Hi all,
I add a comment
#19<https://code.google.com/p/google-guice/issues/detail?id=183#c19> on
this issue since April. I would like to know if it's a stupid comment or
not. In fact we fall in some case in a deadlock situation with the class
level lock used by the Singleton scope. And I think in case of production
stage the lock and event the volatile instance field are not required. For
me only development mode require this kind of protection and we always
create our Injector in production stage.
Does it make sens for you?
Here a code snippet of Singleton Scope (guice 3.0) :
public static final Scope SINGLETON = new Scope() {
public <T> Provider<T> scope(final Key<T> key, final Provider<T>
creator) {
return new Provider<T>() {
/*
* The lazily initialized singleton instance. Once set, this will
either have type T or will
* be equal to NULL.
*/
private volatile Object instance;
// DCL on a volatile is safe as of Java 5, which we obviously
require.
@SuppressWarnings("DoubleCheckedLocking")
public T get() {
if (instance == null) {
/*
* Use a pretty coarse lock. We don't want to run into deadlocks
* when two threads try to load circularly-dependent objects.
* Maybe one of these days we will identify independent graphs
of
* objects and offer to load them in parallel.
*
* This block is re-entrant for circular dependencies.
*/
synchronized (InternalInjectorCreator.class) {
if (instance == null) {
T provided = creator.get();
// don't remember proxies; these exist only to serve
circular dependencies
if (provided instanceof CircularDependencyProxy) {
return provided;
}
Object providedOrSentinel = (provided == null) ? NULL :
provided;
if (instance != null && instance != providedOrSentinel) {
throw new ProvisionException(
"Provider was reentrant while creating a singleton");
}
instance = providedOrSentinel;
}
}
}
Object localInstance = instance;
// This is safe because instance has type T or is equal to NULL
@SuppressWarnings("unchecked")
T returnedInstance = (localInstance != NULL) ? (T) localInstance
: null;
return returnedInstance;
}
Regards,
Romain.
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