I really really don't want to see us modifying the class to indicate a
different class version. This path seems fraught with peril; this type
of "white lie" (actually it's a pretty big lie) seems to me fraught with
peril; who knows what other functionality depends on correct versioning
of the class file?
David
Andrew McIntyre wrote:
On Jan 24, 2006, at 5:13 PM, Daniel John Debrunner wrote:
Are generics represented by new entries in the class file that break on
older VMs?
This question caught my eye, and I did a little research. The answer is
both yes and no.
The bytecode for generics is compatible with older VMs because the
generic type signature is erased at compile time and appropriate casts
added by the compiler. The new methods could be used in the older VM,
but casts which were not expected when the newer class was compiled
would result in a ClassCastException.
But, attempting to load a class with generics compiled with 1.5/1.6 in a
1.4 JVM yields this error:
Exception in thread "main" java.lang.UnsupportedClassVersionError:
Generic (Unsupported major.minor version 49.0)
It is also not possible to compile a class with generics with -target 1.4.
But, you could change the class version so that the class got past the
verifier on the earlier VM and it would still work. I even verified that
this would work with a simple test case. I haven't had time to look at
whether the new methods with generics have counterparts in JDBC 3.0, but
assuming they don't, and the JSR-014 charter says that JDBC 4.0 should
be backwards compatible with JDBC 3.0, altering the class version would
do the trick.
But, then you've entered the dreaded area of modifying class files
post-compilation. :-)
Also, I found this
document: http://homepages.inf.ed.ac.uk/wadler/gj/Documents/gj-oopsla.pdf
very enlightening as to the mechanics of how generic types are handled
in the compiler. It was linked from the JSR-014 page.
andrew
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