index.html

<!-- Include the Polyfill -->
<script src="https://www.javapoly.com/javapoly.js"></script>

<script type = "text/java" src="rhino1.7.7.1/lib/rhino-1.7.7.1.jar"></script>

<!-- Write your Java code -->
<script type="text/java">
  package com.demo;
  import com.javapoly.dom.Window;
  import java.lang.String;
  import org.mozilla.javascript.Context;
  import org.mozilla.javascript.Scriptable;
  import org.mozilla.javascript.ScriptableObject;

  public class HelloWorld
  {
    public static void sayHello()
    {
      Window.alert("Hello World, from Java!");

      String x = "";
      String script = "x = \"Hello from JS\";";

      Context cx = Context.enter();
      try {
        Scriptable scope = cx.initStandardObjects();
        Object wrapped = Context.javaToJS(x, scope);
        ScriptableObject.putProperty(scope,"x",wrapped);
        cx.evaluateString(scope, script, "Script", 1, null);
        x = Context.toString(scope.get("x", scope));
      } catch( Exception e) {
        e.printStackTrace();
      } finally {
        Context.exit();
      }

      Window.alert(x);
    }
  }
</script>

<!-- Invoke your Java code from Javascript -->
<script type="text/javascript">
  com.demo.HelloWorld.sayHello();
</script>