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Dependency Injection - Martin Fowler

Date of publication: 2017-08-26 03:14

This configuration code is typically set up in a different class. For our example, each friend who uses my lister might write the appropriate configuration code in some setup class of their own. Of course it's common to hold this kind of configuration information in separate config files. You can write a class to read a config file and set up the container appropriately. Although PicoContainer doesn't contain this functionality itself, there is a closely related project called NanoContainer that provides the appropriate wrappers to allow you to have XML configuration files. Such a nano container will parse the XML and then configure an underlying pico container. The philosophy of the project is to separate the config file format from the underlying mechanism.

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When these containers talk about how they are so useful because they implement "Inversion of Control" I end up very puzzled. Inversion of control is a common characteristic of frameworks, so saying that these lightweight containers are special because they use inversion of control is like saying my car is special because it has wheels.

The Thesis Statement

The 9 essay delves into the structure of the author’s argument more deeply. The writer not only states the techniques used in the text, but also thoroughly explains their impact on the reader. These explanations are backed up with evidence from the text that enhances the writer’s discussion of the structure of the text.

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When the dependent is a class written for this container, it makes sense for the component to implement the injector interface itself, as I do here with the movie finder. For generic classes, such as the string, I use an inner class within the configuration code.

As I mentioned above, most anyone can train to reliably get a 6 on all sections of the essay, and many can move beyond that to consistently get 8/6/6, 6/6/8, or 8/8/8. Here’s a framework for how to do this:

Of course the testing problem is exacerbated by component environments that are very intrusive, such as Java's EJB framework. My view is that these kinds of frameworks should minimize their impact upon application code, and particularly should not do things that slow down the edit-execute cycle. Using plugins to substitute heavyweight components does a lot to help this process, which is vital for practices such as Test Driven Development.

One of the issues with the simple approach above, is that the MovieLister is dependent on the full service locator class, even though it only uses one service. We can reduce this by using a role interface. That way, instead of using the full service locator interface, the lister can declare just the bit of interface it needs.

For a more sophisticated locator I can subclass service locator and pass that subclass into the registry's class variable. I can change the static methods to call a method on the instance rather than accessing instance variables directly. I can provide thread x7568 specific locators by using thread x7568 specific storage. All of this can be done without changing clients of service locator.

The real point of this article is this finder object, or particularly how we connect the lister object with a particular finder object. The reason why this is interesting is that I want my wonderful moviesDirectedBy method to be completely independent of how all the movies are being stored. So all the method does is refer to a finder, and all that finder does is know how to respond to the findAll method. I can bring this out by defining an interface for the finder.

For service combination, you always have to have some convention in order to wire things together. The advantage of injection is primarily that it requires very simple conventions - at least for the constructor and setter injections. You don't have to do anything odd in your component and it's fairly straightforward for an injector to get everything configured.

You'll notice that since we want to use an interface, we can't just access the services through static methods any more. We have to use the class to get a locator instance and then use that to get what we need.

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