Yes you can create your own javascript engine in Java. This is a cool feature I never knew existed. I don’t know how long it’s been around, I’m guessing Java 6.

Basically, you can do something like this:

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

public class JavaJS {

  public static void main(String[] args) throws ScriptException {
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("JavaScript");
    String script = 
        "function sayHello(name) {"
      + "  print('Hello ' + name);"
      + "}"
      + "sayHello('tom');";
    engine.eval(script);
  }

}

And it will print Hello tom.

You can even mix and match Java and Javascript, here’s how to have Java calling the Javascript function above:

Invocable invocable = (Invocable) engine;
invocable.invokeFunction("sayHello", "Jim");

Cool eh? Read more about this on the Oracle’s Java Scripting Programmer Guide or google JSR-223.