RFR: 8223260: NamingManager should cache InitialContextFactory
Peter Levart
peter.levart at gmail.com
Fri Jan 31 17:16:22 UTC 2020
Hi Seán,
On 1/31/20 2:16 PM, Seán Coffey wrote:
> Thanks for the review Peter. All good points! My latest patch contains
> adjustments based on feedback from you and others to date.
>
> * Incorporate use of ClassLoaderValue -
> -- I'd appreciate feedback on whether I'm using it correctly.
Yes, you used it correctly. You are very verbose about using it, but
that's just style. I, for example, would re-write this:
712 ClassLoaderValue<InitialContextFactory>.Sub<String> key =
FACTORIES_CACHE.sub(className);
713 try {
714 factory = key.computeIfAbsent(loader, (ld, ky) -> {
715 String cn = ky.key();
716 InitialContextFactory fac = getFactory(cn);
717 return fac;
718 });
719 } catch (UndeclaredThrowableException e) {
720 if (e.getUndeclaredThrowable() instanceof
NoInitialContextException) {
721 throw (NoInitialContextException)
e.getUndeclaredThrowable();
722 }
723 }
... into this:
var key = FACTORIES_CACHE.sub(className);
try {
factory = key.computeIfAbsent(loader, (ld, ky) ->
getFactory(ky.key()));
} catch (UndeclaredThrowableException e) {
if (e.getUndeclaredThrowable() instanceof
NoInitialContextException) {
throw (NoInitialContextException)
e.getUndeclaredThrowable();
} else {
throw e;
}
}
Notice also that I added:
} else {
throw e;
}
You have two options. Either UndeclaredThrowableException is possible
only when you wrap NoInitialContextException with it in getFactory() in
which case you could simply do unconditional:
} catch (UndeclaredThrowableException e) {
throw (NoInitialContextException)
e.getUndeclaredThrowable();
}
...or UndeclaredThrowableException is possible also to be thrown from
code called by getFactory() (in theory, it is). In this case you would
want to re-throw it here instead of swallowing it.
> * Use of ServiceLoader.stream()
737 factory = loader
738 .stream()
739 .map(ServiceLoader.Provider::get)
740 .filter(f ->
f.getClass().getName().equals(className))
741 .findFirst()
742 .orElse(null);
Here you instantiate InitialContextFactory instances until you get one
with implementation class with correct className. But Provider.type()
should already return the type of the service without instantiating it.
So you could write the following instead:
factory = loader
.stream()
.filter(p -> p.type().getName().equals(className))
.findFirst()
.map(ServiceLoader.Provider::get)
.orElse(null);
This should instantiate just the service that is found and not any
other. ServiceLoader.Provider.type() is specified to return:
* Returns the provider type. There is no guarantee that this
type is
* accessible or that it has a public no-args constructor. The
{@link
* #get() get()} method should be used to obtain the provider
instance.
*
* <p> When a module declares that the provider class is
created by a
* provider factory then this method returns the return type of its
* public static "{@code provider()}" method.
So in theory this method could return a super-type of the service
implementation class. But one could argue that the name of the service
provider type is the one we should be searching for, not the
implementation class of the service. Perhaps the service declares a
single provider type and then at instantiation time it dynamically
chooses an implementation class depending on the environment
(architecture perhaps). It would be interesting to see whether provider
types in real service implementations differ from service implementation
classes or are they usually the same.
The following is also possible:
// 1st try finding a ServiceLoader.Provider with type() of
correct name
factory = loader
.stream()
.filter(p -> p.type().getName().equals(className))
.findFirst()
.map(ServiceLoader.Provider::get)
.or( // if that doesn't yield any result, instantiate
the services
// one by one and search for implementation class
of correct name
() -> loader
.stream()
.map(ServiceLoader.Provider::get)
.filter(f ->
f.getClass().getName().equals(className))
.findFirst()
).orElse(null);
So what do you think?
Regards, Peter
> * adjusted test for both the ServiceLoader and legacy classpath load
> approach
> * insert use of sleep which testing GC.
>
> http://cr.openjdk.java.net/~coffeys/webrev.8223260.v2/webrev/
>
> regards,
> Sean.
>
> On 30/01/2020 07:59, Peter Levart wrote:
>> Hi Seán,
>>
>> WeakHashMap is not safe to be called concurrently. Even get() method,
>> although it may seem read-only, can modify internal state (expunging
>> stale Weak entries), so holding a READ lock while accessing
>> WeakHashMap it is wrong.
>>
>> getInitialContext() static method is called with an env Hashtable
>> which means it can be called with different keys/values for the same
>> TCCL. So caching of InitialContextFactory is just performed for the
>> 1st call for a particular TCCL. Subsequent calls for the same TCCL
>> and different class names are not cached. Is this the behavior you
>> are pursuing? You could cache the factory using (TCCL, class name) as
>> a compound key.
>>
>> Also, by caching in a WeakHashMap<ClassLoader,
>> InitialContextFactory>, you make a strong reference to
>> InitialContextFactory from class loader of the NamingManager.class
>> and such InitialContextFactory may indirectly reference the
>> ClassLoader key of the same entry, so it will never go away because
>> NamingManager class is never going away. You should at least use a
>> WeakHashMap<ClassLoader, WeakReference<InitialContextFactory>> for that.
>>
>> Shameless plug: there is a JDK internal class
>> jdk.internal.loader.ClassLoaderValue which you might be able to use
>> for caching if a part of your key is a ClassLoader. From the javadoc:
>>
>> * ClassLoaderValue allows associating a
>> * {@link #computeIfAbsent(ClassLoader, BiFunction) computed}
>> non-null value with
>> * a {@code (ClassLoader, keys...)} tuple. The associated value, as
>> well as the
>> * keys are strongly reachable from the associated ClassLoader so
>> care should be
>> * taken to use such keys and values that only reference types
>> resolvable from
>> * the associated ClassLoader. Failing that, ClassLoader leaks are
>> inevitable.
>>
>> So if you know that the InitialContextFactory instance is always
>> resolvable (by class name) from the ClassLoader you are using for the
>> caching key (the TCCL), then this utility might be just right for
>> your purpose.
>>
>> Regards, Peter
>>
>>
>> On 1/29/20 6:22 PM, Seán Coffey wrote:
>>> Thanks for the reviews. I found an issue with the new test also -
>>>
>>> it's loading the custom factory class via the non-serviceloader
>>> approach. I was hoping to exercise ServiceLoader here. I'll address
>>> this and the comments raised and revert with a new patch shortly.
>>>
>>> Regards,
>>> Sean.
>>>
>>> On 29/01/20 16:27, Alan Bateman wrote:
>>>> On 29/01/2020 15:55, Daniel Fuchs wrote:
>>>>> Hi Seán,
>>>>>
>>>>>> http://cr.openjdk.java.net/~coffeys/webrev.8223260.v1/webrev/
>>>>>> A WeakHashKey with the TCCL as the key should be okay here.
>>>>>
>>>>> If the TCCL is the key then there are good chances that the
>>>>> concrete factory class is expected to be loaded by the TCCL.
>>>>>
>>>>> If that happens then the value will reference the key and
>>>>> nothing will ever get garbage collected.
>>>> I don't know how much JNDI is used much beyond LDAP these days but
>>>> you are right that a factory with a strong ref to the TCCL would
>>>> prevent it from being GC'ed. The internal WeakPairMap might be
>>>> useful here.
>>>>
>>>> -Alan
>>>
>>
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