如何从外部模块重写Angular 2提供程序

import { Injectable } from '@angular/core';

@Injectable()
export class MyService {
  // 实现自定义的服务逻辑
}

import { NgModule } from '@angular/core';
import { MyService } from './my-service';

@NgModule({
  providers: [
    { provide: ExternalService, useClass: MyService }
  ]
})
export class AppModule { }

import { Component } from '@angular/core';
import { ExternalService } from 'external-module';

@Component({
  selector: 'app-root',
  template: `
    <h1>{{ message }}</h1>
  `
})
export class AppComponent {
  constructor(private externalService: ExternalService) { }

  get message() {
    return this.externalService.getMessage();
  }
}