Kirill Usov Kirill Usov - 2 months ago 27
Java Question

Java code cannot invoke method from scriptengine with new context

I am trying to implement example invoking method from Javascript in Java.

private static final String JS = "function doit(p) { list.add(p); return true; }";

public static void main(String[] args) throws ScriptException, NoSuchMethodException {
List<String> list = new ArrayList<>();


ScriptEngineManager scriptManager = new ScriptEngineManager();
ScriptEngine engine = scriptManager.getEngineByName("nashorn");

ScriptContext context = new SimpleScriptContext();
context.setBindings(engine.createBindings(), ScriptContext.ENGINE_SCOPE);
Bindings scope = context.getBindings(ScriptContext.ENGINE_SCOPE);

scope.put("list", list);
engine.eval(JS, context);

Invocable invocable = (Invocable) engine;
invocable.invokeFunction("doit", "Hello!!!");

System.out.println(list.size());
}
}


This code throws exception:

Exception in thread "main" java.lang.NoSuchMethodException: No such function doit
at jdk.nashorn.api.scripting.ScriptObjectMirror.callMember(ScriptObjectMirror.java:204)
at jdk.nashorn.api.scripting.NashornScriptEngine.invokeImpl(NashornScriptEngine.java:383)
at jdk.nashorn.api.scripting.NashornScriptEngine.invokeFunction(NashornScriptEngine.java:190)
at testjavascriptinteraction.TestJavaScript.main(TestJavaScript.java:32)


Version



java -version
openjdk version "1.8.0_91"
OpenJDK Runtime Environment (build 1.8.0_91-8u91-b14-3ubuntu1~16.04.1-b14)
OpenJDK 64-Bit Server VM (build 25.91-b14, mixed mode)


What is wrong?

Answer

I can reproduce this problem, and have found a way how to solve it.

I had a hunch that after casting to Invocable, the engines own scripting context is still being used, and not the temporary one you have passed to eval.

I can fix this by calling engine.setContext(...) with this context before casting to Invacable. i.e.:

...
scope.put("list", list);
engine.eval(JS, context); // 'context' now contains the 'doit' function.

engine.setContext(context); // <-- Right here
Invocable invocable = (Invocable) engine; // Now the Invocable will use 'context'.
invocable.invokeFunction("doit", "Hello!!!"); //Runs fine

System.out.println(list.size());

The fact that this works seems to confirm my hunch.