RFR 9: 8068578: test/java/io/Serializable/subclassGC/SubclassGC.java assumes app class loader is a URLClassLoader

[Brent Christian]

Hi,

Please review this test case update.
Bug:
https://bugs.openjdk.java.net/browse/JDK-8068578
Webrev:
http://cr.openjdk.java.net/~bchristi/8068578/webrev.0/

The test relies on creating a new URLClassLoader and using it to load a 
subclass of ObjectOutputStream.  The system classloader is cast to a 
URLClassLoader in order to call getURLs(), but the test can be made to 
work without this URL[].  We can instead create a URL based on the value 
of the "test.classes" system property.

I have confirmed that the test still detects the original bug (6232010) 
in 5u6, and passes with later JDKs.

Thanks,
-Brent