java - 单例如何隐藏其依赖关系？

Question

最近我读了the blog post关于单例的缺点，作者指出:

Why is Singleton evil?

1. Hides dependencies – A component that uses one or more singletons is hiding crucial information about your dependencies. It doesn’t take long for calls to a singleton to creep through your code base like kudzu, slowly attaching itself to every class in the system. Exposing that dependency forces you to think about it as you use a component. It also makes it more reusable as the caller can understand its requirements and how they might be satisfied.

单例究竟是如何隐藏其依赖关系的，这意味着什么？首先我想到了无法注入(inject)依赖项，但我们仍然有 setter 注入(inject)，所以它没有意义。

请尽可能提供代码示例。

Answer 1

1 单例可以有一个setter，但它有一个全局的状态。因此，您可以在应用程序的一个地方设置一个对象，它会影响应用程序另一个地方的行为。

2 当你创建一个对象时，你不知道它的依赖关系，因为构造函数没有它们，你只需要编写 new MyObject();

但是里面有另一个对象的依赖关系。而且您对依赖对象状态一无所知。

public class MyObject {

     private AnotherObject anotherObject = Singleton.getInstance(); 

     public MyObject() {
         //nothing here 
     }
}

相反，这样写更有用:

public class MyObject {

    private AnotherObject anotherObject;

    public MyObject(AnotherObject comesFromOutside) {
        this.anotherObject = comesFromOutside;
    }
}

然后您的代码的用户将立即看到您的所有依赖项，而无需查看您的源代码，只需编写 new MyObject(new AnotherObject())

这里有更多信息: http://www.yegor256.com/2016/06/27/singletons-must-die.html