RFR(XS) 8212612 - Add documentation about Arguments::_exit_hook

[dean.long at oracle.com]

On 10/17/18 2:35 PM, Ioi Lam wrote:
>
>
> On 10/17/2018 02:27 PM, dean.long at oracle.com wrote:
>> On 10/17/18 9:33 AM, Ioi Lam wrote:
>>> http://cr.openjdk.java.net/~iklam/jdk12/8212612-exit-hook-docs.v01/
>>> https://bugs.openjdk.java.net/browse/JDK-8212612
>>>
>>> The following fields are assigned to according to 
>>> JavaVMOption.optionString.
>>>
>>>   abort_hook_t Arguments::_abort_hook = NULL;
>>>   exit_hook_t Arguments::_exit_hook = NULL;
>>>   vfprintf_hook_t Arguments::_vfprintf_hook = NULL;
>>>
>>> However, there's no code in the JDK repo that uses these options.
>>> Instead, they are intended for programs that embed the JVM using
>>> JNI_CreateJavaVM.
>>>
>>> We should add comments in arguments.cpp for clarification. Otherwise
>>> this would look like dead code and someone might be tempted to
>>> remove it.
>>>
>>> Thanks
>>> - Ioi
>>
>> But it's not dead code.  All three are used.  For example, see 
>> jio_vfprintf().
>>
>> dl
>
> Hi Dean, thanks for looking into this.
>
> I was afraid someone would infer that since nowhere in the JDK code 
> would do anything to assign _vfprintf_hook to a non-NULL value, the block
>
>   if (Arguments::vfprintf_hook() != NULL) {
>     jio_fprintf(defaultStream::output_stream(), "%.*s", (int)len, s);
>   }
>
> would be effectively dead code.
>

OK, then this comment was confusing:

These are not used by the JDK itself


I think it's more accurate to say that JDK launchers don't set these, 
but the JVM does use and honor them if set.

dl

> Thanks
> - Ioi