The Scripting API

ScriptEngine engine = new ScriptEngineManager().getEngineByName("groovy");

Bindings bindings = engine.createBindings();
bindings.put("amount", 1500);
Object out = engine.eval("amount > 1000 ? 'high' : 'low'", bindings);

Engines are discovered through ServiceLoader, so adding a language is a matter of putting its jar on the classpath. Groovy, JRuby and Jython all ship JSR-223 engines.

Compiling and reusing

if (engine instanceof Compilable c) {
    CompiledScript script = c.compile(source);   // parse once
    script.eval(bindings);                       // run many times
}

Invocable goes further, letting you call a script function directly or get a Java interface implemented by the script.

Nashorn

The JavaScript engine bundled from Java 8 was deprecated in 11 and removed in 15. If you need JavaScript on the JVM now, that is GraalVM’s js engine, which also implements JSR-223.

Where this earns its keep

User-supplied business rules that change faster than your release cycle. Everything evaluated this way is code, so sandbox it and never eval a string built from user input.