I want to add a maxLength to the custom value on the ComboBox but when I do
getElement().setAttribute("maxLength","30");
Vaadin add it to the wrapping vaadin-combo-box element. How can I access from Java the underlying highlighted input element as show in the picture?
Applying a max-length on the client-side does not guarantee that the value on the server-side is secured.
Instead of mingling with the inner workings of the ComboBox's field, I would suggest to apply this validation on the server-side. In the ComboBox::addCustomValueSetListener or by using a StringLengthValidator on the Binder you have attached to the form/field.
Vaadin ComboBox doesn't support setting a max length to custom values (although TextField does).
You could create a feature request ticket about it here: https://github.com/vaadin/web-components/issues/new?assignees=&labels=&template=feature-request.yml
I found out that you can executejs on client side through element API. I solved using this snippet:
element.executeJs("this.childNodes.item(0).maxLength = 30")
"childNodes.item(0)" is the underlying input element of combobox