No, I don't think you can remove the synchronisation
Assume
Thread Statement
T1 localMap[T1] = new HashMap(clients);
T1 doSomething(localMap[T1]); // Add some stuff
T2 localMap[T2] = new HashMap(clients)
T2 doSomething(localMap[T2]);
T2 clients = localMap[T2];
T1 clients = localMap[T1]; // Oops, lose adds from T2
Regards
/Mikael
>In fact, you can
>remove the synchronized block below and everything is still perfectly
>thread safe. The whole point is that the member variable "clients" is
>*NEVER* modified - only assigned to. In fact, it would drive the
point
>home even more if you did this:
>
>private Map clients = Collections.EMPTY_MAP;
>
>public void someMethod()
>{
> HashMap localMap = null;
> localMap = new HashMap(clients);
> // ... read/write work on local map ...
> clients = Collections.unmodifiableMap(localMap);
>}
>
>public void someOtherMethod()
>{
> HashMap localMap = clients;
> // ... read-only work on local map ...
>}
>
>Here, clients is always immutable, even though someMethod is able to
>update it with a new value. No synchronization, yet perfectly thread
>safe (although very expensive to modify).
>
>-Larry
>
>David Jencks wrote:
>> I think you are wrong, although I have trouble understanding all
the issues
>> with this. I think this is a "double checked locking" idiom and
I think it
>> is just as broken.
>>
>> The guy who calls someOtherMethod will definitely see the correct
hashmap,
>> but there is no guarantee that its state will match any particular
state in
>> which it was put unless access to it is also synchronized.
>>
>> Double checked locking is broken.
>>
>> See Effective Java pp 193 ff and the various Double Checked Locking
is
>> Broken websites.
>>
>> As I understand it, problems with this construct are unlikely
to appear
>> unless you are using something like a multiprocessor alpha box.
>>
>> I think we should prove in some way that this construct is safe
or remove
>> it.
>>
>> david jencks
>>
>> On 2003.02.13 13:00 Larry Sanderson wrote:
>>
>>>If the map is seldom modified, then you can get around synchronization
>>>with techniques like this. It is taking advantage of the fact that
>>>assignement is an atomic operation. If the methods are like this:
>>>
>>>public void someMethod()
>>>{
>>> HashMap localMap = null;
>>> synchronized (clients)
>>> {
>>> localMap = new HashMap(clients);
>>> }
>>> // ... read/write work on local map ...
>>> clients = localMap;
>>>}
>>>
>>>public void someOtherMethod()
>>>{
>>> HashMap localMap = clients;
>>> // ... read-only work on local map ...
>>>}
>>>
>>>Now everyone can call someOtherMethod() without invoking the cost of
>>>synchronization, and once they obtain localMap it is guaranteed
not to
>>>be modified. But someMethod() exists for those rare times when
the map
>>>does need to be modified.
>>>
>>>I don't think this is as useful as it once was - synchronization
is much
>>>faster than it used to be.
>>>
>>>-Larry
>>>
>>>Scott M Stark wrote:
>>>
>>>>I have seen this usage construct in a few places in the code and it
>>>
>>>makes
>>>
>>>>no sense to me:
>>>>
>>>>class X
>>>>{
>>>> HashMap clients = new HashMap();
>>>>
>>>> public void someMethod()
>>>> {
>>>> synchronized(clients)
>>>> {
>>>> HashMap m = new HashMap(clients);
>>>> m.put(dc, cq);
>>>> clients=m;
>>>> }
>>>> ...
>>>> }
>>>> public void someOtherMethod()
>>>> {
>>>> Iterator i = clients.keySet().iterator();
>>>> while( i.hasNext() )
>>>> {
>>>> ...
>>>> }
>>>> }
>>>>}
>>>>
>>>>The unsynchronized clients HashMap is synchronized and copied when
>>>>modified and accessed without synchronization in other contexts.
This
>>>
>>>is
>>>
>>>>not thread safe for the accesses and makes for very expensive
updates.
>>>>Why isn't the HashMap simply synchronized and used without copying?
>>>>
>>>>xxxxxxxxxxxxxxxxxxxxxxxx
>>>>Scott Stark
>>>>Chief Technology Officer
>>>>JBoss Group, LLC
>>>>xxxxxxxxxxxxxxxxxxxxxxxx
>>>>
>>>>
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