Java 6 includes built-in support for JSR 223: Scripting for the Java Platform API classes. This framework can be used to host Script Engines in Java Applications. Numerous Scripting engines are available.

Here is how you can use JSR 223 to talk to Groovy from Java:

// require(url:'https://scripting.dev.java.net', jar:'groovy-engine.jar')
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

public class CalcMain {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager factory = new ScriptEngineManager();
        ScriptEngine engine = factory.getEngineByName("groovy");

        // basic example
        System.out.println(engine.eval("(1..10).sum()"));

        // example showing scripting variables
        engine.put("first", "HELLO");
        engine.put("second", "world");
        System.out.println(engine.eval("first.toLowerCase() + second.toUpperCase()"));
    }
}

The output is:

55
helloWORLD

To make this example work, if you're running Groovy 1.5.x, add groovy-engine.jar from https://scripting.dev.java.net/ to your CLASSPATH and run this program using Java 6. But since Groovy 1.6, Groovy already contains the groovy scripting engine compliant with JSR-223.

This next example illustrates calling an invokable function:

// require(url:'https://scripting.dev.java.net', jar:'groovy-engine.jar')
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.Invocable;

public class FactMain {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager factory = new ScriptEngineManager();
        ScriptEngine engine = factory.getEngineByName("groovy");
        String fact = "def factorial(n) { n == 1 ? 1 : n * factorial(n - 1) }";
        engine.eval(fact);
        Invocable inv = (Invocable) engine;
        Object[] params = { new Integer(5) };
        Object result = inv.invokeFunction("factorial", params);
        System.out.println(result);
    }
}

And when run, results in:

120

Groovy has many mechanisms for integration with Java, some of which provider richer options than available with JSR 223 (e.g. greater configurability and more security control). JSR 223 is recommended when you need to keep the choice of language used flexible and you don't require integration mechanisms not supported by JSR-223.

The engine keeps per default hard references to the script functions. To change this you should set a engine level scoped attribute to the script context of the name "#jsr223.groovy.engine.keep.globals" with a String being "phantom" to use phantom references, "weak" to use weak references or "soft" to use soft references - casing is ignored. Any other string will cause the use of hard references.

See also: JSR-223 access to other JVM languages