On 4/13/2013 2:59 PM, Peter Levart wrote:
I also devised an alternative caching mechanism with scalability in
mind which uses WeakReferences for keys (for example ClassLoader)
and values (for example Class) that could be used in this situation
in case adding a field to ClassLoader is not an option:
I would also consider any alternative to avoid adding the
proxyClassCache field in ClassLoader as Alan commented previously.
My observation of the typical usage of proxies is to use the
interface's class loader to define the proxy class. So is it
necessary to maintain a per-loader cache? The per-loader cache maps
from the interface names to a proxy class defined by one loader. I
would think it's reasonable to assume the number of loaders to define
proxy class with the same set of interfaces is small. What if we
make the cache as "interface names" as the key to a set of proxy
class suppliers that can have only one proxy class per one unique
defining loader. If the proxy class is being generated i.e.
ProxyClassFactory supplier, the loader is available for comparison.
When there are more than one matching proxy classes, it would have to
iterate all in the set.
I would assume yes, proxy class for a particular set of interfaces is
typically defined by one classloader only. But the API allows to
specify different loaders as long as the interfaces implemented by
proxy class are "visible" from the loader that defines the proxy
class. If we're talking about interface names - as opposed to
interfaces - then the possibility that a particular set of interface
names would want to be used to define proxy classes with different
loaders is even bigger, since an interface name can refer to different
interfaces with same name (think of interfaces deployed as part of an
app in an application server, say a set of annotations used by
different apps but deployed as part of each individual app).
Agree. I was tempted to consider making weak reference to the interface
classes as the key but in any case the overhead of
Class.getClassLoader() is still a performance hog. Let's move forward
with the alternative you propose.
The scheme you're proposing might be possible, though not simple: The
factory Supplier<Class> would become a Function<ClassLoader, Class>
and would have to maintain it's own set of cached proxy classes. There
would be a single ConcurrentMap<List<String>, Function<ClassLoader,
Class>> to map sets of interface names to factory Functions, but the
cached classes in a particular factory Function would still have to be
weakly referenced. I see some difficulties in implementing such a scheme:
- expunging cleared WeakReferences could only reliably clear the cache
inside each factory Function but removing the entry from the map of
factory Functions when last proxy class for a particular set of
interface names is expunged would become a difficult task if not
impossible with all the scalability constraints in mind (just thinking
about concurrent requests into same factory Function where one is
requesting new proxy class and the other is expunging cleared
WeakReference which represents the last element in the set of cached
proxy classes).
- one of my past ideas of implementing scalable Proxy.isProxyClass()
was to maintain a Set<Class> in each ClassLoader populated with all
the proxy classes defined by a particular ClassLoader. Benchmarking
such solution showed that Class.getClassLoader() is a peformance hog,
so I scraped it in favor of ClassValue<Boolean> that is now
incorporated in the patch. In order to "choose" the right proxy class
from the set of proxy classes inside a particular factory Function,
the Class.getClassLoader() method would have to be used, or entries
would have to (weakly) reference a particular ClassLoader associated
with each proxy class.
Thanks for reminding me your earlier prototype. I suspect the cost of
Class.getClassLoader() is due to its lookup of the caller class every
time it's called.
Considering all that, such solution starts to look unappealing. It
might even be more space-hungry then the presented WeakCache.
WeakCache is currently the following:
ConcurrentMap<WeakReferenceWithInterfaceNames<ClassLoader>,
WeakReference<Class>>
another alternative would be:
ConcurrentMap<WeakReference<ClassLoader>,
ConcurrentMap<InterfaceNames, WeakReference<Class>>>
...which might need a little less space than WeakCache (only one
WeakReference per proxy class + one per ClassLoader instead of two
WeakReferences per proxy class) but would require two map lookups
during fast-path retrieval. It might not be performance critical and
the expunging could be performed easily too.
I am fine with either of these alternatives. As you noted, the latter
one would save little bit of memory for the cases when several proxy
classes are defined per loader e.g. one per each annotation type.
Mandy
