Unnamed module and duplicate package
Alex Buckley
alex.buckley at oracle.com
Thu Mar 10 21:55:44 UTC 2016
This would entail the module system enumerating every JAR on the
classpath after creating the module graph, in order to detect whether
the unnamed module splits any package exported by any named module.
There's no reason to special case JDK modules -- augmenting a user
module would lead to errors just as confusing.
(Suppose the system image contains just the java.base and spring.boot
modules -- I'm sure the latter's owners are just as concerned about
third-party augmentation as we are about the former. In fact, they
should be more concerned, since their module is in the application
loader so its package-private content would be accessible by a classpath
JAR also in the application loader. In contrast, the java.base module is
safely in the ivory tower of the bootstrap loader, hence a different
run-time package from anything in the application loader.)
There's an obvious bikeshed around this validation behavior -- enabled
by default? can I choose the modules to be coated with anti-split paint?
can I tweak which parts of the classpath get scanned? can I have a
manifest attribute to opt out of being scanned? -- so I suggest you
simply record the proposal on jpms-spec-comments.
Alex
On 3/10/2016 1:40 PM, Paul Benedict wrote:
> My apologies for omitting some key qualifiers in my explanation. Read
> everything as a proposal to prohibit classpath jars from silently
> splitting whatever packages the JDK exports. -- Thanks
>
> On Thu, Mar 10, 2016 at 3:25 PM, Paul Benedict <pbenedict at apache.org
> <mailto:pbenedict at apache.org>> wrote:
>
> Alex,
>
> For the sake of usability, however, it would be nice if the JDK
> verified that jars do not contain any exported JDK packages. This
> would be an RFE. I understand that in order to avoid split packages
> between modules and classpath, the module version takes precedent.
> For developer vs. developer code, I find that reasoning fine.
> However, I really would like to treat the JDK as "special" (your
> words) because in my experience, I see developers constantly
> perplexed by NoClassDefFoundError when something occurred like you
> just detailed.
>
> I'd like to refer you to the Servlet 3.1 spec [1], section 10.7.2,
> as an analogous concern to mine. This is the so-called "prohibited
> classes" violation clause:
>
> "As described in the Java EE license agreement, servlet containers
> that are not part of a Java EE product should not allow the
> application to override Java SE platform classes, such as those in
> the java.* and javax.* namespaces, that Java SE does not allow to be
> modified. The container should not allow applications to override or
> access the container’s implementation classes."
>
> I don't think it's good usability to let JDK packages in the
> classpath go silently unchallenged and unloaded. I recommend they
> are reported as an error.
>
>
> [1] https://java.net/downloads/servlet-spec/Final/servlet-3_1-final.pdf
>
> Cheers,
> Paul
>
> On Thu, Mar 10, 2016 at 2:30 PM, Alex Buckley
> <alex.buckley at oracle.com <mailto:alex.buckley at oracle.com>> wrote:
>
> I see xml-apis.jar (2.0.2) contains:
>
> - a javax.xml.parser package, which includes a FactoryFinder
> class that's not in Java SE, and
>
> - a javax.xml.transform package hierarchy, whose types at first
> glance look identical to those in Java SE except for yet another
> FactoryFinder class in javax.xml.transform.
>
> If you put xml-apis.jar on the classpath, its javax.xml.**
> packages will be ignored. The unnamed module reads the java.xml
> module which exports javax.xml.** packages (assuming java.xml in
> the system image, of course), so the application class loader
> delegates for javax.xml.** packages to the loader responsible
> for the java.xml module. User code that tries to access
> FactoryFinder will get a NoClassDefFoundError.
>
> There's nothing special about JDK modules here. The same
> NoClassDefFoundError would occur if the system image contained a
> module exporting some package, and a JAR on the classpath
> contained the same package with extra classes, and some code on
> the classpath tried to access those extra classes. Since the
> module in the system image is probably the rightful
> owner/exporter of the package, hard questions should be asked
> about the provenance of the JAR on the classpath.
>
> There has been some discussion of a jdeps-like tool that detects
> when a JAR on your classpath is trying to split a JDK package:
> http://mail.openjdk.java.net/pipermail/jigsaw-dev/2015-November/005227.html.
>
> Alex
>
> On 3/10/2016 10:27 AM, Paul Benedict wrote:
>
> Alex, there are JARs that contain javax packages. Anyone in
> the web
> development community knows how many people have included
> xml-apis in
> their WEB-INF :-) only to find out it wasn't loaded or it
> took precedent
> over the JDK versions.
>
> Has Jigsaw introduced any restrictions here on this front?
> Honestly, I
> think the JDK should make it illegal for the classpath to
> contain ANY
> packages that the jdk has. Please opine when it is
> convenient for you.
>
> Cheers,
> Paul
>
> On Wed, Mar 9, 2016 at 5:13 PM, Alex Buckley
> <alex.buckley at oracle.com <mailto:alex.buckley at oracle.com>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>>> wrote:
>
> Paul, thank you for asking. The module system's
> treatment of the
> unnamed module vis-a-vis named modules is probably the
> biggest
> factor affecting usability of the module system. This
> is true almost
> by definition because at JDK 9 GA the only named
> modules in the
> world will be the JDK's while every other class will be
> in the
> unnamed module of the application class loader.
>
> So please, ask more questions about the unnamed module.
> I am
> especially interested to know if anyone has JARs that
> contain javax
> packages (or heaven forbid, sun or com.sun packages)
> found in the
> JDK -- such JARs are a mortal danger to interop between
> unnamed and
> named modules.
>
> Alex
>
> On 3/9/2016 1:47 PM, Paul Benedict wrote:
>
> Thank you Alex. Since it's roughly the same as JDK
> 8, then it's
> also not
> worse. I defer to your explanation on that point.
>
> Cheers,
> Paul
>
> On Wed, Mar 9, 2016 at 3:37 PM, Alex Buckley
> <alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>
>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>>>> wrote:
>
> Presumably you would count the equivalent
> scenario on JDK 8
> -- my
> package A is in Alex.jar on the classpath and
> your package
> A is in
> Paul.jar on the classpath -- as a security
> issue too,
> because some
> of my classes may substitute for yours (or
> some of yours
> for mine,
> depending on how the classpath is constructed).
>
> On JDK 9, we do the "substitution" cleanly.
> Package A is
> not split.
> That avoids one category of error
> (ClassCastException).
> What about
> poor package B that finds itself accessing a
> different
> package A
> than it was compiled with? Well, since package
> A is
> exported by a
> named module, it's reasonable to assume that
> the named
> module "owns"
> package A [*], and that the developer of package B
> co-bundled some
> version of package A without renaming it.
> Dangerous in JDK 8,
> dangerous in JDK 9. (We're trying to
> encapsulate the
> internals of a
> module, which is different from trying to
> isolate modules
> from each
> other.)
>
> [*] Advanced scenario: the named module
> exporting A is
> actually an
> automatic module which happened to co-bundle
> package A. By
> placing
> this JAR on the modulepath to form an
> automatic module, it
> dominates
> the JAR left on the classpath which also
> co-bundled package A.
>
> Alex
>
> On 3/9/2016 1:17 PM, Paul Benedict wrote:
>
> But isn't what your proposing a security
> issue? Let's
> say my
> package A
> is in the unnamed module and your package
> A is in a named
> module. You
> basically took over my code; your classes
> will be
> substituted
> for mine.
>
> Cheers,
> Paul
>
> On Wed, Mar 9, 2016 at 2:38 PM, Alex Buckley
> <alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>>
> <mailto:alex.buckley at oracle.com <mailto:alex.buckley at oracle.com>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>>>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>>
>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>
>
> <mailto:alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>>>>> wrote:
>
> On 3/9/2016 10:36 AM, Paul Benedict
> wrote:
>
> From the doc:
> "If a package is defined in both
> a named
> module and the
> unnamed
> module then
> the package in the unnamed module
> is ignored. This
> preserves
> reliable
> configuration even in the face of
> the chaos of the
> class path,
> ensuring
> that every module still reads at
> most one
> module defining a
> given package.
> If, in our example above, a JAR
> file on the
> class path
> contains
> a class
> file named
> com/foo/bar/alpha/AlphaFactory.class then
> that file
> will never
> be loaded, since the
> com.foo.bar.alpha package is
> exported by the
> com.foo.bar module."
>
> I would like some clarification.
> Correct me if
> wrong, but I
> think this
> entire paragraph is really meant
> to be about the
> perspective from a
> modularized JAR? If a module has
> package A,
> and the unnamed
> module has
> package A, then of course the
> module's package
> A should
> win.
>
> However, if it is meant to be
> absolute
> language, then I
> disagree.
>
> The unnamed module should be
> coherent among
> itself. If the
> unnamed module
> has package B and relies on
> classes from
> package A, it
> should
> still be able
> to see its own package A. I don't
> think
> modules should
> be able
> to impact
> how the unnamed module sees
> itself. That's a
> surprising
> situation.
>
>
> The unnamed module is not a root
> module during
> resolution.
> If your
> main class is in the unnamed module
> (i.e. you did
> java -jar
> MyApp.jar rather than java -m MyApp),
> then the
> module graph is
> created by resolving various root
> modules (what
> are they?
> separate
> discussion) and only then is the
> unnamed module
> hooked up
> to read
> every module in the graph.
>
> Hope we're OK so far.
>
> If some named module in the graph
> exports package
> A (more
> than one
> module exporting A? separate
> discussion), then
> since the
> unnamed
> module reads that named module, the
> unnamed module
> will
> access A.*
> types from that named module.
>
> It's hard to imagine the unnamed
> module NOT
> accessing A.*
> types from
> that named module. Primarily, we need
> to avoid a
> split package
> situation where code in the unnamed
> module sometimes
> accesses A.*
> types from the named module and
> sometimes from the
> unnamed
> module.
>
> You might say, OK, let code in the
> unnamed module
> exclusively access
> A.* in the unnamed module rather than
> exclusively
> access
> A.* in the
> named module. Then you have two problems:
>
> 1. There are issues for named modules
> in the same
> class
> loader as
> the unnamed module -- such named
> modules MUST get
> A.* from
> the named
> module rather than the unnamed
> module, and the
> class loading
> mechanism is incapable of switching
> based on
> accessor. It'll be
> common for named modules to exist in
> the same
> class loader
> as the
> unnamed module, as modular JARs on
> the modulepath and
> non-modular
> JARs on the classpath all end up in the
> application class
> loader
> (modular JARs as named modules;
> non-modular JARs
> jointly as the
> unnamed module).
>
> 2. While the module system is sure
> that package A
> exists in the
> named module, how would the module
> system possibly
> know
> that package
> A exists in the unnamed module?
> Scanning every
> class file
> in every
> non-modular JAR on the classpath at
> startup sounds
> bad.
>
> Alex
>
>
>
>
>
>
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