Re: Java concurrency
puzzlecracker wrote:
On Aug 19, 10:23 pm, EJP <esmond.not.p...@not.bigpond.com> wrote:
puzzlecracker wrote:
The only
instance the rings the bell, if there is a change between "check" and
"act" that break the invariant, and hence violating an operation's
atomic constrain.
Well, that's the reason you don't do it. That's why you don't do it.
That's also why e.g. you don't ping a server then try to connect to it
if it pinged OK - it can go down between the ping and the connect, and
the connect will fail anyway when it needs to, so the ping was
pointless. Same applies to this sort of thing:
if (!targetFile.exists())
sourceFile.renameTo(targetFile);
And to coup with this sort of situation, you add locking mechanism,
hence making the check for existence and an operation on the file
atomic?
That is one way.
But sometimes it is better to just act and then catch an exception
if it was not possible.
Classic simple example: don't lock and check if username already exist
in database - just put a unique index on the field and try an
insert and catch the exception if it was already there.
Arne
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