Possible scope leak in Nashorn

[A. Sundararajan]

[Our emails crossed..]

Yes, script objects returned from ScriptEngine's eval methods are always 
wrapped as ScriptObjectMirrors.

But if you pass a script object to a java method that accepts Object 
type, then it is passed "as is". i.e., no wrapping.  But if your method 
signature uses  JSObject or ScriptObjectMirror, then you get a wrapped 
instance. This is to avoid wrap check for every "Object" accepting Java 
method call.

When you create callback interface implemenations, the interface 
implementation sets right nashorn scope at the entry of method calls.

-Sundar

On Tuesday 10 December 2013 06:49 PM, Tim Fox wrote:
> I did a bit more digging here and inspected (in Java) the classes of 
> objects passed from JavaScript into Java in the following two cases:
>
> 1) Objects explicitly passed from JavaScript to Java, e.g. in a script:
>
> someJavaObject.callJavaMethod(..);
>
> 2) Objects passed to Java from JavaScript by returning them from the 
> execution of a script, i.e. the last statement of a script which is 
> returned to Java via the return value of engine.eval(...)
>
> It seems that objects passed to Java code via 1) do not implement 
> jdk.nashorn.api.scripting.ScriptObjectMirror but objects passed in 2) 
> do implement jdk.nashorn.api.scripting.ScriptObjectMirror
>
> So, it seems that I can workaround the issue by not explicitly passing 
> objects from JS to Java but returning them from the execution of the 
> script.
>
> I have to say that the current behaviour seems a bit inconsistent to 
> me, shouldn't all objects passed to Java from JS be wrapped in 
> ScriptObjectMirror to ensure the correct scope is used whenever they 
> are executed? I'm not sure why some objects passed to Java are wrapped 
> and others aren't...
>
> I'm also concerned about the case of callbacks. Let's say I have a 
> Java object which allows a handler to be set:
>
> class MyObject {
>
>   private Runnable handler;
>
>   void setHandler(Runnable handler) {
>      this.handler = handler;
>   }
>
>   void callHandler() {
>      handler.run();
>   }
>
> }
>
> And I call an instance of this object from inside a JS script:
>
>
> myJavaObject.setHandler(function() {
>    // Will this JS function be called with the correct scope?
> });
>
> The object that gets passed to setHandler does not implement 
> ScriptObjectMirror, so when the JS handler function is called, can we 
> be sure it wil be executed in the correct scope?
>
>
>
>
>
>
> On 10/12/13 12:41, Tim Fox wrote:
>> Hi Sundar,
>>
>> I see you resolved this issue as a non issue here 
>> https://bugs.openjdk.java.net/browse/JDK-8029604
>>
>> Unfortunately I am unable to add comments to the bug so I have a few 
>> questions...
>>
>> >> In either mode, Java level Bindings instance backed Java objects 
>> as script variables are "read only"
>>
>> Can you elaborate what this means? Do you mean the Java object 
>> _reference_ is read only? I.e. JavaScript can't replace the global 
>> variable with anything else. I imagine ensuring that the Java object 
>> itself is read-only would be an extremely difficult thing to do.
>>
>> >> You can get ScriptObjectMirror wrapper of ScriptFunctions and 
>> ScriptObjects and share/call/use it from anywhere (ScriptObjectMirror 
>> takes care of setting right nashorn Global object in thread local 
>> storage).
>>
>> It seems like this is what we need to do to implement CommonJS 
>> require-like functionality as we need to export objects from scopes 
>> such that they are callable from different scopes. This is essential 
>> for implementing any "node-like" functionality using Nashorn.
>>
>> I can't see anything in the javax.script API that mentions 
>> ScriptObjectMirror, ScriptFunctions or ScriptObjects, so I assume 
>> these are in a different API. Is this API documented anywhere, and is 
>> it a public API? If you could give a simple code example of how my 
>> simple test program could be adapted using this other API, that would 
>> be a great help :)
>>
>> I guess this also implies that what I want to do is impossible using 
>> the javax.script API?
>>
>> On 05/12/13 13:55, A. Sundararajan wrote:
>>> Hi,
>>>
>>> Please use "tools" category and include scripting/nashorn in your 
>>> description or title or label somewhere.
>>>
>>> PS. we are working on getting appropriate category.
>>>
>>> Thanks
>>> Sundar
>>>
>>> On Thursday 05 December 2013 06:43 PM, Tim Fox wrote:
>>>> Thanks I am trying to submit a bug here:
>>>>
>>>> http://bugreport.sun.com/bugreport/submit_intro.do
>>>>
>>>> as directed in the link your provided, but I can't see any category 
>>>> I can file it against that seems appropriate for Nashorn...
>>>>
>>>> On 05/12/13 13:00, A. Sundararajan wrote:
>>>>> Please file a bug using web interface. Process explained in this 
>>>>> post by Jim:
>>>>>
>>>>> http://mail.openjdk.java.net/pipermail/nashorn-dev/2013-December/002515.html 
>>>>>
>>>>>
>>>>> Thanks
>>>>> -Sundar
>>>>>
>>>>> On Thursday 05 December 2013 06:14 PM, Tim Fox wrote:
>>>>>> Oops! Forgot to provide a link to the example...
>>>>>>
>>>>>> Here it is:
>>>>>>
>>>>>> https://gist.github.com/purplefox/7804105
>>>>>>
>>>>>> On 05/12/13 12:42, Tim Fox wrote:
>>>>>>> Hello Nashorn folks,
>>>>>>>
>>>>>>> I've been playing around with Nashorn today, and in particular 
>>>>>>> I've been trying to implement commonJS-like require() 
>>>>>>> functionality, but having some problems. Most probably I am 
>>>>>>> doing something stupid...
>>>>>>>
>>>>>>> I'm sure you all know, with commonJS modules you can "require" 
>>>>>>> them from your JS scripts, e.g.
>>>>>>>
>>>>>>> var someObject = require("modulename");
>>>>>>>
>>>>>>> Then in modulename.js
>>>>>>>
>>>>>>> you do your stuff then export the object that ends up being 
>>>>>>> returned from the require, e.g.
>>>>>>>
>>>>>>> var someObject = {
>>>>>>>   foo: "bar"
>>>>>>> }
>>>>>>>
>>>>>>> module.exports = someObject;
>>>>>>>
>>>>>>> One key thing is that modules have their own scope, so any 
>>>>>>> globals defined in modulename.js shouldn't be visible in the 
>>>>>>> script that requires them, and vice versa.
>>>>>>>
>>>>>>> While trying to implement this using Nashorn and running scripts 
>>>>>>> in different scopes I've found it's possible for Nashorn to get 
>>>>>>> confused about what scope it's using, and for scopes to leak 
>>>>>>> from one to another.
>>>>>>>
>>>>>>> I created this simple runnable example to demonstrate the issue.
>>>>>>>
>>>>>>> I wonder if anyone can advise whether this is a bug, or perhaps 
>>>>>>> I am using the API incorrectly. If the latter, could you advise 
>>>>>>> on the proper way to use the API to implement this kind of 
>>>>>>> functionality.
>>>>>>>
>>>>>>> Many thanks
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>