这两种模式似乎都是控制反转原理的实现。也就是说,一个对象不应该知道如何构造它的依赖项。

依赖注入(DI)似乎使用构造函数或setter来“注入”它的依赖项。

使用构造函数注入的例子:

//Foo Needs an IBar
public class Foo
{
  private IBar bar;

  public Foo(IBar bar)
  {
    this.bar = bar;
  }

  //...
}

Service Locator似乎使用了一个“容器”,它连接了它的依赖项并给了foo它的bar。

使用Service Locator的例子:

//Foo Needs an IBar
public class Foo
{
  private IBar bar;

  public Foo()
  {
    this.bar = Container.Get<IBar>();
  }

  //...
}

因为我们的依赖关系只是对象本身,这些依赖关系有依赖关系,依赖关系有更多依赖关系,等等。因此,控制反转容器(或DI容器)诞生了。例如:Castle Windsor, Ninject, Structure Map, Spring等)

但是IOC/DI容器看起来完全像服务定位器。称它为DI容器是一个坏名字吗?IOC/DI容器只是另一种类型的服务定位器吗?当我们有很多依赖时,我们使用依赖注入容器,这是一个细微的差别吗?


当前回答

In this oversimplified case there is no difference and they can be used interchangeably. However, real world problems are not as simple. Just assume that the Bar class itself had another dependency named D. In that case your service locator wouldn't be able to resolve that dependency and you would have to instantiate it within the D class; because it is the responsibility of your classes to instantiate their dependencies. It would even get worse if the D class itself had other dependencies and in real-world situations it usually gets even more complicated than that. In such scenarios DI is a better solution than ServiceLocator.

其他回答

它们都是IoC的实现技术。还有其他实现控制反转的模式:

工厂模式 服务定位器 DI (IoC)容器 依赖注入 (构造函数注入,参数注入(如果不需要),接口注入的setter注入) ...

服务定位器和DI容器看起来更相似,它们都使用容器来定义依赖关系,将抽象映射到具体实现。

主要的区别是依赖关系是如何定位的,在服务定位器中,客户端代码请求依赖关系,在DI容器中,我们使用容器来创建所有对象,并将依赖关系作为构造函数参数(或属性)注入。

当您使用服务定位器时,每个类都依赖于您的服务定位器。依赖注入不是这样的。依赖注入器通常只在启动时被调用一次,以便将依赖注入到一些主类中。这个主类所依赖的类将递归地注入它们的依赖项,直到您有一个完整的对象图。

一个很好的对比:http://martinfowler.com/articles/injection.html

如果你的依赖注入器看起来像服务定位器,类直接调用注入器,那么它可能不是依赖注入器,而是服务定位器。

The difference may seem slight, but even with the ServiceLocator, the class is still responsible for creating its dependencies. It just uses the service locator to do it. With DI, the class is given its dependencies. It neither knows, nor cares where they come from. One important result of this is that the DI example is much easier to unit test -- because you can pass it mock implementations of its dependent objects. You could combine the two -- and inject the service locator (or a factory), if you wanted.

服务定位器隐藏了依赖关系——例如,当一个对象从定位器获取连接时,你不能通过观察对象来判断它是否访问了数据库。使用依赖注入(至少是构造函数注入),依赖关系是显式的。

此外,服务定位器打破了封装,因为它们提供了对其他对象的依赖关系的全局访问点。对于service locator,和任何单例一样:

很难指定前后 客户端对象的条件 接口,因为它的工作方式 可以对实现进行干预 从外面。

使用依赖注入,一旦指定了对象的依赖项,它们就处于对象本身的控制之下。

A class using constructor DI indicates to consuming code that there are dependencies to be satisfied. If the class uses the SL internally to retrieve such dependencies, the consuming code is not aware of the dependencies. This may on the surface seem better, but it is actually helpful to know of any explicit dependencies. It is better from an architectural view. And when doing testing, you have to know whether a class needs certain dependencies, and configure the SL to provide appropriate fake versions of those dependencies. With DI, just pass in the fakes. Not a huge difference, but it is there.

不过,DI和SL可以一起工作。为常见的依赖项(如设置、记录器等)设置一个中心位置是很有用的。给定一个使用这种deps的类,您可以创建一个接收deps的“真实”构造函数,以及一个从SL检索并转发给“真实”构造函数的默认(无参数)构造函数。

EDIT:当然,当您使用SL时,您将向该组件引入一些耦合。这是具有讽刺意味的,因为这种功能的思想是鼓励抽象和减少耦合。这些关注点是可以平衡的,这取决于您需要在多少地方使用SL。如果按照上面建议的那样做,只是在默认的类构造函数中。