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Posted to dev@felix.apache.org by Julian Sedding <js...@apache.org> on 2017/02/02 12:53:28 UTC
Component Property Type with empty array as default value
Hi all
I have been puzzled recently when using DS Component Property Types by
the following situation (see also FELIX-5404[0]):
@interface Config {
String[] whitelist default {};
}
public class WhitelistComponent {
private String[] whitelist;
@Activate
void activate(Config config) {
this.whitelist = config.whitelist(); // throws NullPointerException
}
}
Hopefully, you agree that this behaviour is unexpected and
undesirable. I certainly did not expect a NullPointerException when
calling "config.whitelist()"; I expected an empty array, as that's the
default value I defined. (I actually believe this behaviour is in
violation of the Java spec, see my comment in FELIX-5404).
After the discussion in FELIX-5404 I did some more reading in the
spec, but I am still not entirely sure if the problem is in the spec
or in the implementation.
When I look at the generated metatype information the XML files
contain the empty array as default value. The Metatype Service Spec
1.3 clearly defines the case of an empty array default values: "[...]
The empty string is significant and must be seen as an empty List or
array if specified as the default for an attribute with a cardinality
that is not equal to zero. [...]" (105.7.1 XML Schema of a Meta Type
Resource).
However, the Declarative Services Spec 1.3 is less clear about empty
array default values. As I read it, an array (default) value must be
represented in the <property> tag's body: "[...] If the value
attribute is not specified, the body of the property element must
contain one or more values. The value of the property is then an array
of the specified type. [...]". However, it mandates that there be one
or more values, thus explicitly excluding the possibility to represent
an empty array. Our tooling seems to omit any <property> tags with an
empty default value. It is unclear to me how an empty array default
value can or should be represented in a component descriptor.
AFAIU the metatype does not factor in when component configurations
are concerned, hence the default value present in the metatype is
irrelevant for this situation.
This leaves two possible places where this behaviour could be fixed:
1. My understanding is wrong and an empty array default value can be
represented in the component descriptor XML. Then I believe we need to
fix our tooling.
2. When Component Property Types are used, the annotation's default
values should be consulted as a last resort (i.e. if no service
property is set). We would need to check whether this is compliant
with the spec, however.
The third option is to accept this oddity and leave the implementation
as-is. However, this would likely confuse users of component property
types and negate some of their benefits (i.e. null checks would still
be necessary, even if default values are used).
Thanks for your thoughts and insights!
Regards
Julian
[0] https://issues.apache.org/jira/browse/FELIX-5404
Re: Component Property Type with empty array as default value
Posted by Julian Sedding <js...@gmail.com>.
Thanks for your feedback, Carsten and Ray.
I did as you suggested and opened https://osgi.org/bugzilla/show_bug.cgi?id=208
Regards
Julian
On Fri, Feb 10, 2017 at 9:11 PM, Raymond Auge <ra...@liferay.com> wrote:
> Hello Julien,
>
> As Carsten stated you should post this question on the
> osgi-dev@mail.osgi.org mailing as it relates to specifying behaviour.
>
> However, I tend to agree with your assessment and I think it may be
> reasonable to make some amendments to the specs to clarify the cases you
> mentioned. And as the EGs are in the throws of R7 updates this would be a
> good time to do it, particularly since we are doing other annotation
> related work as we speak :).
>
> In fact the very best thing might be to create a bug report on the OSGi
> Alliance bug tracker https://osgi.org/bugzilla/
>
> Sincerely,
> - Ray
>
> On Fri, Feb 10, 2017 at 2:25 PM, Carsten Ziegeler <cz...@apache.org>
> wrote:
>
>> Hi Julian
>>
>> I think this question is best asked on osgi-dev@mail.osgi.org .
>>
>> Regards
>> Carsten
>>
>> Julian Sedding wrote
>> > Hi all
>> >
>> > I have been puzzled recently when using DS Component Property Types by
>> > the following situation (see also FELIX-5404[0]):
>> >
>> > @interface Config {
>> > String[] whitelist default {};
>> > }
>> >
>> > public class WhitelistComponent {
>> > private String[] whitelist;
>> > @Activate
>> > void activate(Config config) {
>> > this.whitelist = config.whitelist(); // throws
>> NullPointerException
>> > }
>> > }
>> >
>> > Hopefully, you agree that this behaviour is unexpected and
>> > undesirable. I certainly did not expect a NullPointerException when
>> > calling "config.whitelist()"; I expected an empty array, as that's the
>> > default value I defined. (I actually believe this behaviour is in
>> > violation of the Java spec, see my comment in FELIX-5404).
>> >
>> > After the discussion in FELIX-5404 I did some more reading in the
>> > spec, but I am still not entirely sure if the problem is in the spec
>> > or in the implementation.
>> >
>> > When I look at the generated metatype information the XML files
>> > contain the empty array as default value. The Metatype Service Spec
>> > 1.3 clearly defines the case of an empty array default values: "[...]
>> > The empty string is significant and must be seen as an empty List or
>> > array if specified as the default for an attribute with a cardinality
>> > that is not equal to zero. [...]" (105.7.1 XML Schema of a Meta Type
>> > Resource).
>> >
>> > However, the Declarative Services Spec 1.3 is less clear about empty
>> > array default values. As I read it, an array (default) value must be
>> > represented in the <property> tag's body: "[...] If the value
>> > attribute is not specified, the body of the property element must
>> > contain one or more values. The value of the property is then an array
>> > of the specified type. [...]". However, it mandates that there be one
>> > or more values, thus explicitly excluding the possibility to represent
>> > an empty array. Our tooling seems to omit any <property> tags with an
>> > empty default value. It is unclear to me how an empty array default
>> > value can or should be represented in a component descriptor.
>> >
>> > AFAIU the metatype does not factor in when component configurations
>> > are concerned, hence the default value present in the metatype is
>> > irrelevant for this situation.
>> >
>> > This leaves two possible places where this behaviour could be fixed:
>> >
>> > 1. My understanding is wrong and an empty array default value can be
>> > represented in the component descriptor XML. Then I believe we need to
>> > fix our tooling.
>> > 2. When Component Property Types are used, the annotation's default
>> > values should be consulted as a last resort (i.e. if no service
>> > property is set). We would need to check whether this is compliant
>> > with the spec, however.
>> >
>> > The third option is to accept this oddity and leave the implementation
>> > as-is. However, this would likely confuse users of component property
>> > types and negate some of their benefits (i.e. null checks would still
>> > be necessary, even if default values are used).
>> >
>> > Thanks for your thoughts and insights!
>> >
>> > Regards
>> > Julian
>> >
>> > [0] https://issues.apache.org/jira/browse/FELIX-5404
>> >
>>
>>
>>
>> --
>> Carsten Ziegeler
>> Adobe Research Switzerland
>> cziegeler@apache.org
>>
>>
>
>
> --
> *Raymond Augé* <http://www.liferay.com/web/raymond.auge/profile>
> (@rotty3000)
> Senior Software Architect *Liferay, Inc.* <http://www.liferay.com>
> (@Liferay)
> Board Member & EEG Co-Chair, OSGi Alliance <http://osgi.org> (@OSGiAlliance)
Re: Component Property Type with empty array as default value
Posted by Raymond Auge <ra...@liferay.com>.
Hello Julien,
As Carsten stated you should post this question on the
osgi-dev@mail.osgi.org mailing as it relates to specifying behaviour.
However, I tend to agree with your assessment and I think it may be
reasonable to make some amendments to the specs to clarify the cases you
mentioned. And as the EGs are in the throws of R7 updates this would be a
good time to do it, particularly since we are doing other annotation
related work as we speak :).
In fact the very best thing might be to create a bug report on the OSGi
Alliance bug tracker https://osgi.org/bugzilla/
Sincerely,
- Ray
On Fri, Feb 10, 2017 at 2:25 PM, Carsten Ziegeler <cz...@apache.org>
wrote:
> Hi Julian
>
> I think this question is best asked on osgi-dev@mail.osgi.org .
>
> Regards
> Carsten
>
> Julian Sedding wrote
> > Hi all
> >
> > I have been puzzled recently when using DS Component Property Types by
> > the following situation (see also FELIX-5404[0]):
> >
> > @interface Config {
> > String[] whitelist default {};
> > }
> >
> > public class WhitelistComponent {
> > private String[] whitelist;
> > @Activate
> > void activate(Config config) {
> > this.whitelist = config.whitelist(); // throws
> NullPointerException
> > }
> > }
> >
> > Hopefully, you agree that this behaviour is unexpected and
> > undesirable. I certainly did not expect a NullPointerException when
> > calling "config.whitelist()"; I expected an empty array, as that's the
> > default value I defined. (I actually believe this behaviour is in
> > violation of the Java spec, see my comment in FELIX-5404).
> >
> > After the discussion in FELIX-5404 I did some more reading in the
> > spec, but I am still not entirely sure if the problem is in the spec
> > or in the implementation.
> >
> > When I look at the generated metatype information the XML files
> > contain the empty array as default value. The Metatype Service Spec
> > 1.3 clearly defines the case of an empty array default values: "[...]
> > The empty string is significant and must be seen as an empty List or
> > array if specified as the default for an attribute with a cardinality
> > that is not equal to zero. [...]" (105.7.1 XML Schema of a Meta Type
> > Resource).
> >
> > However, the Declarative Services Spec 1.3 is less clear about empty
> > array default values. As I read it, an array (default) value must be
> > represented in the <property> tag's body: "[...] If the value
> > attribute is not specified, the body of the property element must
> > contain one or more values. The value of the property is then an array
> > of the specified type. [...]". However, it mandates that there be one
> > or more values, thus explicitly excluding the possibility to represent
> > an empty array. Our tooling seems to omit any <property> tags with an
> > empty default value. It is unclear to me how an empty array default
> > value can or should be represented in a component descriptor.
> >
> > AFAIU the metatype does not factor in when component configurations
> > are concerned, hence the default value present in the metatype is
> > irrelevant for this situation.
> >
> > This leaves two possible places where this behaviour could be fixed:
> >
> > 1. My understanding is wrong and an empty array default value can be
> > represented in the component descriptor XML. Then I believe we need to
> > fix our tooling.
> > 2. When Component Property Types are used, the annotation's default
> > values should be consulted as a last resort (i.e. if no service
> > property is set). We would need to check whether this is compliant
> > with the spec, however.
> >
> > The third option is to accept this oddity and leave the implementation
> > as-is. However, this would likely confuse users of component property
> > types and negate some of their benefits (i.e. null checks would still
> > be necessary, even if default values are used).
> >
> > Thanks for your thoughts and insights!
> >
> > Regards
> > Julian
> >
> > [0] https://issues.apache.org/jira/browse/FELIX-5404
> >
>
>
>
> --
> Carsten Ziegeler
> Adobe Research Switzerland
> cziegeler@apache.org
>
>
--
*Raymond Augé* <http://www.liferay.com/web/raymond.auge/profile>
(@rotty3000)
Senior Software Architect *Liferay, Inc.* <http://www.liferay.com>
(@Liferay)
Board Member & EEG Co-Chair, OSGi Alliance <http://osgi.org> (@OSGiAlliance)
Re: Component Property Type with empty array as default value
Posted by Carsten Ziegeler <cz...@apache.org>.
Hi Julian
I think this question is best asked on osgi-dev@mail.osgi.org .
Regards
Carsten
Julian Sedding wrote
> Hi all
>
> I have been puzzled recently when using DS Component Property Types by
> the following situation (see also FELIX-5404[0]):
>
> @interface Config {
> String[] whitelist default {};
> }
>
> public class WhitelistComponent {
> private String[] whitelist;
> @Activate
> void activate(Config config) {
> this.whitelist = config.whitelist(); // throws NullPointerException
> }
> }
>
> Hopefully, you agree that this behaviour is unexpected and
> undesirable. I certainly did not expect a NullPointerException when
> calling "config.whitelist()"; I expected an empty array, as that's the
> default value I defined. (I actually believe this behaviour is in
> violation of the Java spec, see my comment in FELIX-5404).
>
> After the discussion in FELIX-5404 I did some more reading in the
> spec, but I am still not entirely sure if the problem is in the spec
> or in the implementation.
>
> When I look at the generated metatype information the XML files
> contain the empty array as default value. The Metatype Service Spec
> 1.3 clearly defines the case of an empty array default values: "[...]
> The empty string is significant and must be seen as an empty List or
> array if specified as the default for an attribute with a cardinality
> that is not equal to zero. [...]" (105.7.1 XML Schema of a Meta Type
> Resource).
>
> However, the Declarative Services Spec 1.3 is less clear about empty
> array default values. As I read it, an array (default) value must be
> represented in the <property> tag's body: "[...] If the value
> attribute is not specified, the body of the property element must
> contain one or more values. The value of the property is then an array
> of the specified type. [...]". However, it mandates that there be one
> or more values, thus explicitly excluding the possibility to represent
> an empty array. Our tooling seems to omit any <property> tags with an
> empty default value. It is unclear to me how an empty array default
> value can or should be represented in a component descriptor.
>
> AFAIU the metatype does not factor in when component configurations
> are concerned, hence the default value present in the metatype is
> irrelevant for this situation.
>
> This leaves two possible places where this behaviour could be fixed:
>
> 1. My understanding is wrong and an empty array default value can be
> represented in the component descriptor XML. Then I believe we need to
> fix our tooling.
> 2. When Component Property Types are used, the annotation's default
> values should be consulted as a last resort (i.e. if no service
> property is set). We would need to check whether this is compliant
> with the spec, however.
>
> The third option is to accept this oddity and leave the implementation
> as-is. However, this would likely confuse users of component property
> types and negate some of their benefits (i.e. null checks would still
> be necessary, even if default values are used).
>
> Thanks for your thoughts and insights!
>
> Regards
> Julian
>
> [0] https://issues.apache.org/jira/browse/FELIX-5404
>
--
Carsten Ziegeler
Adobe Research Switzerland
cziegeler@apache.org