Spring MVC 如何从 Model 获取数据，为什么每次提交表单后都会创建 modelAttribute object？

Question

Everytime I submit form with errors processForm() method is called, errors are checked, BindingResult hasErrors() is true and I return customer-form which have error message next to wrong inputs, but before all that new Customer is always created and setters are called. Why Spring creating new Customer object everytime I submit form and how long Spring keep customer data? Also, why my form inputs become empty if I mark url in browser and hit enter?

I have form:

<form:form action="processForm" modelAttribute="customer">
    First name: <form:input path="firstName" />
    <form:errors path="firstName" cssClass="error" />
    <br><br>
    Last name (*): <form:input path="lastName" />
    <form:errors path="lastName" cssClass="error" />
    <br><br>
    <input type="submit" value="Submit" />
</form:form>

and Controllers:

    @RequestMapping("/showForm")
    public String showForm(Model theModel) {
        theModel.addAttribute("customer", new Customer());
        return "customer-form";
    }

    @RequestMapping("/processForm")
    public String processForm(
            @ModelAttribute("customer") @Valid Customer theCustomer,
            BindingResult theBindingResult) {
       if (theBindingResult.hasErrors()) return "customer-form";
        return "customer-confirmation";
    }

Answer 1

Why Spring creating new Customer object everytime I submit form and how long Spring keep customer data?

Generally speaking, Spring is not creating new Customer object every time you submit the form. It is binding form data to the model object that you sent from the GET handler:

theModel.addAttribute("customer", new Customer());

And in the POST handler, the same object is being resolved with

@ModelAttribute("customer") @Valid Customer theCustomer,

See how this resolution is done with @ModelAttribute in Spring Framework Documentation.

The data will normally live through the requests if not configured otherwise, for example with @SessionAttributes .

Also, why my form inputs become empty if I mark url in browser and hit enter?

This is a new GET request, ie, it's all the same as the first time you enter the URL and press enter!