Question

How can I define a validator for a DataTable? For example to force it at least have 1 row to submit?

Answer 1

You could try the following (not tested):

Bind your h:dataTable to a UIComponent in the backing bean:

<h:dataTable value="#{myBean.myList}" var="item" id="myId" binding="#{myBean.myTable}">

In your backing bean define a javax.faces.component.html.HtmlDataTable myTable

In the action method for the submit button:

if (myTable.getRowCount() < 1) {
   FacesMessage facesMsg = new FacesMessage(FacesMessage.SEVERITY_ERROR, "Your short message", "Your detailed message); 
   FacesContext.getCurrentInstance().addMessage(null, facesMsg);
   return null; // reload view
}