Dependency Injection

Found a good explanation of Dependency Injection, one of the pillars of CDI.

Of course, the original text by Martin Fowler² is also well worth the read, but if you want to get into details of JEE, [1] is very good.

It works great, of course sometimes I screw it up, and then I get errors that need to be resolved, for instance:

org.jboss.weld.exceptions.DeploymentException: Exception List with 8 exceptions:
Exception 0 :
org.jboss.weld.exceptions.DeploymentException: WELD-001443: Pseudo scoped bean has circular dependencies. Dependency path: 
  - Managed Bean [class com.mrbear.OrderService] with qualifiers [@Any @Default],
  - [BackedAnnotatedParameter] Parameter 1 of [BackedAnnotatedConstructor] @Inject public com.mrbear.OrderService(AddressService, x, y, z),
  - Managed Bean [class com.mrbear.AddressService] with qualifiers [@Any @Default],
  - [BackedAnnotatedParameter] Parameter 11 of [BackedAnnotatedConstructor] @Inject public com.mrbear.AddressService(a, b, c, d, e, f, g, h, I, j, OrderSupplier),
  - Managed Bean [class com.mrbear.OrderSupplier with qualifiers [@Any @Default],
  - [BackedAnnotatedParameter] Parameter 7 of [BackedAnnotatedConstructor] @Inject public com.mrbear.OrderFactory(a, b, c, d, e, f, CityTownVillageSearchService),
  - Managed Bean [class com.mrbear.CityTownVillageSearchService] with qualifiers [@Any @Default],
  - [BackedAnnotatedField] @Inject private com.mrbear.CityTownVillageSearchService.OrderSupplier,
  - Managed Bean [class com.mrbear.OrderFactory] with qualifiers [@Any @Default]
	at org.jboss.weld.bootstrap.Validator.reallyValidatePseudoScopedBean(Validator.java:924)
	at org.jboss.weld.bootstrap.Validator.validatePseudoScopedInjectionPoint(Validator.java:971)
	at org.jboss.weld.bootstrap.Validator.reallyValidatePseudoScopedBean(Validator.java:933)
	at org.jboss.weld.bootstrap.Validator.validatePseudoScopedInjectionPoint(Validator.java:971)
	at org.jboss.weld.bootstrap.Validator.reallyValidatePseudoScopedBean(Validator.java:933)
	at org.jboss.weld.bootstrap.Va

Funnily enough, circular dependencies seemed to work fine when using Enterprise Beans or some such. I guess it depends a little on the implementation.

References

[1] JBoss Weld Reference - Chapter 4. Dependency injection and programmatic lookup

https://docs.jboss.org/weld/reference/1.1.0.Final/en-US/html/injection.html

[2] MartinFowler.com - Inversion of Control Containers and the Dependency Injection pattern

https://martinfowler.com/articles/injection.html

Using CDI to extend functionality

I recently wanted to change the behaviour of the framework.

Now, usually this means having to change code in the framework, but as this is something I'd rather not do, I found a way around this.

The framework, just as the code I write, uses CDI.

So it was a simple case of telling CDI to inject my Bean, instead of the Framework Bean.

And it worked right out of the box.

I was pretty surprised.

References

Contexts & Dependency Injection for Java

http://cdi-spec.org/