Keeping ScriptEngine per Thread?

[Sundararajan Athijegannathan]

One engine per worker thread would definitely work - this is maximum 
isolation. This mode even avoids sharing script classers / loaders.

If that is not workable in your use-case, you can use a separate 
ENGINE_SCOPE bindings (or separate ScriptContext) per thread. This will 
separate ECMAScript globals to be thread specific. This mode allows for 
script Class (compiled/loaded script Class objects) to be shared -- but 
global state is not. This is partial sharing - but safe.

If you do share the default ScriptContext of the script engine across 
threads, then it is up to you to take care of MT access/modification of 
globals -- you could still carefully structure code to run script 
functions in a such a way to avoid concurrent global modifications - but 
it'd be practically difficult.

Hope this helps,
-Sundar

On 12/9/2015 2:58 PM, Frantzius, Jörg wrote:
> Hi,
>
> we’re using Oracle JDK’s Nashorn in a web application, where it is used to render HTML markup in HTTP worker threads.
>
> I remember having read somewhere that the creation of a ScriptEngine (i.e. invocation of ScriptEngineFactory.getScriptEngine()) may have considerable performance overhead, so probably it should be avoided to do this with every incoming HTTP request. This is why we’ll go for keeping a ScriptEngine per thread in a ThreadLocal.
>
> On the other hand this may result in a large memory footprint, as there can be 200 HTTP worker threads, but we don’t have any experience with it yet.
>
> The ScriptEngineFactory.getParameter() Javadocs<http://docs.oracle.com/javase/8/docs/jdk/api/nashorn/jdk/nashorn/api/scripting/NashornScriptEngineFactory.html#getParameter-java.lang.String-> does list some interesting parameters for the key „THREADING“, that seem to make it possible to have a singleton ScriptEngine instead, which could be used concurrently from multiple threads. However, I have no clue whether this is either supported by Nashorn, nor how to set this parameter on engine creation.
>
> Is there per chance some recommended usage pattern for ScriptEngine in a web server?
>
> Thanks for any insights,
> Jörg
>
>
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