How to throw ServletException on missing a required field

[janning]

We have a SimpleFormController and a formObject which requires a field "id". If the "id" is not given, a MissingServletRequestParameterException should be thrown.

What is the correct workflow?

We did it like this:

protected void onBindOnNewForm ( HttpServletRequest request, Object command ) throws Exception {
	CreateForm form = (CreateForm) command;
	if (form.getId() == null) {
	   throw new MissingServletRequestParameterException("id", "Long");
	}
}

Another approach would be to override initBinder() like this:

@Override
public void initBinder ( HttpServletRequest request, ServletRequestDataBinder binder ) throws Exception
{
	binder.setRequiredFields(new String[] {"id"});
}

But this way it won't throw an Exception, so we looked at DefaultBindingErrorProcessor to search for a way to throw an exception if a required field is not given.

we are a little confused right now, how to choose the "correct" workflow for this scenario, which seems to me very common.

any suggestions? Your help is very appreciated.

kind regards
Janning

[Marten Deinum]

Why do you want to throw an exception. The initBinder method works fine, however as you noticed it won't throw an exception. It will generate an error message in the binder result. This message can be displayed on your screen.

Check the sample about spring mvc and validation also check the samples they should give you an idea.

[janning]

Why do you want to throw an exception. The initBinder method works fine, however as you noticed it won't throw an exception. It will generate an error message in the binder result. This message can be displayed on your screen.

The Controller and View does not function if an id is not selected. Showing the form view doesn't make sense because i can' get the object which id i need.

Think of "adding a new address for a customer". I click on a link like addNewAddress?customerId=1

the controller prepares referencedata to let the view render a nice screen.

Customer Name:John doe
Add new Address [ ]

I need the id of the customer in NewAddressForm and i need the id to collect referenceData. The view won't work without it and it is an exception beacuse the user shoul duse the correct link. If the user enter the url directly without the id i would like to throws an exception and let an ExceptionHandler handle it.

I hope this example was comprehensible why i need to throw an Exception.
i guess this is what MissingServletRequestParameterException is for, isn't it?

[Marten Deinum]

Well if you need the id then why not simply make sure it is there in the formBackingObject method. You might want to take a look at ServletRequestUtils

protected Object formBackingObject(HttpServletRequest request) throws Exception {
    NewAddressForm form = new NewAddressForm();
    final long customerId = ServletRequestUtils.getRequiredLong(request, "customerId");
    form.setCustomerId(customerId);
    return form;
}

[janning]

Well if you need the id then why not simply make sure it is there in the formBackingObject method.

That's exactly what i asked myself :-)

if i just need an id it seems ok. What if i need something more complex. Wouldn't it make sense to let it do the binder and throw the exception AFTER binding? it WORKS in formBackingObject but is it the place to be? Isn't it something that smells like binding?

it would be fine if i can say (no real code):

initBinder () {
  boolean throwExceptionIdMissing = true;
  binder.setRequiredFields("id", throwExceptionIdMissing)
}

But i am fine with checking in formBackingObject if this is the place to be. Sometimes i think it's rather difficult to understand where to place what in SimpleFormController.

thanks for the clarification

[Marten Deinum]

Well you stated yourself

The Controller and View does not function if an id is not selected. Showing the form view doesn't make sense because i can' get the object which id i need.

So why let the binder handle it? Binding occurs after posting the form (ok you can set the bindOnNewForm to true). If you set the required field on the binder then it will generate an errorcode ('required.id') stating that the id is missing. The form is still being displayed, referenceData is being called.

I would check it in the formbackingobject (and make my object a sessionForm) if not available the getRequiredLong will throw an exception, stopping everything from continuing.