Partial Dependency Injection, Partial Construction??

[Tom Hicks]

Am I correct in assuming that Spring does not allow a bean to be
partially configured using dependency injection and then completed
by construction? That is, can one specify some dependencies for
a bean in the configuration file and then, at run time, provide the
rest of the dependencies by construction?

I am imagining a call to the bean factory in which one provides
an array of constructor arguments, something like this:

ApplicationContext ctx = new .....
Object[] conArgs = new Object[] { arg1, arg2,...}
MyClass partDI = (MyClass) ctx.getBean("myClassBean", conArgs);

I suppose one can use a lifecycle callback to set the remaining
bean properties manually at runtime but this does not seem as clean as
providing them to the bean factory.

Any thoughts on this appreciated,
-tom

[gfaerman]

I'm not sure if I get your point. Assuming some understanding, you can
a. declare bean abean of class ABean in app context.
b. IoC this bean in a let's say a service bean
c. create a new ABean using a copy contructor.
d. changing whatever you need to change in the new ABean instance.

ABean class should hava a no arg constr. and a copy constr.

public class ABean implements MyBeanInterface {
  
public ABean () {}

  public ABean (ABean original) {
       this.aProperty= original.getAProperty();
   }
}

in your service class create a new ABean instance passing to the constructor the IoC Abean instance.

ABean newBean = new ABean(this.injectedABean)

Hope this helps,
Regards,
Gustavo.

[manifoldronin]

How about a MyClassFactoryBean, which can take dependencies in the config file on behalf of MyClass, meanwhile can also call MyClass' constructor at runtime however you like?

[Tom Hicks]

Gustavo,

Thanks for the reply and the interesting suggestion.

If I understand your answer, it would seem to incur
a lot of overhead: two beans for every one I really
want. I've always been told that excess object creation
is very expensive and should be avoided.

Never-the-less your answer is very creative and
worth thinking about. Thanks,
-tom

[Tom Hicks]

Jing Xue,

Thanks for the reply. This sounds very good but
how is it done?

The only example I have of using a factory bean
comes from the book ProSpring. It shows how
to create a factory for beans where the arguments
which determine the bean are part of the configuration
but not specified at runtime. That is, there is no example
of providing any extra information to the bean at runtime.
Is there a way to do this with a factory bean?
regards,
-tom

[manifoldronin]

The only example I have of using a factory bean
comes from the book ProSpring. It shows how
to create a factory for beans where the arguments
which determine the bean are part of the configuration
but not specified at runtime. That is, there is no example
of providing any extra information to the bean at runtime.
Is there a way to do this with a factory bean?

In your original post, you imagined to be able to do:

Object[] conArgs = new Object[] { arg1, arg2,...} 
MyClass partDI = (MyClass) ctx.getBean("myClassBean", conArgs);

However you were planing to come up arg1, arg2, etc., well, just do the same in your FactoryBean. The only difference is on the second line above you would be directly calling MyClass constructor with all the args.

[Tom Hicks]

I'm sorry to be slow but I don't understand your answer.

In your original post, you imagined to be able to do:

Object[] conArgs = new Object[] { arg1, arg2,...} 
MyClass partDI = (MyClass) ctx.getBean("myClassBean", conArgs);

However you were planing to come up arg1, arg2, etc., well, just do the same in your FactoryBean.

OK, but I was planning to provide these arguments at runtime,
as I created the bean. That's why I wanted to provide them
directly to the 'getBean' method of the ApplicationContext.
In other words, I can't wire them into the FactoryBean.

The only difference is on the second line above you would be directly calling MyClass constructor with all the args.

But, if I call the MyClass constructor directly with all the arguments
then how is the BeanFactory involved at all? Are you suggesting that
the code looks like this?:

Object[] conArgs = new Object[] { arg1, arg2,...} 
MyClass partDI = new MyClass(conArgs);

or perhaps even like this:

MyClass partDI = new MyClass(arg1, arg2,....);

This is straight Java object creation and I don't see how the
BeanFactory is involved at all. I need the BeanFactory to
fill in the other, more static, parameters to my bean which
are not given in the constructor.

Anyway, sorry not to understand you. I would love to know
a simple solution, like you hint at. Thanks,
-tom

[gfaerman]

Tom,

Thanks for your comments. Reading them, it comes up the issue about creating objects and I agree with the cost of creating objets. The thing here is Spring beans usually are service type of beans, I would not expect having thousands of Spring "incubated" beans. Usually they are singletons or sometimes prototypes. But, if the number of beans to be created is an issue, we must be talking about domain beans or should I say business domain objects, not service objects. Would you share with us your problem?.

Thanks in advance,
Gustavo.

[Tom Hicks]

Thanks for your comments. Reading them, it comes up the issue about creating objects and I agree with the cost of creating objets. The thing here is Spring beans usually are service type of beans, I would not expect having thousands of Spring "incubated" beans. Usually they are singletons or sometimes prototypes. But, if the number of beans to be created is an issue, we must be talking about domain beans or should I say business domain objects, not service objects. Would you share with us your problem?.

Thanks in advance,
Gustavo.

Ah Gustavo, I must apologize for omitting to tell you that I was, indeed,
talking about domain objects, not service objects.

My question is general but imagine, as an example, that we have many clients who wish to log a message to the same database. I create a
ClientLogger bean which needs a data source, but also requires the client name string and id number. I would like the Spring BeanFactory to wire-up
the data source (since this is fixed) but then, at runtime, each client will create a new ClientLogger giving the logger bean its client name and id.

Perhaps a FactoryBean can be used to do this, but I do not understand how. Or is there some other easy way to do it?
regards,
-tom

[gfaerman]

Tom,

Thanks for sharing your requirement. If you need an instance of ClientLogger per each client, and you have a zillion clients, it seems you need a full blown whatever server. Time constrains wouldn't let me get deeper into it. But again, words like data source, client name, id number (and implicit pooling, threads etc), all of this smell to just one singleton service bean or a service facade, or aiming in some other direction, some prototype thing wrapped in a factory. I´m sorry I cannot help you at this time.

Cheers,
Gustavo.