Hi,
I am going to reiterate. This will lead to maintenance nightmare! Any
"Unsupported Java API" is still an API! Remember Unsafe? Once blessed in
any form, it is very difficult to remove. Exposing hotspot VM internals
as a Java API is very bad idea. No, not even as "unsupported API".
Exposing SA to scripts was a different beast. Scripting languages are
dynamically typed. It is quite common in scripting world to check the
existence of an attribute & then use it (if (!obj["foo"]) / if (typeof
obj["foo"] == 'function') kind of code). That'd be extremely painful in
a statically typed language like Java (reflection or method/var
handles?). Platform debuggers like dbx support a scripting language
along with access to the data structures from the target process. SA
scripting was inspired from that model. Also scripts were never meant to
be written & maintained for long time! Most scripts were expected to be
written for a specific debugging exercise/session (thrown away).
-Sundar
On 20/12/19 6:38 am, Yasumasa Suenaga wrote:
Hi Chris,
Can we treat (part of) jdk.hotspot.agent like jdk.unsupported module?
jdk.unsupported exports unspec'd API like Unsafe.
If we do so, we might need to separate SA API into exported class and
internal class.
I've proposed to export all SA packages in JDK-8157947, but it was
rejected.
Thanks,
Yasumasa
On 2019/12/20 1:28, Chris Plummer wrote:
Hi Yasumasa,
I've had similar thoughts about how to extend clhsdb. Why not export
everything since what we would export is not part of a spec, and the
javascript support had the same issue of potentially breaking when
the SA API changed. But maybe this type of unspec'd API exporting is
considered bad policy. I'm not sure. I'll let the API spec guru's
comment on that aspect of it.
thanks,
Chris
On 12/19/19 12:17 AM, Yasumasa Suenaga wrote:
Hi,
I think we can provide API for SA as following:
Patch: http://cr.openjdk.java.net/~ysuenaga/sa-api/webrev/
Plugin examples:
browse:
http://cr.openjdk.java.net/~ysuenaga/sa-api/plugin-examples/
download:
http://cr.openjdk.java.net/~ysuenaga/sa-api/plugin-examples.tar.gz
I think JS plugin (loading via `jsload` CLHSDB command) was
supported "AS IS".
If HotSpot and/or SA code is changed, the user should follow it if
need.
SA is not part of Java SE. We need not to maintain SA API when it
happen IMHO.
The user who want to expand SA features (includes me!) should have
responsible for it.
So I did not expose jdk.hotspot.agent module - the user need to
build with --add-exports.
My proposal can write SA plugins with pure Java. So we don't need to
depend on script engine.
Comments are welcome.
Thanks,
Yasumasa
On 2019/12/11 21:47, sundararajan.athijegannat...@oracle.com wrote:
Effectively you're asking for SA as API. I don't think that is a
good idea. That implies supporting hotspot data structures as Java
*API*. That will be maintainability nightmare - we've to keep
tracking hotspot data structures in SA code. That itself is
problematic. API would be next level nightmare.
-Sundar
On 11/12/19 11:57 am, Yasumasa Suenaga wrote:
Hi,
IMHO we need to export all packages in SA if we do not provide new
API for SA.
sa.js in jdk.hotspot.agent could access all SA classes until JDK 8
(before Jigsaw), so we could make various functions if we need.
OTOH we cannot know what classes are needed by the SA users. All
packages in jdk.hotspot.agent module provides features, and they
require other packages. For example, sun.jvm.hotspot.oops.Oop
requires sun.jvm.hotspot.types, and it requires
sun.jvm.hotspot.debugger .
It is difficult to track and to export minimally.
(I worked for it in JDK-8157947, but I gave up...)
Thus I guess it is a big challenge to export SA classes without
refactoring.
If we provide new API for SA plugin, I guess we need to work some
refactoring.
Yasumasa
On 2019/12/11 15:00, Chris Plummer wrote:
On 12/10/19 9:56 PM, Yasumasa Suenaga wrote:
On 2019/12/11 14:39, Krystal Mok wrote:
Hi Yasumasa,
That's a very nice idea. Basically what you're asking for is
exposing the Command interface [1] so that plugins can
implement it and get dynamically loaded / registered into
CLHSDB / HSDB, right?
Yes, but we also need proxy API to access internal SA objects
e.g. CodeCache, JavaThread, TypeDataBase, etc...
Yes, or export them. I should have read this email before posting
my previous one.
Chris
Yasumasa
[1]:
http://hg.openjdk.java.net/jdk/jdk/file/c71ec1f09f21/src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/CommandProcessor.java#l246
- Kris
On Tue, Dec 10, 2019 at 9:33 PM Yasumasa Suenaga
<suen...@oss.nttdata.com <mailto:suen...@oss.nttdata.com>> wrote:
Hi Chris,
It's a sad proposal, but I agree with you. To maintain SA
in JS is difficult since Jigsaw.
However I want SA to implement pluggable feature.
I use custom script to list compiled codes in CodeCache.
I guess other troubleshooters also want similar feature
(via jsload) in future if they encounter JVM crash.
Thanks,
Yasumasa
On 2019/12/11 11:52, Chris Plummer wrote:
> Hi,
>
> I like to propose the removal of SA javascript support.
Few people even realize this support exists, and hopefully even
fewer are using it since I'd like to remove it. Since I'm new
to this myself, let me first explain what I know about it's
existence, and then explain why I want to remove it.
>
> If you run "jhsdb clhsdb", there are jsload and jseval
commands. Don't look for them in anything post JDK 8. I'll
explain why later. jsload is used to load a javascript file. In
that file you can register new clhsdb commands that are written
in javascript. You can also evaluate javascript using the
jseval command. Some of this is explained in [1], which is the
only place I can find any reference to this support. It does
not appear to be officially supported, nor is there any oracle
provided documentation.
>
> There also appear to be a few clhsdb commands that are
written in javascript. Doing a grep for "registerCommand" in
sa.js shows the following:
>
> registerCommand("class", "class name", "jclass");
> registerCommand("classes", "classes", "jclasses");
> registerCommand("dumpclass", "dumpclass { address |
name } [ directory ]", "dclass");
> registerCommand("dumpheap", "dumpheap [ file ]",
"dumpHeap");
> registerCommand("mem", "mem address [ length ]",
"printMem");
> registerCommand("sysprops", "sysprops", "sysProps");
> registerCommand("whatis", "whatis address",
"printWhatis");
>
> Once again, don't go looking for these in anything newer
than JDK8. You won't find them. Again the only documentation I
can fine is [1].
>
> The other use of Javascript is the SOQL command (Simple
Object Query Language), a tool used to query the heap, and also
the JSDB command. The only SOQL documentation I could find is
the blog reference [2]. I could not find HSDB documentation,
but I believe is is a javascript support for looking at
hotspot. So once again, neither of these seem to be officially
supported or documented.
>
> The real purpose of the email is to propose removal of
this support. Here are the reasons:
>
> (1) It's broken, and has been since 9. See [3]. This is
why you don't see the javascript related commands in clhsdb.
Javascript fails to initialize, so none of the javascript
related commands are registered.
> (2) Nashorn is deprecated and will be removed eventually.
> (3) We have very little understanding of the javascript
support.
> (4) No resources to work on it (unless there is a
community volunteer).
> (5) Very questionable value (lack of users). The fact
this support has been broken since JDK 9 and no bug was filed
until I did so this week is a good indication of that. Another
is that there are no other SA Javascript related bugs filed.
Lastly, the lack of any official documentation and only minimal
mention of it on the web is another good indication of it's
(lack of) value.
>
> Also, regarding the 7 commands listed above that would
be lost (but currently don't work now anyway), if they are
really wanted, they could be implemented in java instead of
javascript.
>
> I'd like to remove javascript support in two steps. The
first is simply disable the clhsdb code that tries to
initialize the javascript support. I'd like to do this in 14
(actually as soon as possible). I'd like to actually do this
now even if we decide to keep javascript support and eventually
fix it because it will get rid of the warning you see whenever
you attach from clhsdb:
>
> Warning! JS Engine can't start, some commands will
not be available.
>
> This warning will become more of an issue for the clhsdb
tests after I push [4] because then you will also see the full
stacktrace for the underlying exception that caused the
Javascript to fail to start. Besides being unnecessary noise in
passing test cases, it can also be misleading in any test that
fails because the exception will be unrelated to the failure.
This is actually what got me going down this path of what the
javascript support is all about.
>
> The next step would be to strip out all Javascript
related code, including the SOQL and JSDB tools. This would be
done in 15.
>
> Please let me know what you think.
>
> thanks,
>
> Chris
>
> [1]
https://cr.openjdk.java.net/~minqi/6830717/raw_files/new/agent/doc/clhsdb.html
> [2]
http://javatroubleshooting.blogspot.com/2015/12/serviceability-agent-part-3.html
> [3] https://bugs.openjdk.java.net/browse/JDK-8235594
> [4] https://bugs.openjdk.java.net/browse/JDK-8234277
>
