Inject Factories - How and why?

Question

In a code review the following comments came up: "I think, you actually mean to inject the factory as an instance, so that you can rebind the factory if needed." and "important hint: Factories should not be static, but should be injected."

Test.java:

Foo foo = FooFactory.get(argument);

FooFactory.java:

public final class FooFactory {
    public static Foo get(String argument) {
        return new Foo();
    }
}

How should I have done it otherwise? What does "rebind" mean in the first comment of the reviewer?

Answer 1

By doing what you did, you basically ignored dependency injection, and used a static factory instead. So, if in a unit test, you want your factory to return fake instances of Foo rather than real instances, you can't.

Instead, you should use dependency injection (Spring example here):

public class SomeService
    private FooFactory fooFactory;

    @Autowired
    public SomeService(FooFactory fooFactory) {
        this.fooFactory = fooFactory;
    }

    public void someMethod(String arg) {
        Foo foo = fooFactory.create(arg);
        ...
    }
    ....
}

And now, in a unit test, you can inject whatever FooFactory implementation you want (typically, a mock).