One factory to bring them all, and at run-time bind them

Most of us know that to call new makes code hard to test because the piece of code calling new is statically bound to another class. In other words, there is no way to isolate the class under test such that its interactions with other classes can be tested. There is of course a solution to this: factories and/or dependency injection (with or without a framework).

I personly prefer to manually inject the needed dependencies, and seldomly use DI frameworks such as Guice. Why? Well, call my old fashioned, but I find it easier to read and understand non-DI framework code. So this leaves me with writing factories.

I don't know about you, but I find it boring to write these lousy factories (also, how can they be tested?). All they do is calling new; should there be some better way of doing it? Of course there should, and in fact, there are (for some definition of "better" :)). I've implemented a generic factory that can be used to instantiate most classes.

The factory uses some reflection magic and does a fair amount of work at run-time instad of compile-time. Actually, the factory is on the edge of being a primitive DI framework. :) As I usually do in my post I cut to the chase and give a few code examples:

final Factory factory = Factory.of(ConcreteClass.class, AnotherConcreteClass.class);

This creates a new factory that can create two concrete classes. Let's say that these classes implement the interfaces Interface and AnotherInterface respectiveley. To get an instance of the Interface the factory is called like this:

final Interface instance = factory.create(Interface.class);

When the create method is called, the factory finds the class that implements the provided interface (in this case this is the ConcreteClass class). Then the factory instantiates that class and returns the instance. This means that

1. the factory can be used for creating different kinds of objects as long as they implement different interfaces,
2. the client of the factory is unaware of the concrete class implementing the interface, and
   
3. the same Factory class can be used in test-cases testing the client code (the difference is only how the factory instance is created).

I illustrait the last bullet with an example, This code is part of a test-case testing a class which use the factory to create an Interface instance:

final Factory stubbedFactory = Factory.of(Stub.class);

where, of course, Stub implements Interface (compare to the example above where the factory is created using ConcreteClass and AnotherConcreteClass).

Ok, so far so good. But how to instantiate a class that need some parameters to be instantiated, i.e., only has a non-default constructor? Parameters that the factory needs in order to create objects are provided by calling the using method:

final DependencyUser user = factory.using(new Dependency()).create(DependencyUser.class);

Here, the concrete class implementing DependencyUser is instantiated with the Dependency instance as argument to the class' constructor. Any number of parameters can be given to the using method as long as they have different (run-time) type. The factory will use those parameters that are needed for instantiating the class that the client requests.

The using method returns a new Factory instance that "know about" the parameters provided to using (and any parameters known by the factory instance which using was called on). This may sound a bit strange, but it makes it possible to add parameters as they become available in the program flow. For instance, it's quite common that some parameters are only know when the factory is used. Example:

// Outside client code. An instance of 'Two' is not available.
final Factory factory = Factory.of(ClassUsingOneAndTwo.class).using(new One());

// Inside client code. An instance of 'Two' is now available.
final OneAndTwoUser user = factory.using(two).create(OneAndTwoUser.class);

(ClassUsingOneAndTwo is a class implementing the OneAndTwoUser interface and its constructor takes an One instance and a Two instance).

That is! That's the mostly simple, almost generic, factory I hacked together... pardon me ...developed this afternoon. The source is available here. There is also a few test-cases in that Eclipse project.