On 6/18/20 5:43 AM, Aleksey Shipilev wrote:
On 6/17/20 6:50 PM, Mandy Chung wrote:
On 6/17/20 8:11 AM, Aleksey Shipilev wrote:
On 6/15/20 11:58 PM, Mandy Chung wrote:
Amusingly, with the rest of Unsafe available the barn is still full open.
Here's the sketch for the
workaround: get the field list with getDeclaredFields, optionally guess the VM
layout (the rules are
not that hard, and JOL does it routinely), instantiate a Record with whatever
arguments, poke the
test patterns with Unsafe.put*, read them back from record components to
verify. It would get me the
same objectFieldOffset in a round-about way.
I'm aware of these workarounds in the wild. As sun.misc.Unsafe is JDK
unsupported API, this patch
does not attempt to implement a complete solution but adds an obvious
notification informing the
frameworks that `sun.misc.Unsafe::objectFieldOffset` and
`sun.misc.Unsafe::staticField{Offset/Base}`
not to support records.
I understand the intent. My point is that intent is mistimed during this time.
I understand this
tripwire needs to be put while Records are still in preview. My point is that
it cannot be put in
before Records serialization story has the preview-proven answer.
The intent also looks rather opaque. The intent is to notify maintainers, fine.
I am one of the
maintainers (although not of serialization library, but still heavy-Unsafe
one), so hear me out. I
came up with the workaround above in about 15 minutes after someone pointed out
the exception to me.
Have it crossed my mind that maybe JDK developers want some help here? No. It
looks like another
impediment that should be solved on the spot. Does that exception communicate
any intent at all? The
message is generic. There is no comment. How would you expect to push
maintainers to think along the
"we should be collaborating to find a proper way to do this", instead of "this
is set in stone; let
me hack this around too"?
Records is in second preview (JEP 384). As clearly stated in JEP 12 and
[1], introducing a feature as a preview feature enables developers to
provide feedbacks before it becoems permanent in a future Java SE Platform.
We should encourage developers to engage and provide feedbacks and have
the discussion during preview.
For JOL, have you raised an enhancement request to the serviceability
team? Inspecting on an object layout would be useful and I suggest you
file a RFE if you have done so.
[1]
https://docs.oracle.com/en/java/javase/14/language/preview-language-and-vm-features.html
This is the actionable bit:
At very least the exception message should say something about the intent. And
maybe even the
comment in Unsafe.java should point to the discussion about this intent and
maybe even provide the
breadcrumbs to follow, e.g. to ObjectInputStream.readRecord().
To hit this exception, the app must be running with --enable-preview.
There are documentation about preview feature and warning messages
etc. To avoid duplicating the info, the best I could do may be to add
"preview feature" in the exception message?
Chris Hegarty will definitely want to help out w.r.t. the serialization
frameworks in supporting records.
Mandy
