[seam-dev] CDI with generic class and the hell of type erasure.

[José Rodolfo Freitas]

Ales and All,

when

 @Inject
 GenericQuery<String> gqs;

my producer do:

 new GenericQuery(clazz)

And then @Inject inside GenericQuery does not work (since I'm using new).

What's the best approach in this situation?

Using BeanManagerProvider.getInstance() inside GenericQuery constructor to
provide instances for the objects I want to inject?


On Tue, Dec 6, 2011 at 9:29 PM, José Rodolfo Freitas <
joserodolfo.freitas at gmail.com> wrote:

> that´s an awesome solution Ales!
> I didn´t thought that I could obtain the parametizedType that way!
> Testing this will be my first task tomorrow morning.
>
> thank you all for your ideas.
>
>
> On Tue, Dec 6, 2011 at 8:55 PM, Jason Porter <lightguard.jp at gmail.com>wrote:
>
>> Nice solution Ales, wish I'd thought of it :)
>>
>>
>> On Tue, Dec 6, 2011 at 15:45, Ales Justin <ales.justin at gmail.com> wrote:
>>
>>> You could do something like this:
>>>
>>>     @SuppressWarnings({"unchecked"})
>>>     @Produces
>>>     public GenericQuery injectClass(InjectionPoint ip) {
>>>         Annotated annotated = ip.getAnnotated();
>>>         Class clazz = Object.class;
>>>         Type type = annotated.getBaseType();
>>>         if (type instanceof ParameterizedType) {
>>>             ParameterizedType pt = (ParameterizedType) type;
>>>             clazz = (Class) pt.getActualTypeArguments()[0];
>>>         }
>>>          return new GenericQuery(clazz);
>>>     }
>>>
>>> --
>>>
>>>     @Inject
>>>     GenericQuery<String> gqs;
>>>     @Inject
>>>     GenericQuery<Number> gqn;
>>>
>>> --
>>>
>>>         System.out.println("GQS: " + gqs.getClazz());
>>>         System.out.println("GQN: " + gqn.getClazz());
>>>
>>> --
>>>
>>> GQS: class java.lang.String
>>> GQN: class java.lang.Number
>>>
>>> --
>>>
>>>
>>> https://github.com/alesj/cdi-arq-workshop/commit/64860f15197be40a7714dfd8a7da931c1db11411
>>>
>>> HTH
>>>
>>> -Ales
>>>
>>> Hey guys, how you doing?
>>>
>>> I'm trying to achieve something that might be impossible, but before
>>> concluding that, I'd like to ask you, CDI gurus!
>>>
>>> I have the following class:
>>>
>>> public class Foo<T> {
>>>
>>>     public TypedQuery<T> getQuery(){
>>>
>>>     }
>>>
>>> }
>>>
>>> As you can Imagine, inside my getQuery method, I'd have to use "T.class"
>>> to make it TypedQuery. which is impossible due java generics type erasure.
>>>
>>> so I'd have to build a private field to hold the t.class for me.
>>>
>>> public class Foo<T> {
>>>
>>>     private Class<T> klass;
>>>
>>>      public TypedQuery<T> getQuery(){
>>>
>>>     }
>>>
>>>     public void setKlass(Class<T> klass){
>>>         this.klass = klass;
>>>     }
>>> }
>>>
>>> The problem is that forcing this 'setKlass' feels very ugly to the api,
>>> and it's not very error prone, since one could easily forget to set this
>>> configuration.
>>>
>>> So I had an Idea: force the setKlass inside the constructor:
>>>
>>> public class Foo<T> {
>>>
>>>     private Class<T> klass;
>>>
>>>     public Foo(Class<T> klass){
>>>         this.klass = klass;
>>>     }
>>>
>>>     public TypedQuery<T> getQuery(){
>>>
>>>     }
>>>
>>> }
>>>
>>> Unfortunatelly, this breaks cdi, since it cannot inject it anymore. At
>>> least AFAIK.
>>>
>>> So, is there a way out of this? maybe using a secret solder feature?
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>>>
>>>
>>>
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>>>
>>
>>
>> --
>> Jason Porter
>> http://lightguard-jp.blogspot.com
>> http://twitter.com/lightguardjp
>>
>> Software Engineer
>> Open Source Advocate
>> Author of Seam Catch - Next Generation Java Exception Handling
>>
>> PGP key id: 926CCFF5
>> PGP key available at: keyserver.net, pgp.mit.edu
>>
>
>
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