RE: FW: Java Vs RPG on iSeries

["Larry Loen" <lwloen@xxxxxxxxxx>]

>>Any one looking for a short course on this should study java.lang.String
>>and java.lang.StringBuffer and look around for explanations of "immutable
>>object" in the literature.

>Is this because all pass by reference variables are locked ( single
>threaded ) before update?  I am a C++ bigot and all I know about java is
>that .NET may marginalize it in the years to come <g>.  But I am
interested
>in how Java implements built in multi threading.

>thanks,

>Steve Richter

No, it doesn't work that way at all.  The lock never applies to variables
-- it only applies to instances.

For methods, the locking is always in terms of the "this" object whose
synchronized method is entered.  For the synchronize verb, it is the object
whose reference is specified that is locked.  That's the lot of locking in
Java.  So, that means parameters' objects are not locked simply by being in
a method call.

The more interesting features are kind of implied and kind of not.  For
instance, the "new" command (build an object from the heap, very like C++)
is inherently thread safe because only one thread is building the object
(unless you define a _very_ unusual constructor) and no one else knows
about it yet.

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.

Immutable objects are not really possible in C++ thanks to the ever-present
possibility of "well defined casting abuse" (un-const-ing) but Java is
strict enough in its semantics to have them.

There's a lot more to this than I can do justice to in this newsgroup.
But, there's a literature about it that can be discovered somewhere or
other.  Once I was tuned into it, I found out about it rather readily --
just can't remember exactly where offhand.


Larry W. Loen  -   Senior Linux, Java, and iSeries Performance Analyst
                          Dept HP4, Rochester MN