jxls-poi 2.12.0 / JEXLS - Can't get custom functions in expression not resolved

Question

Using jxls, I'm trying to use use a custom processing function in a template with the following code :

Transformer transformer = TransformerFactory.createTransformer(template, outputStream);
JexlExpressionEvaluator evaluator = (JexlExpressionEvaluator) transformer.getTransformationConfig().getExpressionEvaluator();

Map<String, Object> functions = new HashMap<>();
functions.put("functions", new XlsExportUtils());

JexlEngine customJexlEngine = new JexlBuilder().namespaces(functions).create();
evaluator.setJexlEngine(customJexlEngine);
JxlsHelper.getInstance().processTemplate(context, transformer);

My method in the XlsExportUtils class is a simple testing method :

public String test() {
    return "toto";
}

And I'm trying several ways to call it in the template but none of them seem to work :

A dot notation : ${functions.test()}

Or a colon ":" notation as seen in another example on the internet :

${functions:test()}

Both returns :

unsolvable function/method 'test'

Same for a method taking an argument. In this case, the lib tries to resolve the type of argument depending on the expression.

For example, if I hardcode an int argument like : test(1), I get the error

unsolvable function/method 'test(Short)'

If I hardcode a bigger value, the argument in the error message changes to "Long"

If I pass a variable corresponding to an Integer : Ex: ${functions:test(myObject.myInt)}

It wierdly resolves the argument into a Float, returning the error message :

 unsolvable function/method 'test(Float)'

Version : 2.12.0