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From Leo Simons <lsim...@jicarilla.org>
Subject [RT] The big metadata topic (again!)
Date Thu, 04 Mar 2004 13:11:18 GMT
In response to the recent threa(d|t)s, I just wrote:

   http://www.jroller.com/comments/lsd?anchor=rethinking_attributes

for lazy people (though the blog page has embedded links), here's the scoop:

                  === Rethinking attributes ===

The new JDK 1.5 attributes are nice (they're powerful and easy to read 
just like the .Net ones), but if you can't use them yet, you'll have to 
do something else (BeanInfo files, MBeanInfo files, xml files, javadoc 
attributes, bytecode hacking, ...). Its been the topic of (sometimes 
heated) debate over at avalon for far too long now. I think I've just 
figured out why.

Its obvious really. I should've listened to the little voice in the back 
of my head that said "too much magic, too much magic, too much 
magic...". What's the simplest thing that can possibly work to associate 
arbitrary attributes with any class or instance? This:

public interface Attributes
{
   /* stuff */
}
public interface AttributeProvider
{
   public Attributes getAttrbibutes()
}
class MyComponent implements AttributeProvider
{
   public Attributes getAttrbibutes()
   {
     /* implement contract */
   }
}

Who was it that jumped to language extensions, BeanInfo classes, coding 
conventions, javadoc tags, pagelong specifications, and worse? Shame on 
you! You should've thought of the simplest possible thing first!

If that had happened, and basic OO rules about decoupling and 
composition had then been followed, none of all the current attribute 
headache would have ever existed. We would just have

public interface AttributeHelper
{
   MethodAttribute[] getAttributesFor( Object instance, String methodName );
   MethodAttribute[] getAttributesFor( Class class, String methodName );
   /* etc etc */
}
public class AttributeProviderBasedAttributeHelper
   implements AttributeHelper { /* ... */ }
public class JDK15BasedAttributeHelper
   implements AttributeHelper { /* ... */ }
public class QDoxAttributesAttributeHelper
   implements AttributeHelper { /* ... */ }
/** Detects which delegate attribute helper to use based on some 
heuristics. */
public class IntelligentAttributeHelper
   implements AttributeHelper { /* ... */ }

Fortunately, its not too late (its never too late). We can always 
refactor. Let's refactor. Jakarta Commons Attributes is nearly there 
already. We just need to make the internals of the Attributes class into 
a set of proper components (with an interface and multiple 
implementations), and then add a lightweight container (I can think of a 
few ;)) inside the facade.

-- 
cheers,

- Leo Simons

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