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Frequently Asked Questions

Graal JavaScript

Can I replace NashornComplateRenderer with GraalComplateRenderer?

The behavior of GraalVM JavaScript and Nashorn are quite different by default and you wil probably need to adjust a few things. See Graal's Migration Guide from Nashorn to GraalVM JavaScript for details.

When creating a GraalComplateRenderer using the default settings host access is allowed for ALL and host class lookup, experimental options, and the option js.experimental-foreign-object-prototype are enabled. If you have been using NashornComplateRenderer before then you probably want to enable js.nashorn-compat as well, for example

    GraalComplateRenderer.of(source)
        .withAdditionalContextCustomizations(ctx ->
            ctx.option("js.nashorn-compat", "true"))
        .build();

Even after enabling full compatibility a few differences remain. Things that are known to have changed:

  • Java.from in Graal will only translate Java Lists to JavaScript arrays while it used to work for any type of collection with Nashorn.

  • If your view wants to read entries from a Java Map the mapName.key syntax doesn't work with Graal, you have to use mapName.get("key") instead. Alternatively you can work around this by wrapping the Map instance with a org.graalvm.polyglot.proxy.ProxyObject.

Is GraalComplateRenderer thread-safe?

No, the GraalVM JavaScript engine relies on a model where JavaScript runtimes cannot be shared between different threads. For more details see https://medium.com/graalvm/multi-threaded-java-javascript-language-interoperability-in-graalvm-2f19c1f9c37b

When using GraalComplateRenderer in a multi-threaded environment (e.g. using complate-spring-mvc) you may encounter exceptions with a message like

Multi threaded access requested by thread Thread[...] but is not allowed for language(s) js.

You must wrap GraalComplateRenderer with ComplateThreadLocalRenderer when a renderer could be used by multiple threads.