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RE: [equinox-dev] Roadmap?
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First an answer to your last questions.
Yes, anything that makes writing components easier is very much of
interest here. Rather than creating new programming idioms and mechanisms
it would be interesting, as you have been doing, to investigate all possible
avenues. If in the end there is truly something new here, it may
make sense to supply a library/bundle.
I will offer an alternative that *may*
make sense. It is something that we have been playing with. In the
Eclipse extension registry it is possible to create executable extensions
via a factory (see IConfigurationElement.createExecutableExtension() and
IExecutableExtensionFactory). So your IoC framework could publish
an extension point "prereqs" (for lack of a better name). People
who have prereqs (services in your example) to register can then supply
extensions that identify either the actual class to instantiate (if the
simple no-arg constructor is enough) or an IExecutableExtensionFactory
(with args) if more complicated construction is needed.
In the IoC mechanism (com.example.ioc)
<extension-point id="prereqs"/>
In the provider JAR (org.example.provider).
<extension id="prereq1"
point="com.example.ioc.prereqs">
<prereq
class="org.example.Factory:coolObject"
...
This extension says run the org.example.Factory
and tell it to create the thing called "coolObject". The
factory can be as simple or as complicated as you like. You can make
some generic factories and share them or make one per thingie you want
to create. You can also pass in an arbitrary number of args statically
declared on in the extension (see the Javadoc for details).
You then have two choices for when the
contributed prerequisites are actually instantiated and supplied to consumers;
lazily or aggressively. If you go for the lazy approach then your
POJO could access the prereq on demand using something like
Object
IoC.getPrereq(String prereqName, Object objectToInject)
where the prereqName is the id of the
extension to instantiate (org.example.provider.prereq1 in the above example).
(not really a POJO if it is referencing IoC but you seemed fine with
this in your example...) This ends up being implemented as a call
to IConfigurationElement.createExecutableExtension(). The prereq
object is only instantiated as needed. When you stop using it, just
drop the pointer. I believe previously you stated that lifecycle was not
important so the disappearance of the provider should not be an issue.
If you want to create the instances
aggressively then the IoC mechanism should have the sort of registerDependency
method you outlined. It should also implement an IRegistryChangeListener
that listens for new extensions being added to the "prereqs"
extension point. When it sees one that someone has registered an
interest in, it calls createExecutableExtension and uses the supplied setter.
If it later sees the extension going away, it can unset the value.
See IExtensionTracker for some utilities to help manage this.
Some notes:
- You are probably thinking that this
is crazy, the extension registry is buried deep in Eclipse. Wrong!
As of today's integration build it is actually broken out in its
own bundle (org.eclipse.equinox.registry) and the intention it that it
run on any OSGi R4 implementation (we've not tested that part yet).
- So you are still thinking, "but
its based on OSGi". Yes and No. It is pluggable. You
can customize the implementation of the registry to suite different sources
of declarations (e.g., something other than plugin.xml), different namespace
management policies and different mechanisms for implementing createExecutableExtension.
We supply an OSGi registry strategy but you can supply one of your
own. The usecases that are driving this new structure are quite similar
to your hosted and client scenarios.
- The ability to supply dynamic context
objects to createExecutableExtension is currently not implemented. We
discussed this late in 3.1 but opted out. This topic has been raised
again and we are certainly open to adding it.
- I've thought about this exact problem
for about as long as it took me to write this response so I could be completely
out to lunch here...
Jeff
"John Wells"
<jwells@xxxxxxx>
Sent by: equinox-dev-bounces@xxxxxxxxxxx
11/22/2005 04:00 PM
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Yes, the code using the LogService
would have to explicitly check (with a synchronized block) whether or not
the logger exists. The code would be something like:
LogService localLogger;
synchronized (this) {
localLogger = logService;
}
If (localLogger != null) {
localLogger(“This is
fun!!!”);
}
This will work whether or not
we are in the OSGi environment.
Also very interesting “trick”
to getting the bundle. Thanks. I wish there was a more “framework
independent” way to do this.
My more general question remains
though: is this a requirement that more people than me have faced?
Would a Dynamic IoC utility be of interest to the OSGi community?
At this point I’m writing this thing anyway, I just want to know
if people here would be interested in me contributing the code? Since
this is a utility (of course it would be a Bundle as well) I’m not exactly
sure where the proper place would be to put such a thing, or if people
are even interested in such a facility.
John Wells (Aziz)
jwells@xxxxxxx
From: equinox-dev-bounces@xxxxxxxxxxx
[mailto:equinox-dev-bounces@xxxxxxxxxxx] On Behalf Of Jeremy Volkman
Sent: Tuesday, November 22, 2005 2:57 PM
To: Equinox development mailing list
Subject: Re: [equinox-dev] Roadmap?
John,
When writing a component to be environment-agnostic, how do you handle
the case when LogService may not be available
at the time of your POJO's instantiation (only in the OSGi environment,
of course)? Will the programmer have to explicitly check the value
of the logger member before attempting to use it?
If so, doesn't this go against your wishes that the programmer not care
what environment he/she is writing for?
Also, on getting a caller's BundleContext: it is possible, but it get ugly.
I had to do something similar to get per-bundle logging working with
the Apache Commons Logging API. Commons Logging provides the static
LogFactory class to get Log instances, and I wanted Log instances to know
which Bundle had created them (so I could print out the name of the Bundle
logging a message).
One way would be to include a separate copy of my modified LogFactory class
with each bundle, and provide the Bundle's personal LogFactory instance
with a reference to the Bundle at start up, but this seemed like a maintenance
nightmare, and it negated OSGi's nifty code sharing feature.
So i went with the following approach:
1. Subclass SecurityManager and gain access to the getClassContext() method
2. Pick the Class of the caller off the stack
3. Use OSGi's PackageAdmin and its getBundle( java.lang.Class) method to
get the Bundle containing a particular class
Since the BundleContext instance is supposed to be private to its associated
Bundle, OSGi provides no way of getting a BundleContext for a given Bundle
(the DS spec mentions this). With Equinox, this can be done through
reflection, as Equniox's AbstractBundle class defines a method "getContext":
Bundle bundle = ...
Method contextMethod = bundle.getClass().getDeclaredMethod("getContext",
(Class[]) null);
contextMethod.setAccessible(true);
BundleContext context = (BundleContext) contextMethod.invoke(bundle,
(Object[]) null);
Of course there are numerous possible issues with this method (can't create
new SecurityManagers, can't change Method accessibility, etc.), but perhaps
it will be food for thought.
Jeremy Volkman
On 11/22/05, John Wells
<
jwells@xxxxxxx> wrote:
Thanks for your response Olivier.
In an earlier response I talked
about the requirement I would like to see satisfied that would make the
IoC pattern work in both a dynamic and a static environment. Basically,
I want to write code that dynamically tells the IoC system that it wants
to depend on some other type and I want this to work both in a "client"
environment where the classes are linked together via the system class-loader
as well in the dynamic environment afforded by OSGi. To me what is
interesting is to be able to write code that runs in both a "hosted"
and "client" environment without modification.
What I do not want is for the
programmers writing the code to know the difference between the two environments.
Furthermore, I think that this could be a common programming pattern
when writing modular code. I want the service writers to have simple
instructions for how to write their services, and then I want the framework
to do the "correct" thing based on whether or not we are running
in the OSGi framework or on a client that has not started the OSGi kernel.
Yes, I want my cake and I want to eat it too ;-)
Let's take something as simple
as a log service. I want to have a pojo that says something like:
private LogServer logger;
…
// inside some method of my
pojo
IoC.createDependency("com.acme.LogService",
"setLogger", this);
…
public synchronized void setLogger(com.acme.LogService
logger) {
this.logger = logger;
}
When in the OSGi world, the
LogService might be satisified by lots of different implementations. However,
in the client world there may be only the one that is satisfied on the
client classpath.. One problem we run up against right away is that
com.acme.LogService is very likely an interface, and hence cannot be instantiated
directly on the client. In order to solve this problem I was thinking
that it would be necessary to define a generic sort of factory interface
like this:
public interface GenericFactory<T>
{
public <T> create(…);
public <T> find(…)
throws NotFoundException;
}
(excuse any typo's, I am typing
as I write, not pasting from an IDE ;-)
Now instead our IoC object might
look like this:
private LogFactory logFactory
= new LogFactory(); // Implements the GenericFactory interface
private LogServer logger;
…
// note this time I must provide
the factory to use for this service
// just in case this is running
in a "client" environment
IoC.createDependency("com.acme.LogService",
"setLogger", this, logFactory);
…
public synchronized void setLogger(com.acme.LogService
logger) {
this.logger = logger;
}
The signature for createDependency
would be something like:
public Object createDependency(String
serviceName, String setterName, GenericFactory staticFactory, …);
where the extra arguments are
used in the find/create methods of the factory.
What my IoC controller could
do then would be that *if* I was running in OSGi (and could find
the bundle context of this caller) then use the ServiceTracker to hook
up the LogService with the implementation. But *if* I was
running in a "client" environment I would use the factory (first
trying to find, then create the service).
Everything above is doable (with
a big question mark on how to get the code's BundleContext) using home-made
code. I was wondering if anyone else has these sorts of requirements
or if there is any "standard" way to accomplish what I am trying
to achieve? And does this clarify what requirements I have?
John Wells (Aziz)
jwells@xxxxxxx
From: equinox-dev-bounces@xxxxxxxxxxx
[mailto:
equinox-dev-bounces@xxxxxxxxxxx]
On Behalf Of Olivier Gruber
Sent: Tuesday, November 22, 2005 12:26 PM
To: Equinox development mailing list
Subject: RE: [equinox-dev] Roadmap?
John,
Declarative services are mostly about lifecycle and injection.
It is the combination of both that is powerful, especially regarding the
linkage with the bundle lifecycle underneath.
That is, a service is created only when its dependencies are available...
and this means that the creations of class loaders are also delayed.
Without lifecycle, injection becomes service tracking with the use of setter/getter
methods,
especially if you are not using a constructor-based injection.
What I am curious about is that inversion of control becomes really interesting
when used with a lifecycle.
The goal is that a service code does not have to deal with the dynamic
nature of its environment.
When it is activated, it has all the references it needs because they have
been injected.
Before loosing a reference, it will be deactivated.
But you are stating that you have no lifecycle, does this mean that you
have injection but your code is also fully
responsible for dynamically appearing/disappearing references? Who controls
that lifecyle? That is, who creates
your POJO and why.
Just trying to understand.
Best regards,
Olivier Gruber, Ph.D.
Persistent & Distributed Object Platforms and Frameworks
IBM TJ Watson Research Center
Yeah I had thought of that too. I guess I wanted it to be sort of
"standardized" rather than having to write a layer. But
that's ok.
So... then I also need to know if/when the "ServiceTracker"
will be
implemented in Equinox. Or perhaps it already is, since I can remember
ServiceTracker from r3?
John Wells (Aziz)
jwells@xxxxxxx
-----Original Message-----
From: equinox-dev-bounces@xxxxxxxxxxx
[mailto:equinox-dev-bounces@xxxxxxxxxxx]
On Behalf Of Jeremy Volkman
Sent: Tuesday, November 22, 2005 11:21 AM
To: Equinox development mailing list
Subject: Re: [equinox-dev] Roadmap?
It seems as though you may be able to realize your DDS needs right now
by using the already-available ServiceTracker. When
registerDependency() is called, create a new ServiceTracker with the
given service information and start it. Override the addingService()
and removedService() methods (or provide a ServiceTrackerCustomizer)
and make these methods call your POJO's setter/unsetter (remember that
this is a dynamic environment, so you'll probably want an unsetter).
The only issue is that you'll need to provide ServiceTracker with a
BundleContext for it to get a ServiceReference.
Jeremy Volkman
On 11/22/05, John Wells <jwells@xxxxxxx>
wrote:
> Yes, I do like the delayed activation part of DS. Here are some
issues
> I have with DS (since you asked - didn't you? ;-)
>
> I would like to be able to have a POJO that uses a service which gets
> injected. While I think that with DS I can declare a class that
the
DS
> would instantiate, what I want is something more dynamic. I
want to
be
> able to have my own class (that I instantiated myself in whatever
way)
> declare that it wants a service (e.g. "com.acme.Foo") injected
into
it.
> This class would *not* be under the lifecycle control of DS, but would
> still be getting the dependent service injected into it appropriately
as
> the class is available in the OSGi framework.
>
> In my mind I have been calling such a facility "Dynamic DS"
or DDS for
> short. It would be a service or a class with static methods
that had
> methods like the following:
>
> /**
> * This method would call the setter on the object when the
appropriate
> * service becomes "available", but objectToInject
is *not* under the
> * specific control of the DS framework
> * Note: There are likely other "registerDependency"
verbs that
specify
> * all the options available in the DS configuration file and
OSGi
> * service filters
> * @param serviceName The name of the service I would like to
depend
on
> * @param setterName The name of the setter - a public void method
that
> * takes the type as the argument
> * @param objectToInject The object (not under the control of
DS) to
> * "inject"
> */
> public static void registerDependency(String serviceName, String
> setterName, Object objectToInject) throws WhateverException;
>
> /**
> * This method removes the dependency, for when the object is
done
> needing
> * the service.
> */
> public static void unregisterDependency(String serviceName, Object
> objectToInject) throws WhateverException;
>
> Obviously, the above is pseudo-code and I wouldn't mind having the
> "registerDependency" return some form of object that can
be used to
> unregister the dependency later. I also wouldn't mind having
the
> registerDependency take some form of other object (e.g. BundleContext)
> that it might need in order to make it work in OSGi. (However,
one of
> the design goals I have is to make any OSGi specific imports not
> visible, so I would almost prefer some sort of wrapper or even
> name-based mechanism).
>
> The basic idea is that independent object can register for injection
> dynamically, and would not have to muck about in the OSGi API in order
> to do service tracking or the like.
>
> Or perhaps there is already a way to do this with the current DS?
I
> looked at the spec and the API, but it is possible I missed something?
> Thanks for helping me understand this a bit more.
>
> And of course, I still need the DS like yesterday ;-).
>
> Anyway, have a nice day.
>
> John Wells (Aziz)
> jwells@xxxxxxx
> -----Original Message-----
> From: equinox-dev-bounces@xxxxxxxxxxx
> [mailto:equinox-dev-bounces@xxxxxxxxxxx]
On Behalf Of BJ Hargrave
> Sent: Tuesday, November 22, 2005 10:34 AM
> To: Equinox development mailing list
> Subject: Re: [equinox-dev] Roadmap?
>
> IBM is in the process of preparing a contribution of a Declarative
> Services implementation (among other selected services). Stay tuned...
>
> I would have to say Declarative Service is the best to use. But in
the
> interest of full disclosure, I was the designer of Declarative
Services
> :-) I am also not very familiar with GBeans. But DS does fully
integrate
>
> with the OSGi service model and has certain desirable performance
> characteristics such as delayed activation.
>
> BJ Hargrave
> Senior Technical Staff Member, IBM
> OSGi Fellow and CTO of the OSGi Alliance
> hargrave@xxxxxxxxxx
> Office: +1 407 849 9117 Mobile: +1 386 848 3788
>
>
>
> "John Wells" <jwells@xxxxxxx>
> Sent by: equinox-dev-bounces@xxxxxxxxxxx
> 2005-11-22 10:00 AM
> Please respond to
> Equinox development mailing list
>
>
> To
> <equinox-dev@xxxxxxxxxxx>
> cc
>
> Subject
> [equinox-dev] Roadmap?
>
>
>
>
>
>
> Is there a roadmap for Equinox, especially where it concerns the
> compendium services of r4? In particular, I am interested in
using
the
> Declarative Services Specification?
>
> I have been looking around to see if I could find information about
it
> (dss), but haven?t found anything other than a handful of mail in
the
> archive. In particular, I need to have a good idea when (if)
dss is
> going
> to be implemented. I?ve even considered just implementing that
part
of
> the specification myself in order to get it quicker.
>
> Also:
>
> Both DSS and GBeans are IoC frameworks. Does anyone have any
opinions
> on
> which are easier to use? Better? Any pros/cons?
>
> John Wells (Aziz)
> jwells@xxxxxxx
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