A stateless component should be locked to one request/response loop
and thus thread at a given time. No two things should happen
concurrently.
If the stateless component is needed by another thread, a new copy
will be created.
I have had a similar problem that I traced back to having my
stateless component recursively embedded within itself. In that case
the same instance of the component does indeed get used at two
different places (within the same thread). The results of this seem
pretty unpredictable.
Pierre
On 7 Dec 2006, at 16:30, Ulrich Köster wrote:
I've hit the send button to early. The special thing about a
stateless component is that only one instance of the component is
used. With concurrent request handling it's very likely that a
reset and valueForKey are called at eh same time.
Uli
Am 07.12.2006 um 16:25 schrieb Ulrich Köster:
_flag is a state !
public boolean flag() {
Boolean _flag;
if(_flag == null) {
// Initialize _flag to a Boolean instance: it can't be
set to null
[...]
}
return _flag.booleanValue();
}
public boolean isStateless() { return true; }
public boolean synchronizesVariablesWithBindings() { return false; }
Am 07.12.2006 um 16:12 schrieb Francis Labrie:
Hi,
I have several WebObjects 5.3.2 applications deployed in
production running with Java 5, and I've found something worrying
with stateless components. By the way, these applications are set
to allow concurrent request.
I have an heavily used stateless component managing is own
bindings synchronisation:
public class Cell extends WOComponent {
// Private instance variable
private Boolean _flag;
// Code removed for clarity...
[...]
public boolean flag() {
if(_flag == null) {
// Initialize _flag to a Boolean instance: it can't be set to
null
[...]
}
return _flag.booleanValue();
}
public boolean isStateless() { return true; }
public void reset() {
super.reset();
// The only place in the variable is set to null
_flag = null;
}
public boolean synchronizesVariablesWithBindings() { return
false; }
}
But sometimes (less than 1% of concurrent requests), I get a
NullPointerException on the "return _flag.booleanValue();" line!
And this is only an evidence of the problem: sometimes I also get
incorrect values. Normally, this can't be the case according to
the WOComponent documentation:
<< Note that a stateless component's instance variables will
remain valid for the duration of the phase
(takeValuesFromRequest , invokeAction , appendToResponse ); this
lets you use instance variables in the stateless components to
hold things analogous to items in a WORepetition. >>
And IIRC, WebObjects use only one instance of a stateless
component per thread. So I must conclude another thread called
"reset()" on the same component instance, violating the
WOComponent contract quoted above. This seems to be a bug in
WebObjects... Can someone confirm this? Is their any workaround?
I've tried to syncronize the stateless component in the
appendToResponse method, but it leads to deadlocks on concurrent
requests...
Kind regards,
--
Francis Labrie
Saint-Bruno-de-Montarville, Quebec, Canada
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