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From Peter Donald <pe...@realityforge.org>
Subject Re: [Avalon4:PROPOSAL] Context Consensus (version 2)
Date Thu, 12 Dec 2002 21:32:23 GMT
+1

On Fri, 13 Dec 2002 06:59, Leo Sutic wrote:
> All,
>
> I hope I have incorporated all comments. Here's the latest draft
> of the proposal. I'd like to go for a vote on this next Tuesday,
> and I would appreciate if anyone not satisfied with it could say
> so now, so the concerns can be addressed while it is still a
> [PROPOSAL] and not a [VOTE]. I know that the proposal as it
> stands is less-than-optimal for everyone, but it is my gut
> feeling that it is also at least acceptable to everyone, which
> is why I'm going forward with it.
>
>                            -oOo-
>
> Proposal: The following text should (after being HTML-ized) replace
> the current documentation for the Context interface:
>
> NOTE: In the text below there are several requirements that a
>       component may set up for a container. It is understood that
>       a container does not have to satisfy those requirements in
>       order to be Avalon-compliant. If a component says "I require
>       X to run" a container may reply with "I don't have any X, so
>       I'm not running you". The requirements here are the maximum
>       that a component may ask for, not the minimum a container must
>       deliver. However, a container should document what it is and
>       isn't capable of delivering.
>
> The context is the interface through which the component and its
> container communicate. Each Container-Component relationship involves
> defining a contract between the two entities. A Context contract is
> defined by two parameters, both of which are requirements the component
> set up for the container in its metadata:
>
> 1. The first is an interface or a class, called T below. It is
>    required that the component should be able to perform the
>    following operation:
>
>     public void contextualize( Context context )
>         throws ContextException
>     {
>         T tContext = (T) context;
>     }
>
>    This case has two variants:
>
>     a. T is an interface. In this case, the container may choose
>        any method to supply the component with a context instance
>        cast-able to T.
>
>        The container must supply an implementation for all methods
>        in the interface. This may be done via a dynamic proxy
>        that routes calls to appropriate handlers or by any
>        other method. The set of methods that a container must
>        support is defined by the standard context interfaces in
>        Framework (currently none).
>
>     b. T is a class. In this case, the class must be instantiated
>        with the T(Map,Context) constructor, and the instance
>        then be passed to the component's contextualize method.
>
>    WARNING: A component that specifies this requirement will not
>             be as portable as one that doesn't. Few containers
>             support it. It is therefore discouraged for components
>             to require a castable context.
>
> 2. The second parameter is a set of entries accessible via the
>    Context.get() method and their types. The class/interface T
>    above may also have associated metadata that specifies entries,
>    in which case these entries must be supplied by the container
>    in addition to any entries the component itself requires.
>
>    Each entry requirement must specify the canonical key name, may
>    specify a name that the canonical key should be remapped to,
>    and must specify the expected type of the value:
>
>    For an example, where the data is specified in XML:
>
>     <entry intent="avalon:work" type="java.io.File"/>
>
>     <entry key="work" intent="avalon:work" type="java.io.File"/>
>
>    NOTE: The proposal does not cover the DTD, nor does it require
>          that the entries are defined in XML. However, it does
>          require that the above three things *can* be specified.
>
>    The current list of canonical keys are (taken from
>    http://jakarta.apache.org/avalon/excalibur/info/context.html):
>
>    component.name         java.lang.String
>    This entry defines the name of the component.
>
>    component.classloader  java.lang.ClassLoader
>    The classloader via which the component was loaded. May differ
>    from the ClassLoader returned by getClass().getClassLoader() if
>    the component was loaded from parent classloader.
>
>    partition.name         java.lang.String
>    This entry defines the name of the partition.
>
>    application.name       java.lang.String
>    This entry defines the name of the application.
>
>    component.home         java.io.File
>    The location in which the component to store persistent data
>    relevent to the component.
>
>    component.work         java.io.File
>    This directory in which to store temporary or working
>    information. It may not persist over restarts of the component.
>
>    The values placed in the context are runtime values that
>    can only be provided by the container. The Context should
>    NOT be used to retrieve configuration values or services
>    that can be provided by peer components.
>
> /LS

-- 
Cheers,

Peter Donald
*------------------------------------------------------*
| "Nearly all men can stand adversity, but if you want |
| to test a man's character, give him power."          |
|       -Abraham Lincoln                               |
*------------------------------------------------------*


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