Hey Steven - you had the processor in a git repo, right? I haven't had
a chance to fully follow up on it, but we have takers to help get it all
tested. Let's coordinate and get the code into some place where it can
be worked on and tested.
As to breaking changes, we already have some breaking changes if I
recall in Guice 4, so it might be timely (though folks on this list
should pipe up as to whether that's going to rain doom upon them. :)
Christian
On 23 May 2014, at 18:06, Luke Sandberg wrote:
On Friday, May 23, 2014 4:20:30 PM UTC-7, Christian Gruber wrote:
+1. I would even consider making it a warning if it isn't marked
final,
to encourage people to add those constraints.
I'm slightly negative on forcing 'final' modifiers (purely due to
boilerplate issues), though I'm sure i could be convinced.
There's an unfinished (it works, but is not sufficiently tested)
annotation processor for some guice static analysis, this could also
be
implemented there (and have it enabled) so that some of these runtime
errors can be caught at compile-time. (It should remain a runtime
error
as well in case people do not run the annotation processor /
validator).
If someone does need the old behavior, they may mark the error
suppressed.
The annotation processor sounds awesome. If it is just a matter of
writing
tests I would be willing to offer some help to get that out the door
:)
The legacy flag is not pretty, but it's certainly a viable option,
though here also I would spout a warning on startup.
Since all these issues would be detected at configure time, we could
probably just log warnings/severes if the error is disabled. I agree
that
adding a flag for this is kind of gross, but i assume it is neccesary
for
backwards compatibility. Though i don't know what the policy is for
breaking changes like this. I don't think that this is a large issue
(based on a cursory review of googles code base), but still it could
definitely break users.
Christian.
On 23 May 2014, at 12:20, Luke Sandberg wrote:
While preparing a recent
change<
https://code.google.com/p/google-guice/source/detail?r=409e0f578b620c38f6c8626dee78783219d2956e>
to
how @Provides methods are invoked, I came across some confusing
behavior
when @Provides methods are overridden. The initial issue i saw was
for a
case like this:
class Super extends AbstractModule {
@Override void configure() {}
@Provides Number provideNumber() {
return 1;
}
}
class Sub extends Super {
@Provides Integer provideNumber() {
return 2;
}
}
Injector injector = Guice.createInjector(new Sub());
assertEquals(2, injector.getInstance(Number.class));
assertEquals(2, injector.getInstance(Integer.class));
This happens because Sub.provideNumber is a covariant override of
Super.provideNumber. So 2 provider methods are bound, but both call
Sub.provideNumber.
If instead you wrote:
class Sub extends Super {
@Provides Number provideNumber() {
return 2;
}
}
then Guice.createInjector will throw a CreationException for
duplicate
bindings for Key.get(Number.class).
There are a lot of different combinations for how you could override
an
@Provides method depending on whether or not you mark the override
with
@Provides and whether or not you change the key for the method.
This
gets
especially weird if you start adding scoping annotations into the
mix
(e.g.
if the override is marked @Singleton but the parent isn't).
Currently, the
Guice implementation of @Provides doesn't do anything special to try
to
detect these cases.
Proposal:
My proposal is to make it an Error to override an @Provides method
in
all
cases. Guice will essentially enforce that @Provides methods are
private/final (whether or not they are actually declared as such).
The goal of this proposal is to eliminate opportunities for
confusion
and
to make modules easier to understand.
FAQs
Q: What about users who overrides @Provides methods for testing
purposes?
Guice has other mechanisms to override bindings (e.g.
Modules.override), or
you can structure your test so that the parent @Provides method
isn't
installed, maybe by splitting it into a separate Module class that
you
don't install in the test. Or if you really want to use method
overriding
you can use a template method pattern and just override a different
method
and have the @Provides method call that one.
Q: What about migration of legacy code that uses this feature?
I'm not sure yet. Guice has a some precedent for using java system
properties for feature flags, so we may introduce a system property
('guice_allow_provides_overrides'?) to enable/disable this error.
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