Exemplo de execução de função JS com o Nashorn

NashornExample.java

import static java.lang.System.out;

import java.util.logging.Level;
import java.util.logging.Logger;

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

public class NashornExample {

    private static final Logger LOGGER = Logger.getLogger(NashornExample.class.getName());

    private static final ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
    private static final Invocable invocable = (Invocable) engine;

    public static void main(final String[] args) {
        final String[] inputs = {"50kb", "20 Mb", "aa20kb"};
        final String processFunction = processFunction();

        for (final String input : inputs) {
            final String resultado = nashornExecuter(processFunction, "process", input);
            out.println(String.format("Entrada: %s | Resultado: %s", input, resultado));
        }
    }

    public static String nashornExecuter(final String function, final String value, final Object... params) {
        try {
            engine.eval(function);
            return (String) invocable.invokeFunction(value, params);
        } catch (final ScriptException | NoSuchMethodException e) {
            LOGGER.log(Level.SEVERE, e.getMessage(), e);
        }
        return value;
    }

    private static String processFunction() {
        final StringBuilder function = new StringBuilder();
        function.append("function process(val) {");
        function.append("    var isPeso = '(?:k|m|g)b$';");
        function.append("    var normalizado = val.trim().toLowerCase().replace(' ', '');");
        function.append("    var unidade = '';");
        function.append("    var numberPattern = /\\d+(\\.\\d{1,2})?|(Livre)/i;");
        function.append("    var myArray = normalizado.match(isPeso);");
        function.append("    if (myArray != null) {");
        function.append("        unidade = myArray;");
        function.append("        var getNumbers = val.match(numberPattern);");
        function.append("        var i;");
        function.append("        var valores = [];");
        function.append("        for (i = 0; i < getNumbers.length - 1; i++) {");
        function.append("            valores.push(getNumbers[i]);");
        function.append("        }");
        function.append("        var out = '';");
        function.append("        if (normalizado.indexOf('superior') > -1) {");
        function.append("            return valores[0] + unidade + ' ou superior';");
        function.append("        } else if (normalizado.indexOf('até') > -1) {");
        function.append("            if (valores.length == 1) {");
        function.append("                out = 'até ' + valores[0];");
        function.append("            } else {");
        function.append("                out = valores[0] + ' até ' + valores[1];");
        function.append("            }");
        function.append("        } else if (normalizado.indexOf('ou') > -1 || normalizado.indexOf('/') > -1) {");
        function.append("            out = valores[0];");
        function.append("            for (i = 1; i < valores.length - 1; i++) {");
        function.append("                out += valores[i];");
        function.append("            }");
        function.append("        } else if (normalizado.indexOf('*') > -1 || normalizado.indexOf('livre') > -1) {");
        function.append("            out = 'Livre';");
        function.append("        } else {");
        function.append("            if (valores.length > 0) {");
        function.append("                out = valores[0];");
        function.append("            }");
        function.append("        }");
        function.append("        if (out.length == 0 || out == null) {");
        function.append("            return '';");
        function.append("        }");
        function.append("        return out + unidade;");
        function.append("    }");
        function.append("}");
        return function.toString();
    }

}