From anthony.vanelverdinghe at gmail.com  Sun Jul 12 12:10:50 2015
From: anthony.vanelverdinghe at gmail.com (Anthony Vanelverdinghe)
Date: Sun, 12 Jul 2015 14:10:50 +0200
Subject: Example 6.6-5: unhelpful reference to section 8.8.8
Message-ID: <55A2594A.1080906@gmail.com>

Dear Java SE Specification Leads

Example 6.6-5 [1] mentions at the bottom "See ?8.8.8 for an example that 
uses a private constructor."
However, section 8.8.8 contains no such example. I believe the intended 
reference is to section 8.8.10 instead, more specifically Example 8.8.10-1

[1] http://docs.oracle.com/javase/specs/jls/se8/html/jls-6.html#jls-6.6.1

Kind regards,
Anthony Vanelverdinghe


From chenyt at cs.sjtu.edu.cn  Thu Jul  9 00:21:32 2015
From: chenyt at cs.sjtu.edu.cn (Yuting Chen)
Date: Wed, 8 Jul 2015 17:21:32 -0700
Subject: An undefined behavior w.r.t. <clinit>
Message-ID: <CAKra362EUi15bj9KEuSmF4GoxCnyytSRKSKtEqSkmg5Z9umqKA@mail.gmail.com>

Hello, I recently feel puzzling about the JVM behaviors when an illegal
<clinit> is met. The specification may not be complete in this part.

In the specification (jvms-2.9, se8), we have the words: "In a class file
whose version number is 51.0 or above, the method must additionally have
its ACC_STATIC flag (?4.6) set in order to be the class or interface
initialization method."

However, the behavior is undefined if the ACC_STATIC flag is not set. Thus
when a class without an ACC_STATIC flag may either be accepted or be
rejected, depending on the JVM developers. The results can be unpredictable.

I checked this by running a class on OpenJDK8-HotSpot and IBM J9, and found
different results. The class can be run on HotSpot without triggering any
exceptions (i.e., <clinit> is of no consequence). However, IBM J9 can throw
out a FormatError (Exception in thread "main" java.lang.ClassFormatError:
JVMCFRE074 no Code attribute specified; class=M1436188543,
method=<clinit>()V, pc=0). I checked the specification and still could not
decide which JVM behaves correctly.

The class is attached, and the decompiled code is given next:

class M1436188543
  minor version: 0
  major version: 51
  flags: ACC_SUPER
Constant pool:
   #1 = Utf8               java/lang/Object
   #2 = Utf8               <clinit>
   #3 = Utf8               SourceFile
   #4 = Utf8               M1436188543
   #5 = Utf8               main
   #6 = Class              #4             // M1436188543
   #7 = Utf8               Code
   #8 = Utf8               ([Ljava/lang/String;)V
   #9 = Utf8               ()V
  #10 = Utf8               Jasmin
  #11 = Class              #1             // java/lang/Object
{
  public abstract {};
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_ABSTRACT

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
}

From chenyt at cs.sjtu.edu.cn  Sun Jul 12 19:41:12 2015
From: chenyt at cs.sjtu.edu.cn (Yuting Chen)
Date: Sun, 12 Jul 2015 12:41:12 -0700
Subject: An undefined behavior w.r.t. <clinit>
In-Reply-To: <CAKra362EUi15bj9KEuSmF4GoxCnyytSRKSKtEqSkmg5Z9umqKA@mail.gmail.com>
References: <CAKra362EUi15bj9KEuSmF4GoxCnyytSRKSKtEqSkmg5Z9umqKA@mail.gmail.com>
Message-ID: <CAKra360R831YJFqqZC1veCKFw2C4hcfbj_i3K-9bwVv+jYsA_g@mail.gmail.com>

Should the text in jvms 2.9 be changed? The statement "Other methods
named <clinit> in a class file are of no consequence" is not very
clear. I guess the correct representation should be "Methods named
<clinit> are of no consequence if they are not class or interface
initialization methods."

Suggestions:
"
The initialization method of a class or interface has the special name
<clinit>, takes no arguments, and is void (?4.3.3).
Other methods named <clinit> in a class file are of no consequence.
They are not class or interface initialization methods. They cannot be
invoked by any Java Virtual Machine instruction and are never invoked
by the Java Virtual Machine itself.
"
=>
"
The initialization method of a class or interface has the special name
<clinit>, takes no arguments, and is void (?4.3.3).

Other methods named <clinit> are not class or interface initialization
methods. They cannot be invoked by any Java Virtual Machine
instruction and are never invoked by the Java Virtual Machine itself.
Methods named <clinit> are of no consequence if they are not class or
interface initialization methods.
"

On Wed, Jul 8, 2015 at 5:21 PM, Yuting Chen <chenyt at cs.sjtu.edu.cn> wrote:
> Hello, I recently feel puzzling about the JVM behaviors when an illegal
> <clinit> is met. The specification may not be complete in this part.
>
> In the specification (jvms-2.9, se8), we have the words: "In a class file
> whose version number is 51.0 or above, the method must additionally have its
> ACC_STATIC flag (?4.6) set in order to be the class or interface
> initialization method."
>
> However, the behavior is undefined if the ACC_STATIC flag is not set. Thus
> when a class without an ACC_STATIC flag may either be accepted or be
> rejected, depending on the JVM developers. The results can be unpredictable.
>
> I checked this by running a class on OpenJDK8-HotSpot and IBM J9, and found
> different results. The class can be run on HotSpot without triggering any
> exceptions (i.e., <clinit> is of no consequence). However, IBM J9 can throw
> out a FormatError (Exception in thread "main" java.lang.ClassFormatError:
> JVMCFRE074 no Code attribute specified; class=M1436188543,
> method=<clinit>()V, pc=0). I checked the specification and still could not
> decide which JVM behaves correctly.
>
> The class is attached, and the decompiled code is given next:
>
> class M1436188543
>   minor version: 0
>   major version: 51
>   flags: ACC_SUPER
> Constant pool:
>    #1 = Utf8               java/lang/Object
>    #2 = Utf8               <clinit>
>    #3 = Utf8               SourceFile
>    #4 = Utf8               M1436188543
>    #5 = Utf8               main
>    #6 = Class              #4             // M1436188543
>    #7 = Utf8               Code
>    #8 = Utf8               ([Ljava/lang/String;)V
>    #9 = Utf8               ()V
>   #10 = Utf8               Jasmin
>   #11 = Class              #1             // java/lang/Object
> {
>   public abstract {};
>     descriptor: ()V
>     flags: ACC_PUBLIC, ACC_ABSTRACT
>
>   public static void main(java.lang.String[]);
>     descriptor: ([Ljava/lang/String;)V
>     flags: ACC_PUBLIC, ACC_STATIC
>     Code:
>       stack=0, locals=1, args_size=1
>          0: return
> }
>

From alex.buckley at oracle.com  Mon Jul 20 21:08:25 2015
From: alex.buckley at oracle.com (Alex Buckley)
Date: Mon, 20 Jul 2015 14:08:25 -0700
Subject: Example 6.6-5: unhelpful reference to section 8.8.8
In-Reply-To: <55A2594A.1080906@gmail.com>
References: <55A2594A.1080906@gmail.com>
Message-ID: <55AD6349.2080406@oracle.com>

Anthony, thanks for the correction.

Alex

On 7/12/2015 5:10 AM, Anthony Vanelverdinghe wrote:
> Dear Java SE Specification Leads
>
> Example 6.6-5 [1] mentions at the bottom "See ?8.8.8 for an example that
> uses a private constructor."
> However, section 8.8.8 contains no such example. I believe the intended
> reference is to section 8.8.10 instead, more specifically Example 8.8.10-1
>
> [1] http://docs.oracle.com/javase/specs/jls/se8/html/jls-6.html#jls-6.6.1
>
> Kind regards,
> Anthony Vanelverdinghe
>

From alex.buckley at oracle.com  Mon Jul 20 21:26:46 2015
From: alex.buckley at oracle.com (Alex Buckley)
Date: Mon, 20 Jul 2015 14:26:46 -0700
Subject: An undefined behavior w.r.t. <clinit>
In-Reply-To: <CAKra362EUi15bj9KEuSmF4GoxCnyytSRKSKtEqSkmg5Z9umqKA@mail.gmail.com>
References: <CAKra362EUi15bj9KEuSmF4GoxCnyytSRKSKtEqSkmg5Z9umqKA@mail.gmail.com>
Message-ID: <55AD6796.2050206@oracle.com>

Yuting,

In a 51.0 class file, a method called <clinit> whose ACC_STATIC flag is 
not set is NOT a class or interface initialization method. It's just an 
ordinary method that happens to be uncallable. There's nothing 
"undefined" about that. Since it's an ordinary method, it can be 
ACC_ABSTRACT and have no Code attribute, so J9 is wrong to throw a CFE 
for it.

It just so happens that Oracle engineers were investigating methods 
called <clinit> earlier this month. I filed a spec clarification: 
https://bugs.openjdk.java.net/browse/JDK-8130682

Alex

On 7/8/2015 5:21 PM, Yuting Chen wrote:
> Hello, I recently feel puzzling about the JVM behaviors when an illegal
> <clinit> is met. The specification may not be complete in this part.
>
> In the specification (jvms-2.9, se8), we have the words: "In a class file
> whose version number is 51.0 or above, the method must additionally have
> its ACC_STATIC flag (?4.6) set in order to be the class or interface
> initialization method."
>
> However, the behavior is undefined if the ACC_STATIC flag is not set. Thus
> when a class without an ACC_STATIC flag may either be accepted or be
> rejected, depending on the JVM developers. The results can be unpredictable.
>
> I checked this by running a class on OpenJDK8-HotSpot and IBM J9, and found
> different results. The class can be run on HotSpot without triggering any
> exceptions (i.e., <clinit> is of no consequence). However, IBM J9 can throw
> out a FormatError (Exception in thread "main" java.lang.ClassFormatError:
> JVMCFRE074 no Code attribute specified; class=M1436188543,
> method=<clinit>()V, pc=0). I checked the specification and still could not
> decide which JVM behaves correctly.
>
> The class is attached, and the decompiled code is given next:
>
> class M1436188543
>    minor version: 0
>    major version: 51
>    flags: ACC_SUPER
> Constant pool:
>     #1 = Utf8               java/lang/Object
>     #2 = Utf8               <clinit>
>     #3 = Utf8               SourceFile
>     #4 = Utf8               M1436188543
>     #5 = Utf8               main
>     #6 = Class              #4             // M1436188543
>     #7 = Utf8               Code
>     #8 = Utf8               ([Ljava/lang/String;)V
>     #9 = Utf8               ()V
>    #10 = Utf8               Jasmin
>    #11 = Class              #1             // java/lang/Object
> {
>    public abstract {};
>      descriptor: ()V
>      flags: ACC_PUBLIC, ACC_ABSTRACT
>
>    public static void main(java.lang.String[]);
>      descriptor: ([Ljava/lang/String;)V
>      flags: ACC_PUBLIC, ACC_STATIC
>      Code:
>        stack=0, locals=1, args_size=1
>           0: return
> }
>