JPA Expression concatenate more than two columns

You can basically wrap the concat(...) into each other, or use a method like the following (assuming you want to use the same delimiter string between columns):

private CriteriaBuilder criteriaBuilder = /* ... */

// notice the three dots before "expressions", they are no decoration ;-)
private Expression<String> concat(String delimiter, Expression<String> ... expressions) {
    Expression<String> result = null;
    for (int i = 0; i < expressions.length; i++) {
        final boolean first = i == 0, last = i == (expressions.length - 1);
        final Expression<String> expression = expressions[i];
        if (first && last) {
            result = expression;
        } else if (first) {
            result = criteriaBuilder.concat(expression, delimiter);
        } else {
            result = criteriaBuilder.concat(result, expression);
            if (!last) {
                result = criteriaBuilder.concat(result, delimiter);
            }
        }
    }
    return result;
}

Expression<String> userId = root.get(Employee_.userId);
Expression<String> empName = joinDept.get(Employee_.empName);
Expression<String> userName = hist.get(User_.name); // or whatever

Expression<String> stringConcat = concat(" # ", userId, empName, userName);