Unnamed module and duplicate package
Alex Buckley
alex.buckley at oracle.com
Thu Mar 10 20:30:21 UTC 2016
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>> 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>>> 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>>>> 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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