What are the benefits of dependency injection containers?
I understand benefits of dependency injection itself. Let's take Spring for instance. I also understand benefits of other Spring featureslike AOP, helpers of different kinds, etc. I'm just wondering, what are the benefits of XML configuration such as:
<bean id="Mary" class="foo.bar.Female">
<property name="age" value="23"/>
</bean>
<bean id="John" class="foo.bar.Male">
<property name="girlfriend" ref="Mary"/>
</bean>
compared to plain old java code such as:
Female mary = new Female();
mary.setAge(23);
Male john = new Male();
john.setGirlfriend(mary);
which is easier debugged, compile time checked and can be understood by anyone who knows only java. So what is the main purpose of a dependency injection framework? (or a piece of code that shows its benefits.)
UPDATE:
In case of
IService myService;// ...
public void doSomething() {
myService.fetchData();
}
How can IoC framework guess which implementation of myService I want to be injected if there is more than one? If there is only one implementation of given interface, and I let IoC container automatically decide to use it, it will be broken after a second implementation appears. And if there is intentionally only one possible implementation of an interface then you do not need to inject it.
It would be really interesting to see small piece of configuration for IoC which shows it's benefits. I've been using Spring for a while and I can not provide such example. And I can show single lines which demonstrate benefits of hibernate, dwr, and other frameworks which I use.
UPDATE 2:
I realize that IoC configuration can be changed without recompiling. Is it really such a good idea? I can understand when someone wants to change DB credentials without recompiling - he may be not developer. In your practice, how often someone else other than developer changes IoC configuration? I think that for developers there is no effort to recompile that particular class instead of changing configuration. And for non-developer you would probably want to make his life easier and provide some simpler configuration file.
UPDATE 3:
External configuration of mapping between interfaces and their concrete implementations
What is so good in making it extenal? You don't make all your code external, while you definitely can - just place it in ClassName.java.txt file, read and compile manually on the fly - wow, you avoided recompiling. Why should compiling be avoided?!
You save coding time because you provide mappings declaratively, not in a procedural code
I understand that sometimes declarative approach saves time. For example, I declare only once a mapping between a bean property and a DB column and hibernate uses this mapping while loading, saving, building SQL based on HSQL, etc. This is where the declarative approach works. In case of Spring (in my example), declaration had more lines and had the same expressiveness as corresponding code. If there is an example when such declaration is shorter than code - I would like to see it.
Inversion of Control principle allows for easy unit testing because you can replace real implementations with fake ones (like replacing SQL database with an in-memory one)
I do understand inversion of control benefits (I prefer to call the design pattern discussed here as Dependency Injection, because IoC is more general - there are many kinds of control, and we are inverting only one of them - control of initialization). I was asking why someone ever needs something other than a programming language for it. I definitely can replace real implementations with fake ones using code. And this code will express same thing as configuration - it will just initialize fields with fake values.
mary = new FakeFemale();
I do understand benefits of DI. I do not understand what benefits are added by external XML configuration compared to configuring code that does the same. I do not think that compiling should be avoided - I compile every day and I'm still alive. I think configuration of DI is bad example of declarative approach. Declaration can be useful if is declared once AND is used many times in different ways - like hibernate cfg, where mapping between bean property and DB column is used for saving, loading, building search queries, etc. Spring DI configuration can be easily translated to configuring code, like in the beginning of this question, can it not? And it is used only for bean initialization, isn't it? Which means a declarative approach does not add anything here, does it?
When I declare hibernate mapping, I just give hibernate some information, and it works based on it - I do not tell it what to do. In case of spring, my declaration tells spring exactly wht to do - so why declare it, why not just do it?
LAST UPDATE:
Guys, a lot of answers are telling me about dependency injection, which I KNOW IS GOOD. The question is about purpose of DI configuration instead of initializing code - I tend to think that initializing code is shorter and clearer. The only answer I got so far to my question, is that it avoids recompiling, when the configuration changes. I guess I should post another question, because it is a big secret for me, why compiling should be avoided in this case.
For myself one of the main reasons to use an IoC (and make use of external configuration) is around the two areas of:
Testing
If you split your testing into 3 scenarios (which is fairly normal in large scale development):
What you will want to do is for the last two test scenarios (Integration & Black box), is not recompile any part of the application.
If any of your test scenarios require you to change the configuration (ie: use another component to mimic a banking integration, or do a performance load), this can be easily handled (this does come under the benefits of configuring the DI side of an IoC though.
Additionally if your app is used either at multiple sites (with different server and component configuration) or has a changing configuration on the live environment you can use the later stages of testing to verify that the app will handle those changes.
Production
As a developer you don't (and should not) have control of the production environment (in particular when your app is being distributed to multiple customers or seperate sites), this to me is the real benefit of using both an IoC and external configuration, as it is up to the infrastructure/production support to tweak and adjust the live environment without having to go back to developers and through test (higher cost when all they want to do is move a component).
Summary
The main benefits that external configuration of an IoC come from giving others (non-developers) the power to configure your application, in my experience this is only useful under a limited set of circumstances:
In practice I've found that even when developing something that you do have control over the environment it will be run on, over time it is better to give someone else the capabilities to change the configuration:
Note: Application refers to the complete solution (not just the executable), so all files required for the application to run.
Dependency injection is a coding style that has its roots in the observation that object delegation is usually a more useful design pattern than object inheritance (ie, the object has-a relationship is more useful than the object is-a relationship). One other ingredient is necessary however for DI to work, that of creating object interfaces. Combining these two powerful design patterns software engineers quickly realized that they could create flexible loosely coupled code and thus the concept of Dependency Injection was born. However it wasn't until object reflection became available in certain high level languages that DI really took off. The reflection component is core to most of today's DI systems today because the really cool aspects of DI require the ability to programmatically select objects and configure and inject them into other objects using a system external and independent to the objects themselves.
A language must provide good support for both normal Object Oriented programming techniques as well as support for object interfaces and object reflection (for example Java and C#). While you can build programs using DI patterns in C++ systems its lack of reflection support within the language proper prevents it from supporting application servers and other DI platforms and hence limits the expressiveness of the DI patterns.
Strengths of a system built using DI patterns:
Definitely DI code seems more cumbersome, the disadvantages of having all of those XML files that configure objects to be injected into other objects appears difficult. This is, however, the point of DI systems. Your ability to mix and match code objects as a series of configuration settings allows you to build complex systems using 3rd party code with minimal coding on your part.
The example provided in the question merely touches on the surface of the expressive power that a properly factored DI object library can provide. With some practice and a lot of self discipline most DI practitioners find that they can build systems that have 100% test coverage of application code. This one point alone is extraordinary. This is not 100% test coverage of a small application of a few hundred lines of code, but 100% test coverage of applications comprising hundreds of thousands of lines of code. I am at a loss of being able to describe any other design pattern that provides this level of testability.
You are correct in that an application of a mere 10s of lines of code is easier to understand than several objects plus a series of XML configuration files. However as with most powerful design patterns, the gains are found as you continue to add new features to the system.
In short, large scale DI based applications are both easier to debug and easier to understand. While the Xml configuration is not 'compile time checked' all application services that this author is aware of will provide the developer with error messages if they attempt to inject an object having an incompatible interface into another object. And most provide a 'check' feature that covers all known objects configurations. This is easily and quickly done by checking that the to-be-injected object A implements the interface required by object B for all configured object injections.
This is a bit of a loaded question, but I tend to agree that huge amounts of xml configuration doesn't really amount to much benefit. I like my applications to be as light on dependencies as possible, including the hefty frameworks.
They simplify the code a lot of the times, but they also have an overhead in complexity that makes tracking down problems rather difficult (I have seen such problems first hand, and straight Java I would be a lot more comfortable dealing with).
I guess it depends on style a bit, and what you are comfortable with... do you like to fly your own solution and have the benefit of knowing it inside out, or bank on existing solutions that may prove difficult when the configuration isn't just right? It's all a tradeoff.
However, XML configuration is a bit of a pet peeve of mine... I try to avoid it at all costs.
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