Greetings.
I'm not understanding proxies. Can anyone advise me what I'm missing?
I try the following:
user=> (def m (proxy [java.util.HashMap] []
(finalize []
;(proxy-super finalize)
(prn "finalizing..."))
(hashCode []
99)))
#'user/m
user=> (.hashCode m)
99
user=> (.finalize m)
IllegalArgumentException No matching field found: finalize for class
user.proxy$java.util.HashMap$0 clojure.lang.Reflector.getInstanceField
(Reflector.java:289)
user=> (ancestors m)
nil
user=>
(the 'proxy-super' is commented out, just to reassure myself that its presence
is not somehow the root of the problem)
I'm afraid that the documentation of the 'proxy' macro is too terse for me to
work out what's supposed to be happening here.
The documentation says that 'proxy' "creates a instance of a proxy class that
implements the named class/interface(s) by calling the supplied fns." That
suggests (without quite saying so) that 'm' should be an extension of class
java.util.HashMap, in this case, and this is corroborated by the remark that
"If a method fn is not provided for a class method, the _superclass_ methd will
be called" (my emphasis, note the docs' misspelling of 'methd'). However, the
null result of (ancestors m) suggests instead that 'm' extends only Object, and
that it handles _all_ of its method calling by delegation through reflection on
the class being proxied.
It's the second interpretation that seems to explain why I can't override
protected method 'finalize' (although that would suggest that I could create a
new method 'finalize', which shadows Object.finalize without overriding it, and
doesn't really explain why I'm seeing this error). That said, however, the
docs stress that "Note that while method fns can be provided to override
protected methods...", which seems to say that what I'm trying to do, here,
should be possible. The rest of that sentence seems to suggest that something
more complicated is happening, without revealing quite what.
Also Fogus and Houser (in The Joy of Clojure, p211) give an example of using
'proxy', in which they illustrate a function proxying FilterOutputStream, and
say that "The proxy returned by screaming-filter extends the Java class
java.io.FilterOutputStream". That seems to suggest that the proxy is indeed a
subclass of the thing it's proxying.
Finally, the on-line discussion of gen-class at
<http://clojure.org/compilation> mentions that one of the facilities it
provides is "Exposing inherited protected members". However the description of
gen-class, there, does strongly suggest that it's a heavyweight, exotic thing,
intended for fuller JVM interop rather than everyday use, and is telling me, in
effect, "these aren't the macros you're looking for; move along". It sounds as
if getting involved in gen-class in this context would be an unidiomatic way of
doing something fairly simple, namely producing an anonymous extension of a
class which overrides a protected method.
What am I missing?
Thanks for any pointers. Best wishes,
Norman
--
Norman Gray : http://nxg.me.uk
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