[squeaky-clean]

> because there’s only an @Inject annotation without any further information

It sounds like you have a gripe with a particular DI framework and not the idea of Dependency Injection. Because

> Normally, when you instantiate a class, the resulting object’s behavior only depends on the constructor arguments you pass it (= the behavior is purely a function of the arguments)

With Dependency Injection this is generally still true, even more so than normal because you're making the constructor's dependencies explicit in the arguments. If you have a class CriticalErrorLogger(), you can't directly tell where it logs to, is it using a flat file or stdout or a network logger? If you instead have a class CriticalErrorLogger(logger *io.writer), then when you create it you know exactly what it's using to log because you had to instantiate it and pass it in.

Or like Kortilla said, instead of passing in a class or struct you can pass in a function, so using the same example, something like CriticalErrorLogger(fn write)

[layer8]

I don't quite understand your example, but I don't think the particulars make much of a difference. We can go with the most general description: With dependency injection, you define points in your code where dependencies are injected. The injection point is usually a variable (this includes the case of constructor parameters), whose value (the dependency) will be set by the dependency injection framework. The behavior of the code that reads the variable and hence the injected value will then depend on the specific value that was injected.

My issue with that is this: From the point of view of the code accessing the injected value (and from the point of view of that code's callers), the value appears like out of thin air. There is no way to trace back from that code where the value came from. Similarly, when defining which value will be injected, it can be difficult to trace all the places where it will be injected.

In addition, there are often lifetime issues involved, when the injected value is itself a stateful object, or may indirectly depend on mutable, cached, or lazy-initialized, possibly external state. The time when the value's internal state is initialized or modified, or whether or not it is shared between separate injection points, is something that can't be deduced from the source code containing the injection points, but is often relevant for behavior, error handling, and general reasoning about the code.

All of this makes it more difficult to reason about the injected values, and about the code whose behavior will depend on those values, from looking at the source code.

[squeaky-clean]

> whose value (the dependency) will be set by the dependency injection framework

I agree with your definition except for this part, you don't need any framework to do dependency injection. It's simply the idea that instead of having an abstract base class CriticalErrorLogger, with the concrete implementations of StdOutCriticalErrorLogger, FileCriticalErrorLogger, AwsCloudwatchCriticalErrorLogger which bake their dependency into the class design; you instead have a concrete class CriticalErrorLogger(dep *dependency) and create dependency objects externally that implement identical interfaces in different ways. You do text formatting, generating a traceback, etc, and then call dep.write(myFormattedLogString), and the dependency handles whatever that means.

I agree with you that most DI frameworks are too clever and hide too much, and some forms of DI like setter injection and reflection based injection are instant spaghetti code generators. But things like Constructor Injection or Method Injection are so simple they often feel obvious and not like Dependency Injection even though they are. I love DI, but I hate DI frameworks; I've never seen a benefit except for retrofitting legacy code with DI.

And yeah it does add the issue or lifetime management. That's an easy place to F things up in your code using DI and requires careful thought in some circumstances. I can't argue against that.

But DI doesn't need frameworks or magic methods or attributes to work. And there's a lot of situations where DI reduces code duplication, makes refactoring and testing easier, and actually makes code feel less magical than using internal dependencies.

The basic principle is much simpler than most DI frameworks make it seem. Instead of initializing a dependency internally, receive the dependency in some way. It can be through overly abstracted layers or magic methods, but it can also be as simple as adding an argument to the constructor or a given method that takes a reference to the dependency and uses that.

edit: made some examples less ambiguous

[layer8]

The pattern you are describing is what I know as the Strategy pattern [0]. See the example there with the Car class that takes a BrakeBehavior as a constructor parameter [1]. I have no issue with that and use it regularly. The Strategy pattern precedes the notion of dependency injection by around ten years.

The term Dependency Injection was coined by Martin Fowler with this article: https://martinfowler.com/articles/injection.html. See how it presents the examples in terms of wiring up components from a configuration, and how it concludes with stressing the importance of "the principle of separating service configuration from the use of services within an application". The article also presents constructor injection as only one of several forms of dependency injection.

That is how everyone understood dependency injection when it became popular 10-20 years ago: A way to customize behavior at the top application/deployment level by configuration, without having to pass arguments around throughout half the code base to the final object that uses them.

Apparently there has been a divergence of how the term is being understood.

[0] https://en.wikipedia.org/wiki/Strategy_pattern

[1] The fact that Car is abstract in the example is immaterial to the pattern, and a bit unfortunate in the Wikipedia article, from a didactic point of view.

[squeaky-clean]

They're not really exclusive ideas. The Constructor Injection section in Fowler's article is exactly the same as the Strategy pattern. But no one talks about the Strategy pattern anymore, it's all wrapped into the idea of DI and that's what caught on.

[layer8]

I'm curious, which language/dev communities did you pick this up from? Because I don't think it's universal, certainly not in the Java world.

DI in Java is almost completely disconnected from what the Strategy pattern is, so it doesn't make sense to use one to refer to the other there.

[morsecodist]

It was interesting reading this exchange. I have a similar understanding of DI to you. I have never even heard of a DI framework and I have trouble picturing what it would look like. It was interesting to watch you two converge on where the disconnect was.

[rjbwork]

Usually when people refer to "DI Frameworks" they're referring to Inversion of Control (IoC) containers.