How to convert ScriptEngine.eval to Java object

Question

I'm using ScriptEngine to run a Javascript. In my case, the Javascript will always return a boolean. I thought that eval() would return an Object that I can cast, but instead it seems to return a ScriptObjectMirror. toString() returns [Boolean true], so the evaluation is working. How can I convert or cast to a Boolean or boolean?

Here is my code

final ScriptEngineManager manager = new ScriptEngineManager();
final ScriptEngine engine = manager.getEngineByName("javascript");
final Object jsEval = engine.eval(String.format("new Boolean(%s)", jsString));
System.out.printf("*** jsEval: %s %s%n ", jsEval, jsEval.getClass().getName());

Output is

*** jsEval: [Boolean true] jdk.nashorn.api.scripting.ScriptObjectMirror

Gaël J · Accepted Answer · 2021-08-07 16:01:08Z

1

If you know the type for sure, you can use the to method:

jsEval.to(Boolean.class)

answered Aug 7, 2021 at 16:01

Gaël J

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3 Comments

Peter Kronenberg Over a year ago

Thank you. Love when it's such a simple solution. Obviously, I need to cast to ScriptObjectMirror. Why does eval() return an Object instead of returning ScriptObjectMirror, or at least some interface

Gaël J Over a year ago

No idea. Keep in mind that Nashorn is deprecated since Java-11 and removed since Java15.

Peter Kronenberg Over a year ago

I'm unfortunately still on Java 8 :-(. I assume that there is a replacement? Since I'm using final ScriptEngine engine = manager.getEngineByName("javascript");, I assume it will use whatever the current recommended engine is

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