cond-expand feature testing for presence of a Java class

[Jamison Hope <jrh at theptrgroup dot com>]

I recently found myself wanting to conditionalize some code on the
presence of a Java class.  At first, I tried to see if I could have a
module call (provide 'some-feature) only when this class was found,
but then I realized it would be much simpler to modify the feature
testing in IfFeature.java to recognize class names as a last resort.
This has proven quite handy for me, letting me do something like:

(cond-expand
  (com.example.AClass ;; AClass is found in classpath, safe to use
    (define-simple-class MySubclass (com.example.AClass) ...))
  (else (error "AClass not found.")))

It seems unlikely for some unrelated feature's name to exactly match a
fully-qualified class name, so I think this patch is pretty safe.

$ svn diff kawa/standard/IfFeature.java
Index: kawa/standard/IfFeature.java
===================================================================
--- kawa/standard/IfFeature.java	(revision 6942)
+++ kawa/standard/IfFeature.java	(working copy)
@@ -86,7 +86,13 @@
     Declaration decl =  
Compilation.getCurrent().lookup(provide_symbol, -1);
     if (decl!=null && ! decl.getFlag(Declaration.IS_UNKNOWN))
       return true;
-    return false;
+
+    try {
+      Class.forName(name);
+      return true;
+    } catch (ClassNotFoundException ex) {
+      return false;
+    }
   }

   public static final String PROVIDE_PREFIX = "%provide%";


- Jamie

--
Jamison Hope
The PTR Group
www.theptrgroup.com

Attachment: IfFeature.patch
Description: Binary data