A DESCRIPTION OF THE REQUEST :
Tuning memory for Java is a time consuming issue, and for some applications it's mostly trial-and-error guesswork.    You should be able to change the -Xmx, the garbage collection algorithm, the generation sizes, etc after the program is already running.

JUSTIFICATION :
Not all applications can be re-run any time you want to tweak memory settings.  Sometimes you predict poorly, and see an OOM errory coming, but can do nothing about it because you can bump the -Xmx up 64M to get the machine out of load balancing.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I'd expect to be able to use something on the command line to update an existing JVM, e.g.

java -Xmx512m -p 545

Would reset the JVM running at pid '545' to have the maximum heap size of 512m.


ACTUAL -
You watch JVMs die, you waste time trying bad settings.