The document discusses the constructor problem in dependency injection and proposes several solutions. It notes that when a class A has a dependency on class B in its constructor, any changes to B could require changes to A. It suggests using dependency injection through an interface rather than a concrete class, allowing classes to be decoupled. This treats classes as "puzzle pieces" that are assembled through an inversion of control container by declaring their required and provided service interfaces.

1. 15 minutes IoC

2. The Constructor Problem
class A {
private B _b;
public A() { _b = new B(); }
}

3. Dependency to the concrete class B
if B changes the constructor
A must change as well
if B becomes new dependencies, e.g. B(C, D),
A must create these dependencies as well
if C, D of B have their own dependencies, e.g. C(E), D
(F),
A must create them too...

5. A is a 'factory' of Bs
A influences the count created instances of type B
could be partially solved by the MonoState Pattern

6. ServiceLocator: temporary solution
class A {
private B _b;
public A(ServiceLocator locator) {
_b = locator.get(B.class);
}
}

7. ServiceLocator problems
extra dependency to the ServiceLocator
'push vs pull':we ask for a service, instead of just
getting the service
standard API across several libraries:
MyServiceLocator vs YourServiceLocator

8. but A needs *only* the services provided by B,
...
e.g. the interface IB
class A {
private IB _b;
public A(IB b) { _b = b; }
}

9. Classes as puzzle pieces
class A implements IA {
private IB _b;
public A(IB b) {
_b = b;
}
}
in -> required services: IB
out <- provided services: IA

10. IoC Container: puzzle table

11. Flavors
Required services declared/injected on
field
setter
constructor
method (e.g. init(...))
Wiring
automatic
manual: xml, JavaConfig, ...

12. References
Gilad Bracha:
Constructors Considered Harmful
Lethal Injection
Martin Fowler:
IoC Containers & DI
Guice
2 min. Pico
my delicious ioc
Pictures:
http://www.flickr.com/photos/joeyday/147651531/
http://www.flickr.com/photos/jowo/20840165/
http://commons.wikimedia.org/wiki/File:Iceberg_4_1997_08_07.jpg