ModuleClass
Jonathan Gibbons
Jonathan.Gibbons at Sun.COM
Tue Mar 3 12:56:34 PST 2009
On Mar 3, 2009, at 12:31 PM, Alex Buckley wrote:
> I'm confused. In:
>
> ModuleClass_attribute {
> u2 attribute_name_index;
> u4 attribute_length;
> u2 main_class;
> }
>
> attribute_name_index points to a "ModuleClass" string, which is what
> Mark's patch from 21:26 yesterday used and a name he seemed happy
> with. (I think the bit of that patch he was loathe to commit was
> reading a class name from a Utf8.)
>
> main_class points to a CONSTANT_Class, which you agreed at 07:54 was
> the right thing to do.
>
> I can add an array of flags after main_class - no ModuleInfoReader
> code reads them yet anyway. Does this reflect what javac emits:
>
> ModuleClass_attribute {
> u2 attribute_name_index;
> u4 attribute_length;
> u2 main_class;
> u2 flags_length;
> { u2 flag; /*Points to Utf8_info*/ } main_class_flags[flags_length];
> }
>
Yes, this is what I understood we agreed to a couple of weeks back,
and which corresponds to javac/javap/classfile.
-- Jon
> Alex
>
> Jonathan Gibbons wrote:
>> Yes, it's a change. It's not in line with what we previously
>> agreed and which I have already implemented.
>> Yes, there was a bug in the impl such that I was using
>> CONSTANT_Utf8 instead of CONSTANT_Class, and I have spent the
>> morning fixing that and writing related test programs, and now
>> you're telling me you're changing the spec on me.
>> Your proposed spec change does *not* bring this in line with
>> ModuleInfoReader's current parsing, because that still uses an even
>> older version of the spec that uses the "ModuleMain" attribute, not
>> "ModuleClass".
>> -- Jon
>> Alex Buckley wrote:
>>> I guess I should have been clearer: the spec section starts at
>>> "The ModuleClass attribute is defined as follows...". "For now" is
>>> certainly not in the spec!
>>>
>>> Nothing has changed. I wrote up ModuleClass in line with Mark's
>>> comment "I suspect we really want ModuleClass to name a class
>>> rather than just convey a plain utf8 string" and with
>>> ModuleInfoReader's current parsing.
>>>
>>> Alex
>>>
>>> Jonathan Gibbons wrote:
>>>> NOOOOOOOOOOOOOO.
>>>>
>>>> Please stop gratuitous changing of the spec like this!
>>>>
>>>> I guess I distrust a spec that includes the words "For now". --
>>>> Jon
>>>>
>>>> On Mar 3, 2009, at 11:47 AM, Alex Buckley wrote:
>>>>
>>>>> Being able to put flags in the main class attribute would
>>>>> probably be worthwhile if multiple main class attributes were
>>>>> allowed: "Main class M is flagged as an FX 1.1 app", "Main class
>>>>> N is flagged as a bundle activator and can be assumed to
>>>>> implement interface...".
>>>>>
>>>>> However, I think for now one ModuleClass attribute is enough,
>>>>> pointing to a CONSTANT_Class_info, and without flags.
>>>>>
>>>>> The ModuleClass attribute is defined as follows:
>>>>>
>>>>> ModuleClass_attribute {
>>>>> u2 attribute_name_index;
>>>>> u4 attribute_length;
>>>>> u2 main_class;
>>>>> }
>>>>>
>>>>> attribute_name_index
>>>>> The value of the attribute_name_index item must be a valid
>>>>> index into the constant_pool table. The constant_pool entry at
>>>>> that index must be a CONSTANT_Utf8_info structure representing
>>>>> the string "ModuleClass".
>>>>>
>>>>> attribute_length
>>>>> The value of the attribute_length item is the length of the
>>>>> attribute excluding the initial six bytes.
>>>>>
>>>>> main_class
>>>>> The value of the main_class item must be a valid index into
>>>>> the constant_pool table. The constant_pool entry at that index
>>>>> must be a CONSTANT_Class_info structure representing the name of
>>>>> the class which the host system should treat as the entrypoint
>>>>> to the module.
>>>>>
>>>>> Attached is an updated (+ tiny corrections) classfile spec.
>>>>>
>>>>> Alex
>>>>>
>>>>> Jonathan Gibbons wrote:
>>>>>> Alex suggested adding it when I asked him for a spec of the
>>>>>> ModuleClass
>>>>>> attribute. At the time, the thought was to be able to mark the
>>>>>> type of class,
>>>>>> main class, applet etc. A more topical example, from a
>>>>>> separate email
>>>>>> thread going on now, would be to be able to mark a JavaFX class.
>>>>>> [Kumar is working on changes to the launcher to make it capable
>>>>>> of
>>>>>> starting FX programs, given an FX main class.] So the ability
>>>>>> to give
>>>>>> simple attributes regarding the class appears somewhat
>>>>>> reasonable.
>>>>>> I guess the updated classfile spec got pre-empted by FOSDEM ;-)
>>>>>> -- Jon
>>>>>> On Mar 3, 2009, at 9:21 AM, Mark Reinhold wrote:
>>>>>>>> Date: Tue, 03 Mar 2009 07:54:39 -0800
>>>>>>>> From: jonathan.gibbons at sun.com
>>>>>>>> <mailto:jonathan.gibbons at sun.com>
>>>>>>>
>>>>>>>> Point noted about the incorrect use of CONSTANT_Utf8 instead of
>>>>>>>> CONSTANT_Class. I'll check that out.
>>>>>>>
>>>>>>> Okay, thanks.
>>>>>>>
>>>>>>>> Also, if I recall
>>>>>>>> correctly,
>>>>>>>> that item is followed by an array of Utf8 strings
>>>>>>>> representing an open
>>>>>>>> ended set of flags/modifiers, similar to that used for the
>>>>>>>> requires
>>>>>>>> entry. If nothing else, you might need to skip over them if
>>>>>>>> there is
>>>>>>>> any info to follow.
>>>>>>>
>>>>>>> Hmm, didn't know that, and neither does the ModuleInfoReader
>>>>>>> code.
>>>>>>>
>>>>>>> I don't recall having discussed the need for modifiers in the
>>>>>>> main-class
>>>>>>> declaration. Do we have use cases?
>>>>>>>
>>>>>>> - Mark
>>>>>
>>>>>
>>>>> |ClassFile| changes in v51.0 for Modules
>>>>>
>>>>>
>>>>> Access flags
>>>>>
>>>>> Module-private accessibility is expressed with a new |
>>>>> access_flags| flag, |ACC_MODULE|, value 0x8000, description:
>>>>> "Declared |module|; may not be accessed from outside its module.
>>>>> May only be set if the |ClassFile| has a |Module| attribute."
>>>>> The flag appears in JVMS 4.1 (|ClassFile.access_flags|), 4.5 (|
>>>>> field_info.access_flags|), and 4.6 (|method_info.access_flags|),
>>>>> which gain the following rules:
>>>>>
>>>>> 4.1
>>>>> *At most one of the |ACC_MODULE| and |ACC_PUBLIC| flags may
>>>>> be set
>>>>> (JLS3 §6.6.1).* 4.5
>>>>> Fields of classes may set any of the flags in Table 4.4.
>>>>> However,
>>>>> a specific field of a class may have at most one of its
>>>>> |ACC_PRIVATE|, |ACC_PROTECTED|, *|ACC_MODULE|,* and |
>>>>> ACC_PUBLIC|
>>>>> flags set (JLS3 §8.3.1) and must not have both its |
>>>>> ACC_FINAL| and
>>>>> |ACC_VOLATILE| flags set (JLS3 §8.3.1.4). All fields of
>>>>> interfaces must have their |ACC_STATIC| and
>>>>> |ACC_FINAL| and *either |ACC_PUBLIC| or |ACC_MODULE|* flags
>>>>> set;
>>>>> they may have their |ACC_SYNTHETIC| flag set and must not
>>>>> have any
>>>>> of the other flags in Table 4.4 set (JLS3 §9.3). 4.6
>>>>> Methods of classes may set any of the flags in Table 4.5.
>>>>> However,
>>>>> a specific method of a class may have at most one of its
>>>>> |ACC_PRIVATE|, |ACC_PROTECTED|, *|ACC_MODULE|,* and |
>>>>> ACC_PUBLIC|
>>>>> flags set (JLS3 §8.4.3). All interface methods must have
>>>>> their |ACC_ABSTRACT| and *either
>>>>> |ACC_PUBLIC| or |ACC_MODULE|* flags set; they may have their
>>>>> |ACC_VARARGS|, |ACC_BRIDGE| and |ACC_SYNTHETIC| flags set and
>>>>> must
>>>>> not have any of the other flags in Table 4.5 set (JLS3
>>>>> §9.4). A specific instance initialization method (§3.9) may
>>>>> have at most
>>>>> one of its |ACC_PRIVATE|, |ACC_PROTECTED|, *|ACC_MODULE|,* and
>>>>> |ACC_PUBLIC| flags set, and may also have its |ACC_STRICT|,
>>>>> |ACC_VARARGS| and |ACC_SYNTHETIC| flags set, but must not
>>>>> have any
>>>>> of the other flags in Table 4.5 set.
>>>>> In the |InnerClasses| attribute's |inner_class_access_flags|
>>>>> table, |ACC_MODULE| means "Marked |module| in source. May only
>>>>> be set if the |ClassFile| has a |Module| attribute."
>>>>>
>>>>> The |ACC_PUBLIC| flag in all |access_flags| is now defined as
>>>>> "Declared |public|; may be accessed from outside its package or
>>>>> module."
>>>>>
>>>>>
>>>>> 4.8.21 The |Module| attribute
>>>>>
>>>>> The |Module| attribute is a fixed-length attribute in the |
>>>>> attributes| table of a |ClassFile| (JVMS 4.1) structure. The |
>>>>> Module| attribute records the /named module/ (JLS 7.4.5) to
>>>>> which the class belongs. A |ClassFile| structure without a |
>>>>> Module| attribute is considered to be a member of the /unnamed
>>>>> module/ (JLS 7.4.6).
>>>>>
>>>>> There can be no more than one |Module| attribute in the |
>>>>> attributes| table of a given |ClassFile| structure. A |Module|
>>>>> attribute may only appear in a |ClassFile| structure that
>>>>> represents a class whose binary name in internal form (as given
>>>>> by the constant pool entry referenced by the |name_index| item
>>>>> of the |CONSTANT_Class_info| structure referenced by |
>>>>> ClassFile.this_class|) is qualified, i.e. contains at least one
>>>>> |/|.
>>>>>
>>>>> The |Module| attribute has the following format:
>>>>>
>>>>> Module_attribute {
>>>>> u2 attribute_name_index;
>>>>> u4 attribute_length;
>>>>> u2 module_id_index;
>>>>> }
>>>>> The items of the |Module_attribute| structure are as follows:
>>>>>
>>>>> |attribute_name_index|
>>>>> The value of the |attribute_name_index| item must be a valid
>>>>> index
>>>>> into the |constant_pool| table. The |constant_pool| entry at
>>>>> that
>>>>> index must be a |CONSTANT_Utf8_info| structure representing the
>>>>> string "Module". |attribute_length|
>>>>> The value of the |attribute_length| item must be 2. |
>>>>> module_id_index|
>>>>> The value of the |module_id_index| item must be a valid index
>>>>> into
>>>>> the |constant_pool| table. The |constant_pool| entry at that
>>>>> index
>>>>> must be a |CONSTANT_ModuleId_info| structure representing the
>>>>> name
>>>>> and version of the named module of which this |ClassFile| is a
>>>>> member. The module name must be encoded in internal form
>>>>> (4.2.1).
>>>>> If the |version_index| item of the |CONSTANT_ModuleId_info|
>>>>> structure is 0, then this |ClassFile| is a member of any
>>>>> version
>>>>> of the module named by the |name_index| item.
>>>>> The |CONSTANT_ModuleId_info| structure has the following format:
>>>>> CONSTANT_ModuleId_info {
>>>>> u1 tag;
>>>>> // 13
>>>>> u2 name_index;
>>>>> // Points to a CONSTANT_Utf8_info representing module name
>>>>> u2 version_index;
>>>>> // Points to a CONSTANT_Utf8_info representing module version
>>>>> }
>>>>>
>>>>>
>>>>> The module-info.class file
>>>>>
>>>>> A module compilation unit (|module-info.java|) is compiled to a |
>>>>> ClassFile| structure like any other compilation unit. It follows
>>>>> the precedent set by compiling a package compilation unit (|
>>>>> package-info.java|) which has been possible since Java 5.0.
>>>>> Since neither compilation unit describes a class as such, the
>>>>> following conventions are adopted for their |ClassFile|
>>>>> structures.
>>>>>
>>>>>
>>>>> package-info.class
>>>>>
>>>>> * |major_version, minor_version|: >=49.0
>>>>> * |this_class|: [Package name in internal form (4.2.1)]/
>>>>> package-info
>>>>> * |access_flags|: v49: ACC_INTERFACE ACC_ABSTRACT >=v50:
>>>>> ACC_INTERFACE ACC_ABSTRACT ACC_SYNTHETIC
>>>>> * |super_class|, |interfaces_count|, |fields_count|,
>>>>> |methods_count|: 0
>>>>> * |attributes|: except for |Module|, |SourceFile|,
>>>>> |RuntimeVisibleAnnotations| and |
>>>>> RuntimeInvisibleAnnotations|,
>>>>> none of the pre-defined attributes in JVMS 4.7 may appear.
>>>>>
>>>>> module-info.class
>>>>>
>>>>> * |major_version, minor_version|: >=51.0
>>>>> * |this_class|: [Module name in internal form (4.2.1)]/module-
>>>>> info
>>>>> * |access_flags|: ACC_INTERFACE ACC_ABSTRACT ACC_SYNTHETIC
>>>>> * |attributes|: One |Module| attribute must be present, for the
>>>>> module name given in the |this_class| item.
>>>>> * |super_class|, |interfaces_count|, |fields_count|,
>>>>> |methods_count|: 0
>>>>> * |attributes|: One |Module| attribute must be present, for the
>>>>> module name given in the |this_class| item. At most one of
>>>>> each
>>>>> of the |ModuleRequires|, |ModulePermits|, |ModuleProvides|,
>>>>> and
>>>>> |ModuleClass| attributes must be present if the module has
>>>>> metadata. Except for these attributes and |SourceFile|,
>>>>> |RuntimeVisibleAnnotations|, and |
>>>>> RuntimeInvisibleAnnotations|,
>>>>> none of the pre-defined attributes in JVMS 4.7 may appear.
>>>>> * A module name may be referenced by at most one entry in the
>>>>> |requires_table| of a |ModuleRequires| attribute and at
>>>>> most one
>>>>> entry in the |permits_table| of a |ModulePermits| attribute
>>>>> and
>>>>> at most one entry in the |provides_table| of a |
>>>>> ModuleProvides|
>>>>> attribute of a |ClassFile|.
>>>>>
>>>>> Attributes for the module-info.class file
>>>>>
>>>>> The |ModuleRequires| attribute is defined as follows:
>>>>> ModuleRequires_attribute {
>>>>> u2 attribute_name_index;
>>>>> u4 attribute_length;
>>>>> u2 requires_length;
>>>>> { u2 requires_index; u1 flags; } requires_table[requires_length];
>>>>> }
>>>>>
>>>>> |attribute_name_index|
>>>>> The value of the |attribute_name_index| item must be a valid
>>>>> index
>>>>> into the |constant_pool| table. The |constant_pool| entry at
>>>>> that
>>>>> index must be a |CONSTANT_Utf8_info| structure representing the
>>>>> string "ModuleRequires". |attribute_length|
>>>>> The value of the |attribute_length| item is the length of the
>>>>> attribute excluding the initial six bytes. |requires_length|
>>>>> The value of the |requires_length| indicates the number of
>>>>> entries
>>>>> in the |requires_table|. |requires_table|
>>>>> Each |requires_index| must be a valid index into the
>>>>> |constant_pool| table. The |constant_pool| entry at that index
>>>>> must be a |CONSTANT_ModuleId_info| structure representing the
>>>>> target module on which this module depends.
>>>>> flags: 0x01 if this dependency on the target module is optional
>>>>> flags: 0x02 if the target module's types must be loaded by the
>>>>> same defining classloader as the
>>>>> types of the module represented by this ClassFile
>>>>> flags: 0x04 if the target module's public types are NOT
>>>>> observable to
>>>>> types of the module represented by this ClassFile
>>>>>
>>>>> The |ModulePermits| attribute is defined as follows:
>>>>> ModulePermits_attribute {
>>>>> u2 attribute_name_index;
>>>>> u4 attribute_length;
>>>>> u2 permits_length;
>>>>> { u2 permits_index } permits_table[permits_length];
>>>>> }
>>>>>
>>>>> |attribute_name_index|
>>>>> The value of the |attribute_name_index| item must be a valid
>>>>> index
>>>>> into the |constant_pool| table. The |constant_pool| entry at
>>>>> that
>>>>> index must be a |CONSTANT_Utf8_info| structure representing the
>>>>> string "ModulePermits". |attribute_length|
>>>>> The value of the |attribute_length| item is the length of the
>>>>> attribute excluding the initial six bytes. |permits_length|
>>>>> The value of the |permits_length| indicates the number of
>>>>> entries
>>>>> in the |permits_table|. |permits_table|
>>>>> The value of each |permits_index| in this item must be a valid
>>>>> index into the |constant_pool| table. The |constant_pool|
>>>>> entry at
>>>>> that index must be a |CONSTANT_Utf8_info| structure
>>>>> representing
>>>>> the name of a module which is permitted by the host system to
>>>>> have
>>>>> a dependency on the module represented by this |ClassFile|.
>>>>> The |ModuleProvides| attribute is defined as follows:
>>>>> ModuleProvides_attribute {
>>>>> u2 attribute_name_index;
>>>>> u4 attribute_length;
>>>>> u2 provides_length;
>>>>> { u2 provides_index; } provides_table[provides_length];
>>>>> }
>>>>>
>>>>> |attribute_name_index|
>>>>> The value of the |attribute_name_index| item must be a valid
>>>>> index
>>>>> into the |constant_pool| table. The |constant_pool| entry at
>>>>> that
>>>>> index must be a |CONSTANT_Utf8_info| structure representing the
>>>>> string "ModuleProvides". |attribute_length|
>>>>> The value of the |attribute_length| item is the length of the
>>>>> attribute excluding the initial six bytes. |provides_length|
>>>>> The value of the |provides_length| indicates the number of
>>>>> entries
>>>>> in the |provides_table|. |provides_table|
>>>>> The value of each |provides_index| in this item must be a valid
>>>>> index into the |constant_pool| table. The |constant_pool|
>>>>> entry at
>>>>> that index must be a |CONSTANT_ModuleId_info| structure
>>>>> representing a module identity that is an alias for the module
>>>>> represented by this |ClassFile|.
>>>>> The |ModuleClass| attribute is defined as follows:
>>>>> ModuleClass_attribute {
>>>>> u2 attribute_name_index;
>>>>> u4 attribute_length;
>>>>> u2 main_class;
>>>>> }
>>>>>
>>>>> |attribute_name_index|
>>>>> The value of the |attribute_name_index| item must be a valid
>>>>> index
>>>>> into the |constant_pool| table. The |constant_pool| entry at
>>>>> that
>>>>> index must be a |CONSTANT_Utf8_info| structure representing the
>>>>> string "ModuleClass". |attribute_length|
>>>>> The value of the |attribute_length| item is the length of the
>>>>> attribute excluding the initial six bytes. |main_class|
>>>>> The value of the |main_class| item must be a valid index into
>>>>> the
>>>>> |constant_pool| table. The |constant_pool| entry at that index
>>>>> must be a |CONSTANT_Class_info| structure representing the
>>>>> name of
>>>>> the class which the host system should treat as the
>>>>> entrypoint to
>>>>> the module.
>>>>
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