Dependency injection

Giving an object its dependencies instead of letting it create them: constructor and setter injection, the role of the assembler or container, and the effect on tests.

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Dependency: what is it?
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An object a class needs in order to do its work: a sending service, a data store, a clock.
The problem is not having dependencies, it is creating them yourself with new: the class ends up tied to one specific implementation.
Which version of Commande applies constructor injection?
- Commande(Notifieur n) { this.n = n; }
- Commande() { this.n = new ServiceMail(); }
- void setNotifieur(Notifieur n) { this.n = n; }
- Commande() { this.n = Notifieurs.parDefaut(); }, a static factory
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Commande(Notifieur n) { this.n = n; }
The first receives its dependency at creation. The second is setter injection. The other two create or look up their dependency themselves: the class stays tied to an implementation.
A class that creates its concrete dependency itself with … is tied to that implementation and can no longer be tested with a fake.
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A class that creates its concrete dependency itself with new is tied to that implementation and can no longer be tested with a fake.
The new keyword in business code is the signal to look for: creation belongs to the assembler, not the class that uses the object.
Dependency injection: what is it?
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The technique of supplying an object, from outside, with the objects it needs, instead of letting it build them itself.
The class only knows the interface of what it uses; the implementation can change, for a test for instance, without touching it.
Commande creates its service itself with new ServiceMail(). The tests send real mail on every run. Which fix?
- Make ServiceMail a singleton and call it through ServiceMail.instance()
- Define a Notifieur interface, which Commande receives through its constructor
- Move the new ServiceMail() into a private method of Commande
- Switch off sending in ServiceMail when an environment variable asks for it
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Define a Notifieur interface, which Commande receives through its constructor
A singleton or a private method keep Commande tied to ServiceMail. The environment flag puts test code in production code. Only injection allows passing a double that sends nothing.

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