Unnamed module and duplicate package

[Alex Buckley]

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>> 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
>
>