如何在TypeScript的Controller构造函数中注入Service？

我正在用TypeScript开发一个简单的REST API，调用层级为：Controller > Service > Repository。我尝试了几种写法，但前两种总会报错，只有第三种能正常运行，想请教问题出在哪，最初的写法是否可行？

第一种写法（报错） #

Controller #

export class GetNurseController {
  constructor(private getNurseService: GetNurseService) {}

  async handle(req: Request, res: Response): Promise<Response> {
    try {
      const { id } = req.authrocket.currentUser;
      const user = await this.getNurseService.execute(id);

      return res.json({ user });
    } catch (err: any) {
      return res.status(500).json({ err });
      // 返回空err对象，但控制台打印getNurseService未定义错误
    }
  }
}

Router #

const nurseRepository = new NurseRepository();
const getNurseService = new GetNurseService(nurseRepository);
const getNurseController = new GetNurseController(getNurseService);

const nurseRoutes = Router();
nurseRoutes.get('/', requireLogin, getNurseController.handle);

第二种写法（报错） #

Controller #

export class GetNurseController {
  public NurseRepository: INurseRepository;
  public getNurseService: GetNurseService;

  constructor() {
    this.nurseRepository = new NurseRepository();
    this.getNurseService = new GetNurseService(this.nurseRepository);
  }

  async handle(req: Request, res: Response): Promise<Response> {
    try {
      const { id } = req.authrocket.currentUser;
      const user = await this.getNurseService.execute(id);

      return res.json({ user });
    } catch (err: any) {
      return res.status(500).json({ err });
    }
  }
}

Router #

const getNurseController = new GetNurseController();

const nurseRoutes = Router();
nurseRoutes.get('/', requireLogin, getNurseController.handle);

第三种写法（正常运行） #

Controller #

export class GetNurseController {
  async handle(req: Request, res: Response): Promise<Response> {
    try {
      const nurseRepository = new NurseRepository();
      const getNurseService = new GetNurseService(nurseRepository);

      const { id } = req.authrocket.currentUser;
      const user = await getNurseService.execute(id);

      return res.json({ user });
    } catch (err: any) {
      return res.status(500).json({ err });
    }
  }
}

Router #

const getNurseController = new GetNurseController();

const nurseRoutes = Router();
nurseRoutes.get('/', requireLogin, getNurseController.handle);

1. 第一种写法的核心问题：this指向丢失 #

当你把getNurseController.handle直接作为路由处理函数传递时，handle方法内部的this不再指向GetNurseController实例，而是变成了路由回调的上下文（通常是req或undefined），所以会出现getNurseService未定义的错误。

修复方案（任选其一）： #

• 手动绑定this：在Router中传递方法时绑定实例上下文：

  nurseRoutes.get('/', requireLogin, getNurseController.handle.bind(getNurseController));
  

• 用箭头函数包裹：通过箭头函数保留this指向：

  nurseRoutes.get('/', requireLogin, (req, res) => getNurseController.handle(req, res));
  

• 在类中用箭头函数定义方法：直接让方法的this绑定到实例：

  export class GetNurseController {
    constructor(private getNurseService: GetNurseService) {}
  
    handle = async (req: Request, res: Response): Promise<Response> => {
      try {
        const { id } = req.authrocket.currentUser;
        const user = await this.getNurseService.execute(id);
        return res.json({ user });
      } catch (err: any) {
        return res.status(500).json({ err });
      }
    }
  }
  

2. 第二种写法的问题：属性命名不一致 #

你在类中声明的属性是public NurseRepository: INurseRepository;（首字母大写），但构造函数中赋值的是this.nurseRepository = new NurseRepository();（首字母小写），导致this.getNurseService依赖的this.nurseRepository未定义，进而引发错误。

修复方案： #

统一属性命名的大小写：

export class GetNurseController {
  public nurseRepository: INurseRepository; // 改为小写开头
  public getNurseService: GetNurseService;

  constructor() {
    this.nurseRepository = new NurseRepository();
    this.getNurseService = new GetNurseService(this.nurseRepository);
  }

  // 同时需解决this指向问题，参考第一种写法的修复方案
  async handle(req: Request, res: Response): Promise<Response> {
    // ... 原有逻辑
  }
}

3. 关于最初写法的可行性 #

你的第一种写法完全可行且更符合依赖注入原则，它通过构造函数注入Service依赖，比第三种在方法内部实例化依赖的写法更利于测试和解耦。只要修复this指向的问题，就能正常运行。

第三种写法虽然能运行，但每次请求都会创建新的Repository和Service实例，存在性能损耗，且不利于单元测试（无法Mock依赖），不推荐长期使用。

内容的提问来源于stack exchange，提问作者Heitor Danilo