[jsr294-modularity-eg] .spkg file format

[Alex Buckley]

Hello,

Recall that superpackage com.foo.test is declared in 
com/foo/test/super-package.java and is compiled to 
com/foo/test/super-package.spkg. We need to decide on the form of the 
.spkg file.

The file will need to contain:
- name of superpackage
- name of enclosing superpackage
- names of member packages, and possibly the types in those packages
- names of exported types
- annotations on superpackage

The file format should consider these requirements:
- ease of production by Java compilers
- ease of adoption by classfile parsing tools
- speed of parsing by VMs
- ability to completely express annotations

Many formats are possible, but I think we should stay fairly close to 
the familiar binary ClassFile structure (JVMS 4.1). This lets us reuse 
existing attributes for annotations, which would be non-trivial to 
recreate in an alternative format.

Some options for formats:

1) Encode a superpackage declaration into ClassFile, with member and 
export declarations transformed to (for example) fields with pre-defined 
names and specially-formatted values.

2) Use the existing ClassFile structure as far as possible, redefining 
and zeroing out items as necessary. Membership and export declarations 
could be represented with new attributes (JVMS 4.8).

3) Design a new binary structure inspired by ClassFile. We could drop 
irrelevant items like superinterfaces and fields. An example structure:

SuperpackageFile {
   u4 magic;          // 0xDECAFDAD ? :-)
   u2 major_version;  // 51 ?
   u2 minor_version;  // 0
   u4 constant_pool_count;  // Not u2 like ClassFile
   cp_info constant_pool[constant_pool_count-1];
   u2 this_superpackage;
   u2 enclosing_superpackage;
   u4 members_count;  // Allow lots of members
   member_info members[members_count];
   u4 exports_count;  // Allow lots of exports
   export_info exports[exports_count];
   u4 attributes_count;
   attribute_info attributes[attributes_count];  // for annotations
}

Please let us know what you think. If you can privately consult authors 
of classfile tools, please do so and summarize their comments here.

Alex