On 03/23/2016 07:12 AM, David M. Lloyd wrote:
On 03/22/2016 08:59 AM, Daniel Fuchs wrote:
Hi David,
On 22/03/16 13:53, David M. Lloyd wrote:
Am I understanding it correctly that you're pointing the system
property
to a "proxy" as stated in JDK-8152063, not an actual implementation? So
it's sort of a provider-locating mechanism outside of jaxp, that
locates
the actual implementation, and if it cannot find one, it attempts to
fall back to and directly instantiate the JDK's default. Is that why
"it will fail because we can't access the JDK's default
implementations"?
Correct.
Was it because the ServiceLoader mechanism was not meeting your
requirement that you needed a proxy?
Right. Most user code using JAXP factories will just call e.g.
XMLInputFactory.newFactory() with no arguments. So the ServiceLoader
mechanism is fine when we want to allow the user to override the
implementation with one they've bundled in their WAR or whatever. But
if they do not do so, we are forced to ensure that another
implementation is on the deployment class path. It's even worse in
contexts where we cannot control what the TCCL is set to when the
factory methods are called. There's just no practical way to ensure
that our implementation is visible from every class loader. We can
however "hack" our proxy implementations on to every class loader, and
then later set the default factory class loader to use (which then just
uses the same ServiceLoader-style load again, but from the class loader
we choose).
If I understand well, what you need is to be able to
install a service provider that can create an instance
of the JDK default implementation in order to wrap it.
I assume such a service provider would have to be deployed
in the System ClassLoader - so that it could be shadowed
by whichever provider is declared in the context class
loader.
In order to create an instance of the JDK default implementation,
have you explored using the factory method that takes a
ClassLoader, and passing the extension class loader as
parameter?
I can definitely try this out to see if we can use it. How can I get
the extension class loader though? Weird as it sounds, I haven't found
any idioms which can do so reliably (and it has to work across JVMs as
well).
Daniel, I have hacked together a way to use the system class loader
(which should be "good enough", I hope), but there is one further
problem: org.xml.sax.helpers.XMLReaderFactory does not have a factory
method which accepts a class loader like the others do. Any thoughts on
this one? It looks like it might be pretty easy to add this method...
but then, it's also pretty easy to just add setters to set the default
Class :)
I was thinking of setting TCCL for the duration of the wrapped call
using my system class loader trick where I construct a "dummy" class
loader in the event that the system class loader is null. But to do
this I cannot rely on setting the system-wide system property for JAXP
factory, which means I cannot uninstall my proxy at any time (because
service loader descriptors are not dynamic).
--
- DML
