Re: [sca-j] Why is @Property.required attribute MUST be true forconstructor parameter?

[Mike Edwards <mike_edwards@uk.ibm.com>]

Folks,

When using constructor parameters for
injection, there are a couple of considerations to bear in mind:

1) Each constructor parameter MUST be
annotated with either @Reference or with @Property, depending
on the type of the parameter.  This
is required in order that each parameter has a NAME which then appears
in the componentType - without this,
it is impossible to get a name for the constructor parameter (none is
available via introspection)

2) Clearly, each constructor parameter
MUST supply a value, even if that value is NULL.  There is no
avoiding this.  The runtime MUST
pass in some value when it invokes the constructor.  This is the reason
behind the requirement for required=true
on the @Property annotation.  The using component can set
any value it likes for the property
- potentially including "nil" - but it must set something.

On the point that Raymond makes that
the property value "can be inherited from the componentType"
- this
is not correct.  The componentType
is entirely derived from the implementation itself, not from some other
metadata.  In fact, there is no
separate componentType file any longer for Java implementations.  So
the
only place for a property value to come
from for a constructor parameter is from the component configuration
that uses the Java implementation.


Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com

From:	Raymond Feng <rfeng@us.ibm.com>
To:	sca-j@lists.oasis-open.org
Date:	23/02/2009 21:09
Subject:	Re: [sca-j] Why is @Property.required attribute MUST be true for constructor parameter?

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My understanding is that @required=true means
the property MUST be set with a value which can be either configured at
the component level or inherited from the componentType. 

For example, if we have a constructor as follows: 

public MyServiceImpl(@Property(name="myProp") String myProp)
{ 
        ... 
} 

The injection for "myProp" is required no matter what value it
is set. 

Thanks, 
Raymond

From:	C Vamsi <vamsic007@in.ibm.com>
To:	sca-j@lists.oasis-open.org
Date:	02/23/2009 10:41 AM
Subject:	[sca-j] Why is @Property.required attribute MUST be true for constructor parameter?

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sca-javacaa-1.1-spec-cd02-rev2.doc:

Sec 9.15 @Property

Lines 1622-1623:
required (optional) – specifies whether injection is required, defaults
to
true. For a constructor parameter annotation, this attribute MUST have
the
value true.

Q: Why is, "For a constructor parameter annotation, this attribute
MUST
have the value true." required?

The componentType can define a default value for the property. In this
case, even if a property is not specified in the component definition,
the
runtime will still have a value to pass as an argument to the constructor.


++Vamsi
Apache Tuscany Committer  http://tuscany.apache.org
Apache Geronimo Committer and Member of PMC  http://geronimo.apache.org

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