An undefined behavior w.r.t. <clinit>

[Yuting Chen]

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
}