ScriptObjectMirrors

[A. Sundararajan]

There is no notion of wrap/unwrap of user implemented JSObject - so 
those won't be affected. JSObject itself is just another Java type and 
so would be between script and Java code "as is".

Only for Object param types of Java methods will receive wrapped 
ScriptObjects - argument filter will be inserted for such parameters. 
Actually a filter is already installed for such params to handle 
ConsString objects - so ScriptObject wrapping is just another 
incremental check+wrap in that filter. And Object returning Java methods 
will have unwrapper filter on return value (on the script side). Other 
Java parameter types/return type should not be affected at all. 
Providing another command line option implies we keep checking it in 
argument/return type filter methods - which would be costly.

-Sundar

On Friday 17 October 2014 09:23 AM, Serguei Mourachov wrote:
> Sundar
>
> Will it also affect classes implementing JSObject?
> IMO, we should have an option to disable this wrap/unwrap behavior in 
> cases when it significantly affecting performance.
>
> SM
>
> On 10/16/2014 6:44 AM, A. Sundararajan wrote:
>> There were many questions in this list and elsewhere on 
>> ScriptObjectMirror. This email is to clarify those.
>>
>> Nashorn represents JavaScript objects as instances of implementation 
>> class called jdk.nashorn.internal.runtime.ScriptObject or one of it's 
>> subclasses (like NativeArray, NativeRegExp etc. - or even generated 
>> ones like jdk.nashorn.internal.scripts.JO4 etc)
>>
>> When ScriptObjects are returned from a script function or evaluated 
>> script code, ScriptEngine.eval returns an instanceof 
>> jdk.nashorn.api.scripting.ScriptObjectMirror.
>>
>> http://cr.openjdk.java.net/~sundar/jdk.nashorn.api/9/javadoc/jdk/nashorn/api/scripting/ScriptObjectMirror.html 
>>
>>
>> Example:
>>
>>     ScriptEngine e = new 
>> ScriptEngineManager().getEngineByName("nashorn");
>>     Object obj = e.eval("var obj = { foo: 23 }"); // obj is an 
>> instance of ScriptObjectMirror
>>
>> Caller can cast the result to ScriptObjectMirror to access properties 
>> of that script object or call methods on it. All javax.script 
>> interface methods returning Object (ScriptEngine.eval, 
>> Invocable.invokeFunction, Invocable.invokeMethod) return 
>> ScriptObjectMirror if underlying script or script function/method 
>> returns a JS object.
>>
>> But, if you call any Java method accepting Object type param or 
>> assign to element of Object[], then Nashorn was not wrapping 
>> ScriptObject in the past. i.e., 'raw' ScriptObject (or subclass) 
>> instances "escaped" to Java layer. If you try to cast those to 
>> ScriptObjectMirror from Java code, you got ClassCastException. Also, 
>> if you passed such raw object as "self" parameter for 
>> Invocable.invokeMethod, you would  IllegalArgumentException. This was 
>> causing a lot of confusion - script objects got to java code 
>> sometimes as wrapped mirror objects and sometimes as 'raw' objects!
>>
>> With a recent change
>>
>>     http://hg.openjdk.java.net/jdk9/dev/nashorn/rev/a8d44c7c2ac0
>>
>> in jdk9 and the corresponding backport to jdk8u-dev
>>
>> http://hg.openjdk.java.net/jdk8u/jdk8u-dev/nashorn/rev/a35c8136c045
>>
>> the way nashorn wraps internal ScriptObjects to ScriptObjectMirror 
>> has changed. Script objects are always wrapped to ScriptObjectMirror 
>> - even when you're calling Java method that accepts "Object" type 
>> value. Also, return values from java methods returning Object are 
>> "unwrapped" (if the return value is a ScriptObjectMirror) when it 
>> gets to script execution.
>>
>> Example:
>>
>>     // list gets ScriptObjectMirror as element
>>     engine.eval("var m = new java.util.HashMap(); l.put('myobj', { 
>> foo: 33 });");
>>     engine.eval("var obj = m.get('myobj'); // obj gets unwrapped as 
>> ScriptObject here");
>>
>> With this change, raw ScriptObjects don't escape to Java layer at all.
>>
>> Hope this helps,
>> -Sundar
>