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RE: [equinox-dev] Roadmap?


That's correct. The UI component framework is not UI specific.  Generally speaking, it provides a loosely-coupled way to instantiate and destroy objects.

Concretely, here was our driving use-case:
- API Clients are allowed to implement views and editors (or "parts").
- We wanted to add API such that clients could host parts.
- Parts have a number of local APIs available to them, provided by their containing context. We called these local API "services", but they are more like Eclipse adapters than OSGi services.
- These service APIs change over time.
- Clients that host parts can offer an extended set of services to the parts they host.
- All parts need to work in all contexts. For example:
     - If a part updates itself to use the latest APIs, it should continue to work when hosted in a context that still implements the old API.
     - If a host is updated to implement the latest APIs, it should continue to be able to host parts that expect the old version.
     - If a part uses an extended set of features, it should still work in a context that does not know about the extended features.
- We wanted to be able to act on an open-ended set of services. For example, it should be possible to write a wrapper part that overrides a closed set of services and delegates everything else to another part.

Put simply, we wanted to make all parts interchangeable, but still leave avenues open for API evolution. These are issues that apply to any object that gets created from an extension point, and our implementation was similarly generic. Although the IoC framework works with any POJO, I will continue using the word "part" since I find the UI metaphor easiest to visualize.

We wanted our implementation to have the following properties:
- It must be simpler (require less code + XML) to implement a component-based part than a traditional part.
- There must not be a significant memory or CPU cost to using IoC.
- It must be self-documenting in the following sense: given a set of plugins, a developer should be able to generate the list of services that they could use in the implementation of their part.
- Services can depend on other services. Since all are created through the framework, there is no real distinction between the implementation of a "part" and a "service".

Although the framework was originally intended for views and editors, it is very well suited for any extension point that calls createExecutableExtension and I have used it for many non-UI extension points. I do not believe it would work as a replacement for OSGi's DS, as DS is mainly concerned with dynamic connections between singletons, whereas the UI IoC stuff is concerned with creating static connections to objects that can be instantiated anywhere.


Here's how our implementation works:
- Our implementation does NOT require dependencies to be declared anywhere (no need for a prereqs extension point).
- You only need to say "when someone asks for service X, create it using factory Y". When it comes time to create an X, the factory will ask for any services it needs, which get created dynamically.
- One standard factory, "ReflectionFactory" creates objects using constructor injection. It would also be possible to write a setter-injection factory or factories that are specific to a particular service.
- Although the factories are framework-aware, the objects they create can truly be POJOs with no framework awareness.

I hope that explains what it can and can't do. If you want to play with it, you'll find the main IoC container in org.eclipse.ui.internal.components.framework.Container.


My two cents:

I would not suggest trying to use the same IoC framework for OSGi services and for createExecutableExtension. The former are dynamic, so it makes sense to formally declare dependencies between services in order to defer creation of a service until all of its dependancies exist. In the case of createExecutableExtension, everything the object needs will be available immediately if the code is correctly written. This means that an IoC framework for createExecutableExtension wouldn't need all the tiresome metadata for describing service dependencies. Services can just ask for their dependencies the first time they need them.

If this were to be seriously considered for inclusion in core, I would want to trim a lot of fat first.


Example usage:

class Cat {
      String catName;
     
      Cat(String name) {
          catName = name;
      };
 }
 
 class Dog {
      Cat theCat;
 
      Dog(Cat toChase) {
          theCat = toChase;
      }
     
      public chaseCat() {
          System.out.println("Chasing " + theCat.getName());
      }
 }
 
 ...
 
public static void main(String[] args) {
      // Create a context that knows how to create Dogs. Any time it needs a Cat,
      // it will refer to poor Fluffy.
      FactoryMap context = new FactoryMap()
          .add(Dog.class, new ReflectionFactory(Dog.class))
          .addInstance(Cat.class, new Cat("Fluffy"));
     
      // Create a container for a particular Dog
      Container container = new Container(context);
     
      try {
          // Create a Dog
          Dog myDog = (Dog)container.getService(Dog.class);
     
          // Chase Fluffy around
          myDog.chaseCat();
         
      } catch (ComponentException e) {
          // We weren't able to create the Dog
          System.out.println(e.toString());
      } finally {
          // Clean up the container when no longer needed
          container.dispose();
      }
 }






Pascal Rapicault/Ottawa/IBM

11/22/2005 10:04 PM

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If I remember correctly, the design of the support for nesting UI component was based of an IoC container and it had nothing UI specific.
It would be good to see if we could actually reuse some of the work that had been done there.
Stephan, am I completly smoking? Would you be able to provide us with some usefull pointers so people can explore?

Thanks,

PaScaL



Jeff McAffer/Ottawa/IBM@IBMCA
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11/22/2005 09:52 PM

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RE: [equinox-dev] Roadmap?






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>
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11/22/2005 04:00 PM

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RE: [equinox-dev] Roadmap?







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

  "John Wells" < jwells@xxxxxxx>
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      Subject:        RE: [equinox-dev] Roadmap?




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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