Why not use the Manifest?

[Peter Kriens]

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