On 09/05/2013 05:39 AM, Neil Bartlett wrote:
On Thu, Sep 5, 2013 at 8:26 AM, Jaroslav Tulach
<jaroslav.tul...@oracle.com> wrote:
Dne St 4. září 2013 19:50:34, David M. Lloyd napsal(a):
* Transitive-by-default causes problems in mid to large projects due to
extensive conflicts [can't find the discussion...];
We run into this all the time. Just recently I was trying to use maven-antrun-
plugin which depends on (and exposes) Ant 1.7.1 while using features of Ant
1.8. Conflicts everywhere. Resolution fragile. But it seems to run at the end.
fix is to use and
verify exclusions, specify "provided" scope,
Provided scope is good for "compile only dependency" - e.g. one that is not
used during runtime. I use it for APIs with annotation processors that just
generate something and are no longer needed during runtime.
However I've noticed that grizzly used the "provided" scope for optional
runtime dependency. E.g. I could use grizzly-http-server, and add in optional
support for websockets - but it was a nightmare to find out all the JARs that
were needed (like javax.servlet-3.0 API) and I got a lot for
NoClassDefFoundErrors before I collected them all. I double this is a practice
to follow.
and use
maven-enforcer-plugin [5] to ban transitive dependencies
That might work. Explicitly specify what you compile against is a good idea.
NetBeans Platform (Ant based) build system is using it for a decade (after
switching from the previously transitive mode) and there are no problems with
it as far as I can tell.
I continue to believe that the root of this problem is that the Maven
dependency model cannot work for runtime dependencies. "Whole module"
dependencies such as used in Maven, in previous Jigsaw/JSR277
prototypes, and in OSGi's discouraged Require-Bundle instruction
always lead to excessive fanout and version clashes.
We use whole module dependencies all the time in JBoss Modules without
such problems; however we don't always transitively pull in all imports
either, which might be a critical difference, and we also provide
mechanisms to limit what is exported from the source module as well.
All the OSGi arguments I can find seem to stem from only having two
levels of granularity: packages, or *everything*. Really I guess it's
probably more fair to say that our "whole module" dependencies are
really a midway point between OSGi's per-package and whole-bundle
dependencies; they really mean "the exported subgroup of all of the
module's packages".
In any case this has worked better than package granularity for us
because it allows APIs to grow new packages without having to repackage
every dependent (something that happens a lot with rapidly developing
projects like ours), while still allowing us to hide packages and
transitive imports that should not be imported into the target.
This is one of those fine details that would make for an interesting
discussion on its own, but doesn't really relate much to the original
topic :)
But the #1 reason Maven dependencies don't work in practice is simply
that they're designed for a flat class path. More than one of my
colleagues have naively tried to come up with a mechanism that
automatically just maps Maven dependencies on to JBoss Modules, using
the exact literal dependency resolution from Aether or whatever
mechanism they try (both with multiple versions and flattened versions),
and frequently they have run into NCDFE/CNFE and other linkage errors,
CCE, etc. The dependencies in the existing repository are not adequate
(let alone deterministic [version ranges!]), and the dependency
specification language is not expressive enough to accommodate even the
most common use cases we have found (e.g. establishing what packages are
non-public). Being transitive by default is a big part of the reason
why this space is "poisoned", in my opinion, but also the side-effects
of a flat class path (i.e. forgotten dependencies magically still work
through luck) will always prevent this database from working in an
isolated module system until/unless Maven itself moves to an isolated
system.
* range dependencies can cause resolution
to be NP-complete; best practice is to use ranges in a more restricted
manner [6]
Thanks for the quote. Btw. the article's final suggestion is somewhat similar
to what the maven-enforcer-plugin does. The write up suggest to record the
exact versions of libraries we compile against - either specified directly as a
compile dependency, or inferred by the compiler for the (hidden) transitive
ones.
Although David quoted you in the context of OSGi resolution, your
article is not really considered OSGi best practice. In fact we do use
wide ranges according to the rules of Semantic Versioning[1]. While
NP-complete resolutions can technically occur, the problem rarely
arises in realistic scenarios -- if they do ever occur then the
solution is to narrow the search space and/or pre-resolve and persist
the resolution state.
In any case though, the way OSGi uses version constraints to dynamically
wire up dependencies (and verify compatibility) works in a container
(where you don't actually know what's going to come in and from where)
but in a static environment like Jigsaw and the default JBoss Modules
loader, you want all those questions to be answered up front (much like
in an OS distribution), so while version ranges can be useful to help
ensure correctness in your original module repository, I think making
them part of the runtime resolver is a mistake.
This is where the OSGi people talk how OSGi can pre-cache resolution
results, and yes I acknowledge this is a solution, but only if you
actually *do* it. :-) Using this information to build the static module
repository in the first place essentially gives the same result though.
But in any case, for a Java SE module system, I still maintain that
modules should always load in constant time (algorithmically speaking),
and that means no version ranges at run time, period (other than perhaps
for run time verification purposes, if the integrity of the original
repository is in doubt for some reason; this might be a core assumption
though if we're talking about using a Maven repository as a source for
modules).
However, I really feel there is no point in rehashing all these
arguments yet again, since we don't have a clue about the requirements
of the rebooted Jigsaw project other than Mark's vague implication
that it will be "simpler". Oracle needs to clarify the purpose and
goals of the project before any progress can be made.
Indeed, once again we don't know what worked and what didn't work and
why, and what Oracle is *really* after. Instead we are expected to be
lulled by the disclaimer that "this is just another prototype [that you
know nothing about]".
--
- DML
