On 11/12/13 12:53, Attila Szegedi wrote:
On Dec 11, 2013, at 1:13 PM, Tim Fox <[email protected]> wrote:
Confused...
I assumed that if two scripts where run with their own script context, then
they would already have separate globals, i.e. if I do
myglobal = 1
in module 1 that won't be visible in module 2.
That's true, but then you also end up with the need for ScriptObjectMirrors
between them, and that was what I suggested you try to avoid.
So I'm not sure really what --global-per-engine really means, if the modules
have their own globals anyway. I guess my understanding must be wrong somewhere.
Well, it will mean that modules won't have their own globals…
--global-per-engine will make it so that the Global object is specific to the
ScriptEngine, and not to the ScriptContext, e.g. even if you replace the
ScriptContext of the engine, when scripts are run, they'll still see the same
global object as before the replacement. The gist of it is:
a) without --global-per-engine, the Global object is specific to a
ScriptContext, each ScriptContext has its own. ENGINE_SCOPE Bindings object of
the context is actually a mirror of its Global.
b) with --global-per-engine, the Global object lives in the ScriptEngine.
ENGINE_SCOPE Bindings object of the context is just a vanilla SimpleBindings
(or whatever you set it to), and Global will delegate property getters for
non-existent properties to it (but it'll still receive property setters, so new
global variables created by one script will be visible by another; no isolation
there).
What I was suggesting is that your module loading code would look something
like:
// The engine that you use
ScriptEngine engine = new NashornScriptEngine().getEngine(new String[] {
"--global-per-engine" });
...
// when loading a module
Bindings moduleVars = new SimpleBindings();
moduleVars.put("require", requireFn);
moduleVars.put("module", moduleDescriptorObj);
moduleVars.put("exports", exportsObj);
Bindings prevBindings = engine.getBindings(ENGINE_SCOPE);
engine.setBindings(moduleVars, ENGINE_SCOPE);
try {
engine.eval(moduleSource);
} finally {
engine.setBindings(prevBindings, ENGINE_SCOPE);
}
return exportsObj;
NB: your modules would _not_ run in isolated globals.
That's the problem. CommonJS modules need to run in their own globals to
avoid pollution.
I thought they do, but I just spoke to Sundar and he explained the mechanism to
me so now I see they won't -- see above the case b).
They could pollute each other's global namespace (since it's shared). Hopefully
they'd adhere to Modules/Secure recommendation and refrain from doing so, but
you can't really enforce it.
Just about every CommonJS module I have seen uses globals, e.g. pretty
much all the node.js ones (and the vert.x ones). Here's an example:
https://github.com/joyent/node/blob/master/lib/http.js
Notice the liberal use of top level vars, which are globals. This is the
norm in CommonJS AIUI. Any require() implementation that doesn't provide
global isolation isn't going to fly.
My require() implementation in Rhino could provide real isolation, but this is
unfortunately impossible in Nashorn. Nashorn makes an assumption that the
global object during execution of a script is an instance of
jdk.nashorn.internals.objects.Global; in Rhino, it could have been anything so
there I was able to run a module in a new scope that had the actual Global
object as its prototype, so it could catch all variable assignments in itself
and essentially make the Global read only (albeit objects in it mutable - e.g.
a module could still extend Array prototype etc.).
In Nashorn, it's the other way round with "--global-per-engine" - Global object is the
one immediately visible to scripts, and ENGINE_SCOPE Bindings object is used as source of
properties that aren't found in the Global. Here, Global catches variable assignments and
ENGINE_SCOPE Bindings object ends up being immutable (although objects in it are obviously still
mutable, so the module can build up its "exports" object).
Basically, you have a choice between having shared globals (no isolation)
without mirrors (and then you won't run into any issues with mirrors), or
separate globals (with real isolation), but then also mirrors, and then you
might run into limitations of mirrors (e.g. they can't be automatically used as
Runnable etc. callbacks from Java and so on.)
Mirrors also don't seem to work in the way required for a require()
implementation, as described in my previous experiments :(
So... the way it looks right now, I can't see anyway I can create a
working commonJS require() implementation with Nashorn as it stands
currently. This would be a showstopper for having Nashorn support in
Vert.x which is dissappointing as I was impressed by the good performance :(
Having said that, apparently avatar.js does implement require() so I'm
puzzled how it manages that. I did look at the avatar.js code but
couldn't find their require implementation. Does anyone have any pointers?
I'm not trying to justify or otherwise qualify any of the design decisions
here, just trying to help you understand its constraints.
Attila.
