Question

Auto-binding of request params seems like default behaviour, but I can’t find a lot of documentation on it. Here is my example:

<form:form modelAttribute="test" action="testsubmit.do" method="POST">
  Name: <form:input path="name" />
  Nested Name: <form:input path="test.nestedName"/> 
<input type="submit"/>

---

public class Test {
    public String name;
    public String name2;
    public TestNested test;
    ...

---

public class TestNested {
    public String nestedName;
    ...

Now with my mapping:

@RequestMapping(value = "/testsubmit")
public String testSubmit(Test test){
    ...

The test object is binding the form values including the nested value. This seems to me like expected behavior, but I am a bit confused by the @ModelAttribute annotation and its use with respect to objects specified as mapped method parameters.

15.3.2.8 Providing a link to data from the model with @ModelAttribute says:

When you place it on a method parameter, @ModelAttribute maps a model attribute to the specific, annotated method parameter (see the processSubmit() method below). This is how the controller gets a reference to the object holding the data entered in the form.

When I bind the object test to the form on load, I set a value to name2.

@RequestMapping(value = "/test")
public String test(Model model) {       
    Test test = new Test();
    test.setName2("test name2");
    model.addAttribute("test", test);
    return "test";
}

This doesn’t get passed through on the submit method when I annotate the test parameter with @ModelAttribute("test"):

@RequestMapping(value = "/testsubmit")
public String testSubmit(@ModelAttribute("test") Test test) {
    ...

This is expected to me as name2 was not specified as a form field/request param, but it doesn’t help me understand the point of the @ModelAttribute("test") usage. Can anyone shed some light on this for me?

Answer 1

@ModelAttribute just lets you specify a different name/key for your object in the model. If you don’t use it, Spring will automatically assign a name based on the object’s class, e.g. “test” in the case of your “Test” class. With @ModelAttribute you could change the name/key of your object in the model to, for example, “whatever” by specifying “@ModelAttribute(“whatever”)”.