Angular依赖注入的一个常见错误NullInjectorError,No provider for XXX

测试代码：

export abstract class GreetingService {
    abstract greet(name: string): string;
 }

加了@Injectable注解的实现类：

import { Injectable } from '@angular/core';
import { GreetingService } from './greeting.service';

@Injectable({ providedIn: 'root'})
export class EnglishGreetingService extends GreetingService {
   greet(name: string): string {
      return 'Hello ' + name;
   }
   constructor(){
      super();
      console.log('English class created!');
   }
}

试图通过构造函数参数的方式注入：

错误消息：

core.js:6241 ERROR NullInjectorError: R3InjectorError(AppModule)[GreetingService -> GreetingService -> GreetingService]: 
  NullInjectorError: No provider for GreetingService!
    at NullInjector.get (http://localhost:4201/vendor.js:8310:27)
    at R3Injector.get (http://localhost:4201/vendor.js:22317:33)
    at R3Injector.get (http://localhost:4201/vendor.js:22317:33)
    at R3Injector.get (http://localhost:4201/vendor.js:22317:33)
    at NgModuleRef$1.get (http://localhost:4201/vendor.js:39618:33)
    at Object.get (http://localhost:4201/vendor.js:37352:35)
    at getOrCreateInjectable (http://localhost:4201/vendor.js:12112:39)
    at Module.ɵɵdirectiveInject (http://localhost:4201/vendor.js:26132:12)
    at NodeInjectorFactory.AppComponent_Factory [as factory] (http://localhost:4201/main.js:122:289)
    at getNodeInjectable (http://localhost:4201/vendor.js:12257:44)

解决办法

在module的providers区域里，为GreetingService维护具体的实现类：

providers: [{ provide: JerrySandBoxService },
  { provide: GreetingService, useClass: EnglishGreetingService}]

问题即可解决：

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