Re: [osgi-dev] DS Reference injection order

Wed, 31 Jan 2018 06:43:29 -0800

Tim correctly cites the spec regarding the order SCR must process the <reference> elements in the XML. Since you are using annotations, there is one more thing to know. The javadoc for the Reference annotation states:
 

In the generated Component Description for a component, the references must be ordered in ascending lexicographical order (using String.compareTo ) of the reference names.

 
So you can control the order of the reference elements in the xml by the names of the references.
 
The order of processing reference can be useful if you are using method injection and a method needs to use a previously injected reference. But since you examples show field injection, your component cannot not really observe the order of injection.
 
With 'volatile' you have a dynamic reference which can be updated at any time, so there is no order with respect to other reference.
 
--

BJ Hargrave
Senior Technical Staff Member, IBM // office: +1 386 848 1781
OSGi Fellow and CTO of the OSGi Alliance // mobile: +1 386 848 3788
hargr...@us.ibm.com
 
 
----- Original message -----
From: Tim Ward via osgi-dev <osgi-dev@mail.osgi.org>
Sent by: osgi-dev-boun...@mail.osgi.org
To: Thomas Driessen <thomas.driessen...@gmail.com>, OSGi Developer Mail List <osgi-dev@mail.osgi.org>
Cc:
Subject: Re: [osgi-dev] DS Reference injection order
Date: Wed, Jan 31, 2018 6:43 AM
 
Firstly - why do you need to rely on this? It sounds like very fragile code to me and you should probably consider rewriting so that you don’t need to care. However...
 
Section 112.5.7 of the compendium says that:

When binding services, the references are processed in the order in which they are specified in the component description. That is, target services from the first specified reference are bound before services from the next specified reference.
 
A static optional service will not be set if it is not satisfied, or will if it is. A dynamic optional service will behave the same way, but it may be unset and reset many times afterwards on other threads. Dynamic services should always be treated with care, whether they are optional or not.
 
Tim
 
On 31 Jan 2018, at 10:50, Thomas Driessen via osgi-dev <osgi-dev@mail.osgi.org> wrote:
 
Hi,
 
I searched the compendium spec but didn't find what I was looking for.
 
If I have a DS with multiple references to other DS
 
@Component
class Example1{
 
    @Reference
    Example2 e2
 
    @Reference
    Example3 e3
}
 
in which order are the references injected? Is there even a deterministic order?
 
In the above example both references are static/mandatory. What happens if I have a static/mandatory and a dynamic/optional one like this:
 
@Component
class Example1{
 
    @Reference
    Example2 e2
 
    @Reference
    volatile Example3 e3
}
 
Kind regards,
Thomas
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