Adding multiple validators using initBinder

You need to set the value of the @InitBinder annotation to the name of the command you want it to validate. This tells Spring what to apply the binder to; without it, Spring will try to apply it to everything. This is why you’re seeing that exception: Spring is trying to apply the binder – with your UserValidator – to a parameter of type CustomerPayment.

In your specific case, it looks like you need something like:

@InitBinder("user")
protected void initBinder(WebDataBinder binder) {
    binder.setValidator(new UserValidator());
}

To your second question, as Rigg802 explained, Spring does not support attaching multiple validators to a single command. You can, however, define multiple @InitBinder methods for different commands. So, for example, you could put the following in a single controller and validate your user and payment parameters:

@InitBinder("user")
protected void initUserBinder(WebDataBinder binder) {
    binder.setValidator(new UserValidator());
}

@InitBinder("customerPayment")
protected void initPaymentBinder(WebDataBinder binder) {
    binder.setValidator(new CustomerPaymentValidator());
}