This past Monday, for example, a good friend of mine, a lawyer by profession, telephoned me late at night, desparate to rescue his home computer from the malware that he had inadvertently downloaded. I managed to walk him through the steps necessary to get his computer working again. That is, I thought I had, until a couple of nights later, he called me again asking my advice in buying a new computer.
A few days later, I was reading a printed copy of Martin Fowler's paper "Inversion of Control Containers and the Dependency Injection pattern" [1] when a friend of mine commented, "inversion of control?" Dude, that's the story of my life. My kids rule the house."
The term "dependency injection" can at first be bit more intimidating than the reality of what it is and how it's used. The simplest explanation that I've ever seen for it is in James Shore's excellent blog "The Art of Agile" [2] when he states that:
'...dependency injection means giving an object its instance variables..'
OK. This is all interesting, but what does it have to do with software testing? Quite a bit, actually. In the blog entry from a few weeks ago titled "Parameterized Testing - Very Easy with TestNG's DataProvider" we talked about designing tests that accept parameters, so that the same test could be run against multiple sets of data. In this post, we'll revisit that test design pattern.
But first, let's take a deeper look at inversion of control and dependency injection.
To begin, let's disect the phrase "inversion of control." In this context, who wants control, and just how is this control inverted?
In an object oriented language such as Java, you solve programming problems by designing classes. The classes contain variables (to hold data) and methods (functions that manipulate the data). To actually perform the tasks that you want the programs to do, you create discrete instances of the objects. When you create an object instance, you work with instances of the variables.
Let's say that you've defined a Java class of type Car, and within the class you set a Manufacturer variable to, say, "Audi."
What have we just done in terms of control and dependencies?
* Who has control? The "Car" class has the control. In order to build a new car, it defines a variable of type "Manufacturer." Or more precisely it defines an instance variable of type "Manufacturer." In this case, the class only allows for a Manufacturer of "Audi."
* The "Car" class needs the "Manufacturer" variable to do its work. In other words, it depends on this variable. That's right, this instance variable is a "dependency."
However, there's a problem with the "Car" class. How can build a test for the class, and test multiple car manufacturers? To build a test for Toyota, for example, we will have to build another class that's almost identical to the "Car" class, except that it will create a different "Manufacturer" object.
The answer is easy, of course, we just add another constructor to the Car class:
public Car(Manufacturer targetManufacturer) {
carManufacturer = targetManufacturer;
}
So, now what have we just done in terms of control and dependencies?
* The value of the dependency, that is, the "carManufacturer" variable is not hardwired into the "Car" class. It is "injected" into the "Car" class through its new constructor.
* Who has control now? Not the "Car" class anymore. The class can now be used to create any type of car. Who controls the decision of the type of car to create? The program (or test program) that creates the instance of the "Car" class. That's the "inversion" of control.
One important use of this model is that it emables us to isolate the class being tested. For example, if we want to test the Car class with a mock or stub[3] version of the Manufacturer class, we can pass that stub or mock object to the Car class during the test. This will let the test concentrate on verifying the operation of the Car class, independent of the function (or lack of function) provided by the Manufacturer class.
When you couple this design pattern with ability to pass a parameter into a test, such as is supported in TestNG, this enables us to inject the dependecies into the classes being tested. This makes it possible for the tests to control not only the test data, but also the defined profile and characteristics of the classes under test. And to do it without hardcoding data into the tests or the classes under test. Now, that's control!
References:
[1] http://martinfowler.com/articles/injection.html
[2] http://jamesshore.com/Blog/Dependency-Injection-Demystified.html
[3] Next Generation Java Testing by Cedric Beust and Hani Suleiman, pages 95-96. http://testng.org/doc/book.html
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