[slf4j-dev] Service Provider Interface (using jcl104-over-slf4j.jar with axis.jar)

[Ceki Gülcü]

Dave,

Thank you for the clarifications. Looking at [1] and given that 
o.a.c.logging.LogFactory class as shipped in jcl104-over-slf4j.jar does not 
define FACTORY_PROPERTIES and FACTORY_DEFAULT, do you have an idea how can 
the code run? The code should barf before the find() method in 
DiscoverSingleton can be invoked...

Have you modified jcl104-over-slf4j.jar?

Anyway, at this point, the correct thing to do would be to define the 
missing variables in jcl104-over-slf4j. Would you like to give a modified 
copy of SLF4J a try?

[1] 
http://svn.apache.org/viewcvs.cgi/webservices/axis/trunk/java/src/org/apache/axis/components/logger/LogFactory.java

At 06:00 PM 3/21/2006, Dave Wallace wrote:
>Hi Ceki,
>I knew I did I horrible job of explaining, there's just too many levels
>of indirection involved !  :) Anyhow:
>
>I am pretty certain that I've gotten rid of commons-logging, I believe
>the code that caused the problem is actually in
>  org.apache.axis.components.logger.LogFactory in the _axis.jar_. In
>which there is a bit of code:
>-----
>  return DiscoverSingleton.find(org.apache.commons.logging.LogFactory.class,
>
>org.apache.commons.logging.LogFactory.FACTORY_PROPERTIES,
>
>org.apache.commons.logging.LogFactory.FACTORY_DEFAULT);
>----
>Since my copy of axis.jar was compiled against commons-logging.jar,
>org.apache.commons.logging.LogFactory.FACTORY_*  must refer to constants
>that were _folded into the axis.jar_ code (these constants do not appear
>in jcl104-over-slf4j.jar) with these values (from commons-logging source):
>----
>    public static final String FACTORY_PROPERTY =
>"org.apache.commons.logging.LogFactory";
>    public static final String FACTORY_DEFAULT =
>"org.apache.commons.logging.impl.LogFactoryImpl";
>----
>And my original failure resulted precisely because I do not have
>"org.apache.commons.logging.impl.LogFactoryImpl" in my classpath.
>
>DiscoverSingleton is from org.apache.commons.discovery.tools:
>http://jakarta.apache.org/commons/discovery/commons-discovery-0.2/docs/api/org/apache/commons/discovery/tools/DiscoverSingleton.html
>.
>Those docs describe how the discovery process works, first looking for
>the property, then attempting an SPI lookup, then falling back to the
>default.
>At any rate, my take on the /META-INF/services SPI mechanism is that it
>allows jcl104-over-slf4j.jar to announce: "I am the provider of the
>service named org.apache.commons.logging.LogFactory and the
>implementation is org.apache.commons.logging.impl.SLF4FLogFactory"
>
>-Dave
>
>Ceki Gülcü wrote:
> >
> > Hello Dave,
> >
> > I am a little puzzled by your message. The code contained in
> > jcl104-over-slf4j.jar makes no use of meta-information or the
> > org.apache.commons.logging.LogFactory system variable. The fact that
> > setting JCL related variables has an effect means that you have a copy
> > of commons-logging.jar or commons-logging-api.jar in your environment.
> > Have you checked that?
> >
> > Cheers,
> >
> > At 04:25 PM 3/21/2006, Dave Wallace wrote:
> >> Hello,
> >> I recently encountered some trouble adding axis.jar to an existing
> >> project that uses slf4j and jcl104-over-slf4j.jar to take over logging
> >> for hibernate and a few other apache commons jars.
> >> The application failed to load axis.jar until I set this property on the
> >> Java VM command line:
> >> 
> -Dorg.apache.commons.logging.LogFactory=org.apache.commons.logging.impl.SLF4FLogFactory.
> >>
> >>
> >> As it turns out, axis.jar uses commons-logging but through a
> >> commons-discovery mechanism that looks for a factory implementation
> >> based on the value of a property with a name from
> >> org.apache.commons.logging.LogFactory.FACTORY_PROPERTY. Hence the
> >> property is required, but this led me to look a little deeper into the
> >> discovery mechanism.
> >>
> >> Following
> >> http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#Service%20Provider
> >> I added a file named
> >> "/META-INF/services/org.apache.commons.logging.LogFactory" to
> >> jcl104-over-slf4j.jar. The file contained a single line naming
> >> org.apache.commons.logging.impl.SLF4FLogFactory as a provider of the
> >> LogFactory service. With this modification the property is no longer
> >> necessary.
> >>
> >> Do you think this might be a good thing to add to the jar permanently so
> >> that whenever jcl104-over-slf4j.jar is dropped in the classpath it
> >> becomes a provider of the org.apache.commons.logging.LogFactory service?
> >>
> >> - Dave
> >
>
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-- 
Ceki Gülcü
http://ceki.blogspot.com/