Using === across different contexts

Tue Dec 22 04:04:20 UTC 2015

Thanks for the response! I understand that in general it would be difficult
for foreign objects. But this seems like surprising behavior given that
these are both JavaScript objects. That they are unequal seems to be an
artifact of the implementation (the fact that JS objects from different
contexts are treated as foreign). Could JS objects be treated differently?

After going through the Nashorn source, I decided to try this very naive
approach adding the following test to ScriptRuntime#equalSameTypeValues:

if(x instanceof ScriptObjectMirror && y instanceof ScriptObjectMirror) {
    return x.equals(y);
}

After this change, the code now returns true, because
ScriptObjectMirror#equals compares the actual objects. I am not sure if
this breaks anything though (been trying to run the test suite, but end up
getting some errors from make). Is there a reason this particular fix is a
bad idea? I can't think of a particular reason why. From the perspective of
the runtime, I can't see a reason why those two objects should be
considered different.

I will investigate the eval approach, but ideally I would like something
that doesn't impose any changes on the JS code. I am developing a simple
runtime that exposes some objects and utilities, and where custom scripts
can run in their own contexts. The fact that === returns false in these
cases leads to some very strange behavior.

On Mon, Dec 21, 2015 at 8:48 PM, Sundararajan Athijegannathan <
sundararajan.athijegannathan at oracle.com> wrote:

> Unless we create mirrors as weak refs internally (i.e., maintain 1:1 with
> underlying foreign object reference), there is no easy solution. And
> maintaining such weak refs is unnecessarily complex. "Foreign object"
> call/access is mean to be just a "lightweight wrapper" based access. That
> said, you can do the === on the foreign context itself. You can call
> ScriptObjectMirror's eval to evaluate === test in that foreign context.
> That would get right object identity.
>
> -Sundar
>
>
> On 12/22/2015 4:26 AM, Vivin Suresh Paliath wrote:
>
>> One more thing I noticed is that apparently a new ScriptObjectMirror
>> instance is probably being created each time x is dereferenced, so "e.x
>> ===
>> e.x" also returns "false".
>>
>> On Mon, Dec 21, 2015 at 3:49 PM, Vivin Suresh Paliath <
>> vivin.paliath at gmail.com> wrote:
>>
>> I ran into an issue where === returns false even when both should be
>>> pointing to the same object. I'm assuming this is because one of the
>>> objects is wrapped by a ScriptObjectMirror, because it was defined in a
>>> different context.
>>>
>>> Here's some code that demonstrates this:
>>>
>>>          ScriptEngine engine = new
>>> NashornScriptEngineFactory().getScriptEngine(
>>>              new String[] { "-strict" }
>>>          );
>>>
>>>          try {
>>>              engine.eval("function Foo(src) { this.src = src }; var e = {
>>> x: new Foo(\"what\") };");
>>>
>>>              ScriptContext c = new SimpleScriptContext();
>>>              c.setBindings(engine.createBindings(),
>>> ScriptContext.ENGINE_SCOPE);
>>>
>>>
>>> c.getBindings(ScriptContext.ENGINE_SCOPE).putAll(engine.getBindings(ScriptContext.ENGINE_SCOPE));
>>>
>>>              System.out.println(engine.eval("var z = e.x; z === e.x;",
>>> c));
>>>          } catch(Exception e) {
>>>              throw new RuntimeException(e);
>>>          }
>>>
>>> This prints out "false". Is there a way around this? I am also explicitly
>>> copying over all the bindings from the parent scope into the new scope so
>>> that I have access to "e". Could this be the source of the problem, and
>>> if
>>> so, is there a better way to achieve what I'm trying to do?
>>>
>>> --
>>> Ruin untold;
>>> And thine own sadness,
>>> Sing in the grass,
>>> When eve has forgot, that no more hear common things that gleam and pass;
>>> But seek alone to lip, sad Rose of love and ruin untold;
>>> And thine own mother
>>> Can know it as I know
>>> More than another
>>> What makes your own sadness,
>>> Set in her eyes.
>>>
>>> map{@n=split//;$j.=$n[0]x$n[1]}split/:/,"01:11:02".
>>> ":11:01:11:02:13:01:11:01:11:01:13:02:12:01:13:01".
>>> ":11:04:11:06:12:04:11:01:12:01:13:02:12:01:14:01".
>>> ":13:01:11:03:12:01:11:04:12:02:11:01:11:01:13:02".
>>> ":11:03:11:06:11:01:11:05:12:02:11:01:11:01:13:02".
>>> ":11:02:12:01:12:04:11:06:12:01:11:04:12:04:11:01".
>>> ":12:03:12:01:12:01:11:01:12:01:12:02:11:01:11:01".
>>> ":13:02:11:01:02:11:01:12:02";map{print chr unpack"
>>> i",pack"B32",$_}$j=~m/.{8}/g
>>>
>>>
>>
>>
>

-- 
Ruin untold;
And thine own sadness,
Sing in the grass,
When eve has forgot, that no more hear common things that gleam and pass;
But seek alone to lip, sad Rose of love and ruin untold;
And thine own mother
Can know it as I know
More than another
What makes your own sadness,
Set in her eyes.

map{@n=split//;$j.=$n[0]x$n[1]}split/:/,"01:11:02".
":11:01:11:02:13:01:11:01:11:01:13:02:12:01:13:01".
":11:04:11:06:12:04:11:01:12:01:13:02:12:01:14:01".
":13:01:11:03:12:01:11:04:12:02:11:01:11:01:13:02".
":11:03:11:06:11:01:11:05:12:02:11:01:11:01:13:02".
":11:02:12:01:12:04:11:06:12:01:11:04:12:04:11:01".
":12:03:12:01:12:01:11:01:12:01:12:02:11:01:11:01".
":13:02:11:01:02:11:01:12:02";map{print chr unpack"
i",pack"B32",$_}$j=~m/.{8}/g