为了在Jest中成功测试一个service，需要用到Jest的mock功能，将该service的构造函数mock掉。下面是示例代码：

// 实际的service代码
@Injectable()
export class MyService {
  constructor() {}
  getSomething(): Observable {
    // get something
  }
}

// 在测试中，将构造函数mock掉
const mockService = {
  getSomething: jest.fn().mockReturnValue(of({}))
};

describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ MyComponent ],
      providers: [
        { provide: MyService, useValue: mockService } // 将service替换成mockService
      ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should create', () => {
    expect(component).toBeTruthy();
  });
});

在上面的示例中，我们将MyService中的构造函数mock掉，并把mockService注入到了测试中。这样，当测试中需要调用MyService中的getSomething方法时，会调用mockService中的方法，而不会出现构造函数相关的问题。