RE: FW: Java Vs RPG on iSeries

On 09/19/2002 at 01:28:33 AM, java400-l-admin@midrange.com wrote:
What happens if you _define_ an object such that its contents _never
change_ after the "new" (constructor) finishes?  Answer:  It is
automatically thread safe and everyone and his brother can have references
to it without any synchronization.  That's an immutable object.  Very
relevantly, "String" is an immutable object and so is always thread safe.
--- end of excerpt ---

Careful... Which JVM release? Pre or Post JSR 133?
Multithreading is a bit too complex even for this statement. Its almost
always
more complex than someone thinks it is. JSR 133 tries to address it in some
pieces, but
I don't know details about when that JSR was/is going to be complete
implemented, but
this is NOT true in general.  Do you know?

For others on the list...
On most hardware the statement about immutable objects is true.
However, on hardware that is weakly  consistent (i.e. allows unordered
storage access), this is
not true.

To really be safe, you simply CANNOT access objects/storage in one thread
that were
created/initialized/modified in any other thread WITHOUT synchronization.
PERIOD.
There are many tricks and exceptions that DON'T WORK. In a multi-threaded
application,
anything that can theoretically fail, WILL, and you can't rely on it.

Here's one reference, see this.
http://servlet.java.sun.com/javaone/resources/content/sf2002/conf/sessions/pdfs/1073.pdf
In the "Idioms and Pitfalls" sections, see "Safe Immutable Objects".
You'll need to be a member of JDC (Java Developer Connection) but see this:
The one I liked most was a similar presentation in the 2000 JavaOne.





"The stuff we call "software" is not like anything that human society
  is used to thinking about. Software is something like a machine, and
  something like mathematics, and something like language, and
  something like thought, and art, and information...
  but software is not in fact any of those other things."
Bruce Sterling - The Hacker Crackdown

Fred A. Kulack - IBM eServer iSeries - Enterprise Application Solutions
ERP, Java DB2 access, Jdbc, JTA, etc...
IBM in Rochester, MN  (Phone: 507.253.5982   T/L 553-5982)
mailto:kulack@us.ibm.com   Personal: mailto:kulack@magnaspeed.net
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