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From David Blevins <>
Subject Re: [xbean] annotation based dependency injection
Date Wed, 05 Apr 2006 10:12:23 GMT

On Apr 5, 2006, at 2:30 AM, James Strachan wrote:

> On 4/5/06, David Blevins <> wrote:
>> On Apr 4, 2006, at 11:37 PM, James Strachan wrote:
>>> On 4/4/06, David Blevins <> wrote:
>>> Good point - though before JSR 250 & EJB 3, when Pico was just
>>> starting, I tended to use the convention that contructor arguments
>>> were 'mandatory' properties and setter methods were for optional  
>>> stuff
>>> (or stuff that can be configured after construction).
>>> i.e.  its kinda implicit that any constructor arguments must be
>>> injected to be able to construct the object - so they are mandatory
>>> already. I guess a container could shove in null objects or zero
>>> values if it can't find a value, but maybe its easier to just say  
>>> that
>>> for constructor injection, the parameters should be considered
>>> mandatory?
>> I get it -- you're using @Resource to imply whether or not an object
>> attribute is optional or required.
> Yeah; Ideally there would be an @Mandatory in JSR 250 on properties...
>> We're talking about different
>> things then.  Was more going after EJB 3 perspective where @Resource
>> is primarily used to label an object attribute with an identifier and
>> optional mapping the injector understands (such as a jndi name, an
>> xml attribute or element name, or whatever).
> Yeah. Though the EJB 3 injection is more like auto-JNDI lookup; the
> POJO is kinda pulling resources into itself using the name it defines.
> In Spring style, the XML config injects into the POJO without its
> particular pre-knowledge. So AnDI is a little attempt to reclaim the
> @Resource annotation for non-JNDI style injection where we can just
> use it as a marker for what are the mandatory properties.

Exactly.  Lot's of stuff could fall under the "whatever" category...  
kernel references or just about anything could be named via @Resource 
(name="foo").  Heck, if you use a URI in there like I know you would,  
you could do anything with it :)   I'd be likely to endorse something  
sick and twisted like that ;)

>> Seems like in your scenario, someone couldn't use the annotation to
>> specify how object attribute would be populated via an injectable
>> resource but still let it be an optional dependency.  Guessing you
>> don't really care though :)
> This is a good point. I'd assumed in EJB 3 land that @Resource implies
> mandatory injection; what happens if there is no value available in
> JNDI? Is that an error or is null used? If its the latter then maybe
> we do need a @Mandatory or @NotNull or something.

Right, EJB 3 says they are mandatory.  But thinking along your lines  
of slightly repurposing it for better use, you could theoretically  
allow someone to label an object attribute via @Resource and still  
annotate or flag it as optional somehow.  Just a thought.


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