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From: Neil Coffey on 23 Jul 2008 00:04
Greg Stasica wrote:

> public class DBQuery{
>
> private static DBQuery dbQuery = null;
>
> public static DBQuery getInstance()
> {
> if (dbQuery == null) {
> dbQuery = new DBQuery();
> }
> return dbQuery;
> }

You do know that this getInstance() method isn't thread-safe, don't you?

Neil
From: Arne Vajhøj on 27 Jul 2008 15:59
Neil Coffey wrote:
> Greg Stasica wrote:
>
>> public class DBQuery{
>>
>> private static DBQuery dbQuery = null;
>>
>> public static DBQuery getInstance()
>> {
>> if (dbQuery == null) {
>> dbQuery = new DBQuery();
>> }
>> return dbQuery;
>> }
>
> You do know that this getInstance() method isn't thread-safe, don't you?

At least he can get multiple DBQuery objects, but he should always
get a valid DBQuery reference back.

Arne
From: Arne Vajhøj on 27 Jul 2008 16:37
Mike Schilling wrote:
> Arne Vajh�j wrote:
>> Neil Coffey wrote:
>>> Greg Stasica wrote:
>>>
>>>> public class DBQuery{
>>>>
>>>> private static DBQuery dbQuery = null;
>>>>
>>>> public static DBQuery getInstance()
>>>> {
>>>> if (dbQuery == null) {
>>>> dbQuery = new DBQuery();
>>>> }
>>>> return dbQuery;
>>>> }
>>> You do know that this getInstance() method isn't thread-safe, don't
>>> you?
>> At least he can get multiple DBQuery objects, but he should always
>> get a valid DBQuery reference back.
>
> I don't think that's guaranteed. There is some possibility that he'll
> get a DBQuery that is not fully initialized, since the JVM is within
> its rights to reorder the assignment to dbQuery with the statements in
> the constructor. (Disclaimer; that was true in the original JVM
> memory model. I don't know if the current one changes that.)

I don't think the MM has changed regarding that.

And that could be a problem as well. Could be, because I don't
think we have seen the constructor.

Arne
From: Neil Coffey on 11 Aug 2008 23:33
Mike Schilling wrote:

>> At least he can get multiple DBQuery objects, but he should always
>> get a valid DBQuery reference back.
>
> I don't think that's guaranteed. There is some possibility that he'll
> get a DBQuery that is not fully initialized, since the JVM is within
> its rights to reorder the assignment to dbQuery with the statements in
> the constructor. (Disclaimer; that was true in the original JVM
> memory model. I don't know if the current one changes that.)

No, it doesn't -- au contraire, as time moves on we'll probably see more
VMs implementing such optimisation.

It's hard to say if this was what actually caused the anomaly the
poster reported -- I have to admit you'd be extremely unlucky (but
then out in the wild, the extremely unlikely does eventually occur...).
It *is* a bug, so in absence of anything else to go on, I'd
recommend correcting it as the first course of action.

Neil
From: edmund yau on 12 Aug 2008 03:39
Neil Coffey 提到:
> Mike Schilling wrote:
>
>>> At least he can get multiple DBQuery objects, but he should always
>>> get a valid DBQuery reference back.
>>
>> I don't think that's guaranteed. There is some possibility that he'll
>> get a DBQuery that is not fully initialized, since the JVM is within
>> its rights to reorder the assignment to dbQuery with the statements in
>> the constructor. (Disclaimer; that was true in the original JVM
>> memory model. I don't know if the current one changes that.)
>
> No, it doesn't -- au contraire, as time moves on we'll probably see more
> VMs implementing such optimisation.
>
> It's hard to say if this was what actually caused the anomaly the
> poster reported -- I have to admit you'd be extremely unlucky (but
> then out in the wild, the extremely unlikely does eventually occur...).
> It *is* a bug, so in absence of anything else to go on, I'd
> recommend correcting it as the first course of action.
>
> Neil

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