Why not use the Manifest?

Remi Forax forax at univ-mlv.fr
Wed Oct 14 13:41:09 UTC 2015

I don't think so, it's part of the tools descriptions,
the manifest is described as part of the java/jar commands.

Anyway, it's important that the module descriptor is checked at compile time (by javac) and at runtime (by java),
thus, it has to be a part of Java the language with a syntax understandable by javac.


----- Mail original -----
> De: "Peter Kriens" <peter.kriens at aqute.biz>
> À: "Mark Reinhold" <mark.reinhold at oracle.com>
> Cc: jpms-spec-experts at openjdk.java.net
> Envoyé: Lundi 12 Octobre 2015 18:51:52
> Objet: Re: Why not use the Manifest?
> Isn’t the manifest already part of the JVM specification?
> Kind regards,
> 	Peter Kriens
> > On 5 okt. 2015, at 20:50, mark.reinhold at oracle.com wrote:
> > 
> > 2015/9/28 9:49 -0700, peter.kriens at aqute.biz:
> >> The encoding of the module data in a binary class file remains hard to
> >> understand and seems to go against the best practices we learned the
> >> hard way over the past decades. (Debugging!)
> >> 
> >> Especially since the current proposal leaves the heavy lifting to
> >> build tooling. In the proposal, build tools cannot use the
> >> module-info.java since it does not contain sufficient
> >> information. (Versions being the most glaring omission for a build
> >> too.) Nor is the format extensible to contain build tool specific
> >> information. Ergo, the build tool will have to generate the
> >> module-info.class file.
> >> 
> >> This make the class file makes these tools unnecessary hard and slower
> >> because it must either use the compiler as an intermediate step or use
> >> a library like ASM to create the unreadable version of its metadata.
> >> 
> >> This is the first time in a very long time that I see a regression to
> >> binary files for meta data. Especially because there is already a good
> >> place that all build tools are already using: the manifest.
> > 
> > The reasons for expressing module declarations in source files and
> > compiling them into class files are mentioned in SotMS, but here's a
> > longer take on just this topic.
> > 
> > The Java programming language, at present, provides for the definition
> > of three kinds of program components: classes, interfaces, and packages.
> > They are defined in Java source files, and compiled into class files.
> > Such files govern, among other things, the mechanisms of symbolic
> > resolution and access control implemented by every Java compiler and
> > JVM.  A developer need not reason about any other type of file in order
> > to understand the actions of these mechanisms.  A compiler or JVM need
> > not consume or produce any information other than that found in source
> > and class files in order to implement these mechanisms.
> > 
> > The module system extends the mechanism of symbolic resolution with the
> > concept of readability in order to provide reliable configuration.  It
> > extends the mechanism of access control, in part by relating it to
> > readability, in order to provide strong encapsulation.  To support these
> > extensions, every Java compiler and JVM must be able to locate and
> > interpret descriptions of modules which convey, at least, each module's
> > name, dependences, and exported packages.  Such descriptions, regardless
> > of their form, will govern symbolic resolution and access control in both
> > compilers and JVMs, so they will have to be specified in both the Java
> > Language Specification (JLS) and the Java Virtual Machine Specification
> > (JVMS).
> > 
> > Regardless of how they are described, modules are, fundamentally, a new
> > kind of Java program component.  The present proposal therefore treats
> > them as such, in both the language and the JVM: Module descriptions are
> > expressed and encoded in the same way as the other kinds of information
> > that define program components and govern symbolic resolution and access
> > control -- that is, as Java source and class files.
> > 
> > This approach is immediately familiar to developers, who already think
> > about program components, symbolic resolution, and access control in
> > terms of the Java programming language.  It is easy to retrofit into the
> > JLS and the JVMS, since those specifications are already centered upon
> > source and class files.  It is, finally, straightforward for existing
> > Java implementations and tool chains to support, since they need not be
> > revised to handle an entirely new type of file.
> > 
> > We could choose another format for module descriptions such as XML, JSON,
> > YAML, or JAR-file manifests.  Such formats may be more convenient for
> > tool maintainers but we'd have to bake that format into the JLS and the
> > JVMS, thereby increasing the complexity of those specifications and their
> > implementations.  If the format depends upon external standards (XML,
> > JSON, YAML) then these foundational specifications, and their
> > implementations, would become dependent upon those standards.  If the
> > format proves unsuitable over time then replacing it with something else
> > would require major revisions to these specifications, and to their
> > implementations.
> > 
> > Source and class files are fundamental to the Java platform, to the JLS
> > and the JVMS, and to their implementations and supporting tools.  Other
> > formats come and go; these will be with us forever.
> > 
> >                                  * * *
> > 
> > As to debugging, in the prototype we've already enhanced the jar tool
> > with an option to print the descriptor of a modular JAR file.  I expect
> > similar support to show up in related tools such as jmod and javap, and
> > eventually in IDEs and other tools outside the JDK.
> > 
> > As to ease of tooling, most tools will only need to read module
> > descriptors, not write them, and the java.lang.module.ModuleDescriptor
> > class already provides convenient static methods for doing that.  Adding
> > similar methods to write descriptors would be straightforward, though it
> > would complicate the API a bit.
> > 
> > As to extensibility, class files are already in a precisely-specified and
> > extensible format.  If a build tool needs to add information to a module
> > descriptor then it can do so via non-standard class-file attributes,
> > using an existing popular library such as ASM.
> > 
> > To make it easier for tools to manipulate module descriptors we could
> > consider enhancing the java.lang.module.ModuleDescriptor class, and
> > related tools, to read and write custom, non-standard class-file
> > attributes.  This might be a bit tricky depending on how the attributes
> > are defined, but for simple property-style attributes it's likely
> > straightforward and would obviate the need to use ASM directly.
> > 
> > - Mark

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