Module name check is too restrictive

[Remi Forax]

While testing ASM,
i've found that you can not currently load a module that has a '+' in its name.

/usr/jdk/jdk-9-jigsaw/bin/java --module-path tmp --module "foo+bar"
Error occurred during initialization of VM
java.lang.module.ResolutionException: Error reading module: tmp/foo
	at java.lang.module.Resolver.findWithBeforeFinder(java.base at 9-ea/Resolver.java:735)
	at java.lang.module.Resolver.resolveRequires(java.base at 9-ea/Resolver.java:86)
	at java.lang.module.Configuration.resolveRequiresAndUses(java.base at 9-ea/Configuration.java:370)
	at java.lang.module.ModuleDescriptor$1.resolveRequiresAndUses(java.base at 9-ea/ModuleDescriptor.java:2081)
	at jdk.internal.module.ModuleBootstrap.boot(java.base at 9-ea/ModuleBootstrap.java:263)
	at java.lang.System.initPhase2(java.base at 9-ea/System.java:1927)
Caused by: java.lang.module.InvalidModuleDescriptorException: foo+bar: Invalid module name:  Illegal character at index 3
	at java.lang.module.ModuleInfo.invalidModuleDescriptor(java.base at 9-ea/ModuleInfo.java:804)
	at java.lang.module.ModuleInfo.read(java.base at 9-ea/ModuleInfo.java:92)
	at java.lang.module.ModuleDescriptor.read(java.base at 9-ea/ModuleDescriptor.java:1902)
	at java.lang.module.ModulePath.readExplodedModule(java.base at 9-ea/ModulePath.java:591)
	at java.lang.module.ModulePath.readModule(java.base at 9-ea/ModulePath.java:268)
	at java.lang.module.ModulePath.scanDirectory(java.base at 9-ea/ModulePath.java:234)
	at java.lang.module.ModulePath.scan(java.base at 9-ea/ModulePath.java:188)
	at java.lang.module.ModulePath.scanNextEntry(java.base at 9-ea/ModulePath.java:145)
	at java.lang.module.ModulePath.find(java.base at 9-ea/ModulePath.java:109)
	at java.lang.module.ModuleFinder$2.lambda$find$0(java.base at 9-ea/ModuleFinder.java:359)
	at java.util.stream.ReferencePipeline$3$1.accept(java.base at 9-ea/ReferencePipeline.java:195)
	at java.util.Spliterators$ArraySpliterator.tryAdvance(java.base at 9-ea/Spliterators.java:958)
	at java.util.stream.ReferencePipeline.forEachWithCancel(java.base at 9-ea/ReferencePipeline.java:127)
	at java.util.stream.AbstractPipeline.copyIntoWithCancel(java.base at 9-ea/AbstractPipeline.java:502)
	at java.util.stream.AbstractPipeline.copyInto(java.base at 9-ea/AbstractPipeline.java:488)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(java.base at 9-ea/AbstractPipeline.java:474)
	at java.util.stream.FindOps$FindOp.evaluateSequential(java.base at 9-ea/FindOps.java:152)
	at java.util.stream.AbstractPipeline.evaluate(java.base at 9-ea/AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.findFirst(java.base at 9-ea/ReferencePipeline.java:476)
	at java.lang.module.ModuleFinder$2.find(java.base at 9-ea/ModuleFinder.java:361)
	at java.lang.module.Resolver.findWithBeforeFinder(java.base at 9-ea/Resolver.java:732)
	at java.lang.module.Resolver.resolveRequires(java.base at 9-ea/Resolver.java:86)
	at java.lang.module.Configuration.resolveRequiresAndUses(java.base at 9-ea/Configuration.java:370)
	at java.lang.module.ModuleDescriptor$1.resolveRequiresAndUses(java.base at 9-ea/ModuleDescriptor.java:2081)
	at jdk.internal.module.ModuleBootstrap.boot(java.base at 9-ea/ModuleBootstrap.java:263)
	at java.lang.System.initPhase2(java.base at 9-ea/System.java:1927)

According to the spec [1] section 2.1, the module name is a name in its internal form (JVMS 4.2.1)
so it's a sequence of unicode characters (beside '.', ';', '[' and '/') separated by '/'.
So '+' is a valid character.

The code that checks the name of a module is in jdk.internal.module.Checks::requireModuleName and it checks that the name is a valid Java identifier which is obviously wrong.
The module name of a module-info.java is a java identifier because it is parsed by javac but a module name in a module-info.class is encoded a binary name to not add restrictions to module names defined by other languages than Java.

There is also a pending spec issue [2] to make a module name a unicode sequence of any characters and not a binary name removing the restriction around the characters ('.', ';', '[').
For '/', it's a little different because if a module name allows '/', the encoding of the option '--module' of the java executable becomes ambiguous. 

regards,
Rémi

[1] http://cr.openjdk.java.net/~mr/jigsaw/spec/lang-vm.html
[2] http://openjdk.java.net/projects/jigsaw/spec/issues/#ClassFileModuleName