Re: Numpty "synchronized" question with ArrayList
On 10/25/2010 6:50 PM, Richard Maher wrote:
Hi Eric,
"Eric Sosman"<esosman@ieee-dot-org.invalid> wrote in message
news:ia3shk$mf7$1@news.eternal-september.org...
On 10/25/2010 7:25 AM, Richard Maher wrote:
[...]
Can someone please explain why locking/synchronizing on the ArrayList
instance itself is not sufficent to serialize access?
No one can explain, because it *is* sufficient -- provided you
remember to do it every time,
Ok, no problem.
and provided any clients that get a
view of your list also remember to do it every time,
Are you using view in the regular english sense of the word here or are
there specific Java constructs that you had in mind?
I was making an attempt at English. Some specific Java constructs
that qualify as "views of your list" might be:
- An actual reference to your List
- An Iterator or ListIterator for your List
- An unmodifiableList() or other "wrapper" of your List
- An externally-accessible method that accesses the List
.... and there are probably a few others I haven't thought of.
In my case the ArrayList is used internally, privately, and never return-ed,
Then how can two different threads get access to it? If the List
is used by only one thread, there's no need to synchronize -- one thread
can do only one thing at a time, so there's no way for it to interfere
with itself.
Much hinges, I guess, on exactly what you mean by "internally,
privately, and never return-ed."
but even if the opposite was the case then as long as the consumer(s) also
lock the ArrayList via their reference variables then all is serialized yes?
If more than one thread accesses the List, all must synchronize on
the same object while doing so. Commonly, that object is the List
itself, but in fact any object would do, so long as all threads use
the same one.
Special case: If you create an populate a List in one thread before
you "publish" it to others, and if it is purely read-only thereafter,
synchronization might not be needed. It depends on the implementation
of the List, though: For example, a "self-organizing" List might change
its internal state in response to a query, and the state change would
require synchronization. I don't think any of the Java-provided
collections do such things, but I could be wrong.
--
Eric Sosman
esosman@ieee-dot-org.invalid