Example of how to call functions from a JavaScript snippet running inside the Java8 Nashorn engine

exampleScript.js

function testMe() {
    ui.showMessage('this should show up in the UI this is running in');
}

expectedOutput.txt

[main] INFO  a.p.d.blogpost.TestScriptExecutor - trying to run: testMe
[main] INFO  a.p.d.blogpost.TestScriptExecutor - showMessage called: this should show up in the UI this is running in

TestScriptExecutor.java

package at.paukl.dynclient.blogpost;

import jdk.nashorn.api.scripting.NashornScriptEngine;
import org.apache.commons.io.IOUtils;
import org.slf4j.Logger;

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

import java.io.FileReader;

import static org.slf4j.LoggerFactory.getLogger;

/**
 * @author Paul Klingelhuber
 */
public class TestScriptExecutor {

    private static final Logger LOG = getLogger(TestScriptExecutor.class);

    private NashornScriptEngine engine;

    public static void main(String[] args) throws Exception {
        final TestScriptExecutor test = new TestScriptExecutor();
        test.loadScript(IOUtils.toString(new FileReader("exampleScript.js")));
        test.invokeScriptFunction("testMe");
    }

    public TestScriptExecutor() {
        ScriptEngineManager engineManager = new ScriptEngineManager();
        engine = (NashornScriptEngine) engineManager.getEngineByName("nashorn");
    }

    public void loadScript(String script) {
        Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
        bindings.put("ui", this);
        try {
            engine.compile(script).eval(bindings);
        } catch (ScriptException e) {
            LOG.warn("scripterror: ", e);
        }
    }

    public void invokeScriptFunction(String functionName) {
        try {
            LOG.info("trying to run: {}", functionName);
            engine.invokeFunction(functionName);
        } catch (ScriptException e) {
            LOG.warn("script error!");
        } catch (NoSuchMethodException e) {
            LOG.info("method not defined");
        }
    }

    public void showMessage(String message) {
        LOG.info("showMessage called: {}", message);
    }

}