Why does getEngineByName(“js”) return null?

Question

Whatever I try, getEngineByName() keeps returning null.

Here's my code:

final ScriptEngineManager manager = new ScriptEngineManager();
final ScriptEngine engine = manager.getEngineByName("js");

But engine is null after these lines.

I also tried:

• getEngineByName("javascript")
• getEngineByName("nashorn")

They all return null. Actually, manager.getEngineFactories() shows an empty array - meaning there are no Factories at all.

These 2 answers suggest passing null to the constructor, but it didn't work for me:

• https://stackoverflow.com/a/25332851/900394
• https://stackoverflow.com/a/25332851/900394

And this answer says it's a bug that has been fixed.

---

Update:

That was an Android Application Project in eclipse.

I didn't know it differs that much from a Java Project.

Now I just opened a new Java Project, wrote these lines, and I'm getting some results:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.script.SimpleBindings;

public class TestClass {

    public static void main(String[] args) throws ScriptException {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = scriptEngineManager.getEngineByName("js");
    }
}

engine isn't null !

Also, javax.script.* was imported successfully as if it's already there (built in). Is this because I'm using Java 8?

In that previous (android) project I was using javax.script downloaded from here.

So what would be the problem in the Android Project and how do I solve it?

Answer 1

Apparently when running in an IDE even if you are on a recent version of Java that includes the Nashorn javascript engine you have to pass null into the ScriptEngineManager constructor or else it often won't have engines. It'd be fine when actually compiled, seems to be an IDE error. For me it was resolved by changing this code:

mgr = new ScriptEngineManager();
engine = mgr.getEngineByName("JavaScript");

to this code:

mgr = new ScriptEngineManager(null);
engine = mgr.getEngineByName("JavaScript");

Note the null passed into the ScriptEngineManager constructor call.

Answer 2

You have to write the argument with Capital Letters, as shown below:

getEngineByName("JavaScript");
                 ^   ^

Answer 3

Your version of Java does not include a JavaScript engine. Java 8 includes the Nashorn javascript engine and has in general better support for Java <-> JavaScript interoperability. For older versions of Java you can put Rhino on the classpath and use that.