Hello Paulex,
Please refer to JDiff (http://javadiff.sourceforge.net/), maybe this is
tool can meet your requirement more. :-D
Richard Liang
China Software Development Lab, IBM
Dalibor Topic wrote:
On Mon, Mar 06, 2006 at 02:12:16PM +0800, Paulex Yang wrote:
Stuart,
Stuart Ballard wrote:
Tim Ellison <t.p.ellison <at> gmail.com> writes:
Stuart: is there some way to avoid JAPI taking inherited API signatures
into account when doing the diff? kinda like the javadoc tool.
This is something of a Japi FAQ (I should probably put together a Japi FAQ
page
somewhere now that it's no longer the case that pretty much everyone using
Japi
is on the Classpath list ;) ).
The answer is no, it doesn't do this, and I'm not going to implement it.
The
reason is that whether a method is overridden or not is *not* part of the
public
API. Javadoc documents it, sure, but that's not really sufficient to say
whether
it's part of the API or not - it's quite likely it was simply easier to
write
Javadoc that way (so it didn't have to go and find the doc comments in the
superclass's source file, etc).
The rules of binary compatibility say that moving a method up or down the
class
hierarchy is a completely compatible change.
A method may be overridden, or not, purely for performance reasons and the
behavior could be completely identical. For example, LinkedList.subList is
not
documented as being overridden, but if it's not, that guarantees that the
iterator() method will always be at least O(N) where N is the startIndex
that
the sublist was created with. An overridden LinkedList.subList method
could hang
on to the start and end nodes of the list and provide an O(1)
implementation of
iterator(). As long as some effort was taken to ensure that serialization
stayed
compatible, there's no reason why a Free runtime shouldn't do that
optimization
even if Sun doesn't.
I've also heard of cases where Sun documents a method as being overridden
even
though as far as any testing can determine, all it does is call super.
There
should be no obligation for any Free runtime to duplicate this
inefficiency.
It's also possible to implement a method that needs changed behavior in a
subclass by making the superclass delegate some parts of its behavior to
non-public methods, so the superclass method is unchanged but the subclass
just
overrides the non-public methods (obviously, this only works if the
subclass is
in the same package, but that's pretty common).
Finally, it's possible (albeit unlikely) that even if there *was* an
apparent
need to override a method in a subclass because it has different behavior,
there
might be some weird optimization-related reason why somebody might want to
actually write it as "if (this instanceof Whatever)" in the superclass. I
can't
see why that would happen, but if someone did that and got all the behavior
right, they shouldn't be penalized for it in Japi.
Most of above is true and has good reason, but as a practicer trying to
implement a RI compatible class library, I still feel strongly that the
method override information has its value, maybe a most common example
is equals(Object), most classes don't override this method and only call
j.l.Object's, but others DO has their own criteria, it's very reasonable
to just remind programmer to implement this method, otherwise it will
introduce serious compatibility issue.
It won't introduce a binary compatibility issue. That's what really
matters. You can't get semantics like who overloads what right via a tool,
you need to read the specs to see if you should override equals/hashCode or
not, and how.
Just as a proposal, if you still feel that override difference is
another story with common difference, how about to separate them into
another category, similar with Compiler's warning, which at least can be
as reference to programmer, and this information even can be switched on
only if some user want to check the difference more *strictly*.
Basically, Japi's business is to test the coverage of the API, and it's a
fundamental design decision that Japi will *not* be testing implementation
details. I've done quite a lot of work fixing bugs where implementation
details
inadvertently get exposed in the Japi results and if I find any more I'll
fix
them too.
I think this is *extremely* important because, among other things, it's the
*only* reason I feel legally secure in distributing Japi files for the
JDK. As
long as they only contain functional information necessary for
interoperability,
they're uncopyrightable. If they start exposing copyrightable
implementation
details, they become derived works of the JDK, and illegal to distribute
(or,
technically, to even create in the first place). And potentially so do the
results of the comparisons. And if you then use those comparisons to make
decisions on what to implement in Harmony, it jeopardizes Harmony's
cleanroom
status too. (IANAL; the people who actually decide whether legal risks are
acceptable might think this isn't actually a problem for Harmony, but *I*
don't
want to be responsible for it ;) )
The right way to ensure that you've overridden all the right methods is to
test
the *behavior* of those methods. Obviously this is a lot more work than a
simple
automated run via Japi or equivalent, but it's the only way to get
meaningful
results.
It is definitely the right way, but maybe not the most practical one.
Still the example of equals(), do we need a testEquals for every single
class? or do we still need to check the JavaDoc one by one to find
difference on this method? why not the tool just give some warning to
some possible incompatible?
Because a tool can't know if an implementation detail outside of the
binary compatibility spec is relevant for an implementation of the J2SE
API spec or not. AI hasn't caught up with that goal yet. :)
For example, different compatible implementations
of the spec may or may not chose to override equals for a class: as long
as they are binary compatible (trivial if we are not overriding), and fulfil
both the contract of the spec for equals in their class, and the
contract for equals in the superclasses, they are OK.
You could for
example, implement Object.equals as a giant instanceof if-else cascade
for all the defined types in J2SE, and save yourself all the wasted
bytes of storage going into having many little trivial methods in many
classes. Such an implementation would be convenient for embedded uses,
as it shaves off unnecessary bytecodes very nicely, and still be fully
compatible with the specs.
You could also implement a generic,
reflection based equals method in Object that does the right thing in
99% of the cases, and just override the few cases where that's not
good enough. Or use AOP, or something else, as long as your code is
binary compatible.
So yes, you need to wade through the specs one by one to implement them
faithfully. On the other hand, the specs also give you a lot of leeway
to do the right thing efficiently, without having to explicitely mimic
the RI. :)
cheers,
dalibor topic
Obviously since Japi is Free Software I can't prevent others from making a
version that does this, but I hope I've shown why I think it would be a
bad idea.
There is another issue of JAPI, pls. correct me if I'm wrong, seems the
*final* information is also ignored. I also think this difference of
method signature is a compatibility issue which could not be overlooked,
because the spec writer might have consider very carefully about which
methods can be overridden and which should be immutable. Comments?
Stuart.
--
Paulex Yang
China Software Development Lab
IBM
