I haven't taken a look at the the code so this is off the cuff, but
couldn't you use a boolean field for each of the interfaces on the
Resource class to indicate whether it is supported? Then have any
methods that implement that interface check the boolean flag to see
whether to proxy the request to the decorated resource or to throw an
exception. In code which needs to determine the abilities of any
resource, it can call an is() method rather than using instanceof. The
actual resource implementations would typically hard code the is()
return values and ignore or not implement the set() methods.
Something like:
public static MappedResource map(Resource r) {
MappedResource decorator = new MappedResource(r);
decorator.setTouchable(r.isTouchable());
decorator.setAppendable(r.isAppendable());
decorator.setFileProvider(r.isFileProvider());
return decorator;
}
Stefan Bodewig wrote:
I need help!
I'm trying to write a mapped-resource that is a decorator for a
different resource and changes the name of the decorated resource
using a mapper.
Resource itself is a class, so my base class is set. There are
several "optional" behaviors available via interfaces, namely
Touchable, Appendable and FileProvider (I hope I didn't miss one).
Our tasks use instanceof checks in several places, so if the mapped
resource wraps a FileResource it should also implement all three of
those interfaces. OTOH it must not implement any of these interfaces
if wrapping a StringResource.
Any attempt of using anonymous subclasses of MappedResource or
reflection proxies leads to objects that either no longer extend
Resource or contain the methods required by an interface but don't
implement it.
The current solution is really really ugly (seven subclasses with many
of them identical except for the class they extend):
On 2008-11-14, <[EMAIL PROTECTED]> wrote:
public static MappedResource map(Resource r) {
if (r instanceof FileProvider) {
if (r instanceof Touchable) {
if (r instanceof Appendable) {
// most probably FileResource
return new AppendableTouchableFileProviderMR(r);
}
return new TouchableFileProviderMR(r);
}
if (r instanceof Appendable) {
return new AppendableFileProviderMR(r);
}
return new FileProviderMR(r);
}
if (r instanceof Touchable) {
if (r instanceof Appendable) {
return new AppendableTouchableMR(r);
}
return new TouchableMR(r);
}
if (r instanceof Appendable) {
return new AppendableMR(r);
}
// no special interface
return new MappedResource(r);
}
The other approach I could take is to clone the wrapped resource (all
our resources are cloneable) and change the clone's name - and not
have a decorator at all.
The main problem with this approach is that some resources are
immutable and will throw an exception in setName (StringResource for
example) while others will not do what I want - FileResource will
simply ignore setName().
Any other ideas (apart from not using Java?)
Stefan
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