeOrmModule.forFeature([
            ...
          ]),
    ],
    providers: [
        PmsConnectionService,
    ],
    exports: [
        PmsConnectionService,
    ],
})
export class PmsModule {}

@Module({
  imports: [
    PmsModule,
    TypeOrmModule.forFeature([
      User,
      ...
    ]),
  ],
  controllers: [
    UserController,
  ],
  providers: [
    UserService,
  ],
  exports: [UserService],
})
export class UserModule {
  public configure(consumer: MiddlewareConsumer) {
    consumer
      .apply(ActivityMiddleware)
      .exclude({
        path: '*',
        method: RequestMethod.OPTIONS,
      })
      .forRoutes({ path: '*', method: RequestMethod.ALL });
  }
}

@Module({
  imports: [
    UserModule,
    TypeOrmModule.forFeature([
      User,
      DoctorProfile,
      DoctorClinic,
      ...
    ]),
    forwardRef(() => BookingModule),
  ],
  controllers: [
    DoctorProfileController,
  ],
  providers: [
    DoctorLoaderService,
    DoctorClinicService,
  ],
  exports: [
    DoctorClinicService,
    DoctorLoaderService,
  ],
})
export class DoctorModule { }

Module({
  imports: [
    PmsModule,
    UserModule,
    DoctorModule,
    TypeOrmModule.forFeature([
      ...
    ]),
  ],
  controllers: [
   ...
  ],
  providers: [
    ReviewService,
  ],
  exports: [
    ReviewService,
  ],
})
export class BookingModule { }

启动时，应用程序将引发以下错误：

[Nest] 17921   - 9/26/2019, 9:44:15 PM   [NestFactory] Starting Nest application...
[Nest] 17921   - 9/26/2019, 9:44:15 PM   [ExceptionHandler] Nest can't resolve dependencies of the ReviewService (?, ReviewManagerService, NotiIssuerService). Please make sure that the argument at index [0] is available in the current context. +90ms
Error: Nest can't resolve dependencies of the ReviewService (?, ReviewManagerService, NotiIssuerService). Please make sure that the argument at index [0] is available in the current context.
    at Injector.resolveSingleParam (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:115:19)
    at Promise.all.dependencies.map (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:85:49)
    at Array.map (<anonymous>)
    at Injector.resolveConstructorParams (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:83:58)
    at Injector.loadInstance (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:63:20)
    at Injector.loadInstanceOfComponent (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:40:20)
    at Promise.all.map (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/instance-loader.js:39:102)
    at Array.map (<anonymous>)
    at InstanceLoader.createInstancesOfComponents (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/instance-loader.js:39:59)
    at Promise.all.map (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/instance-loader.js:26:24)

尽管我认为问题出在PmsConnectionService与DoctorClinicService之间的连接中，但我不知道如何解决此问题。

回答如下：

似乎您只设置了一半的前向引用。由于ReviewService（BookingModule的一部分）取决于DoctorLoaderService（DoctorModule的一部分），而DoctorLoaderService取决于ReviewService，因此在模块和每个受影响阶层的服务。在这种情况下，forwardRef需要DocotrModule，forwardRef(() => BookingModule)需要BookingModule，以及具有相似forwardRef(() => DoctorModule)设置的ReviewService和DoctorLoadingService。

通常，最好避免这些循环依赖性，因为它们会造成难以调试的麻烦。也许看看体系结构是否有变化，可以使代码更简单/更少依赖其他部分。