Wednesday, January 12, 2011

Clojure: Using Java Inner Classes

Recently, I wanted to add some specific behavior to my Clojure application when any uncaught exception occurred. This is easy enough by calling the setDefaultUncaughtExceptionHandler (sDUEH); however, it's worth noting that the sDUEH takes a Thread.UncaughtExceptionHandler argument. Clojure has no problem giving you access to inner classes, but the syntax is slightly different: Outer$Inner in Clojure is the same as Outer.Inner in Java.

It's easy to use the REPL and determine if you have things correct.
user=> Thread$UncaughtExceptionHandler
java.lang.Thread$UncaughtExceptionHandler
This works without issue, because java.lang is (generally) available in Clojure.

If you want access to an inner class that is not in java.lang you'll want to import it directly. For example, the following REPL session shows how you can access the ThreadPoolExecutor.DiscardPolicy class.
user=> ThreadPoolExecutor$DiscardPolicy
java.lang.Exception: Unable to resolve symbol: ThreadPoolExecutor$DiscardPolicy in this context (NO_SOURCE_FILE:0)
user=> (ns my-ns (:import [java.util.concurrent ThreadPoolExecutor$DiscardPolicy]))
java.util.concurrent.ThreadPoolExecutor$DiscardPolicy
my-ns=> ThreadPoolExecutor$DiscardPolicy
java.util.concurrent.ThreadPoolExecutor$DiscardPolicy
Returning the entire class in the REPL should suffice as proof that everything is working fine; however, the code below is a brief example of using Thread$UncaughtExceptionHandler in a larger context.
jfields:Documents jfields$ cat inner.clj
(def handler (proxy [Thread$UncaughtExceptionHandler] []
(uncaughtException [thread exception]
(println thread exception))))

(Thread/setDefaultUncaughtExceptionHandler handler)

(/ 12 0)


jfields:Documents jfields$ java -cp ../dev/clojure-current/clojure.jar clojure.main -i inner.clj
#<Thread Thread[main,5,main]> #<CompilerException java.lang.ArithmeticException: Divide by zero (inner.clj:0)>
As you can see from the command-line output, the exception is handled by the proxy of Thread$UncaughtExceptionHandler.
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