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`@ModelAttribute`

You can use the @ModelAttribute annotation on a method argument to access an attribute from the model or have it be instantiated if not present. The model attribute is also overlain with values from HTTP Servlet request parameters whose names match to field names. This is referred to as data binding, and it saves you from having to deal with parsing and converting individual query parameters and form fields. The following example shows how to do so:

Java
Kotlin

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute Pet pet) { (1)
	// method logic...
}

1	Bind an instance of `Pet`.

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
fun processSubmit(@ModelAttribute pet: Pet): String { (1)
	// method logic...
}

1	Bind an instance of `Pet`.

The Pet instance above is sourced in one of the following ways:

Retrieved from the model where it may have been added by a @ModelAttribute method.
Retrieved from the HTTP session if the model attribute was listed in the class-level @SessionAttributes annotation.
Obtained through a Converter where the model attribute name matches the name of a request value such as a path variable or a request parameter (see next example).
Instantiated using its default constructor.
Instantiated through a “primary constructor” with arguments that match to Servlet request parameters. Argument names are determined through JavaBeans @ConstructorProperties or through runtime-retained parameter names in the bytecode.

One alternative to using a @ModelAttribute method to supply it or relying on the framework to create the model attribute, is to have a Converter<String, T> to provide the instance. This is applied when the model attribute name matches to the name of a request value such as a path variable or a request parameter, and there is a Converter from String to the model attribute type. In the following example, the model attribute name is account which matches the URI path variable account, and there is a registered Converter<String, Account> which could load the Account from a data store:

Java
Kotlin

@PutMapping("/accounts/{account}")
public String save(@ModelAttribute("account") Account account) { (1)
	// ...
}

1	Bind an instance of `Account` using an explicit attribute name.

@PutMapping("/accounts/{account}")
fun save(@ModelAttribute("account") account: Account): String { (1)
	// ...
}

1	Bind an instance of `Account` using an explicit attribute name.

After the model attribute instance is obtained, data binding is applied. The WebDataBinder class matches Servlet request parameter names (query parameters and form fields) to field names on the target Object. Matching fields are populated after type conversion is applied, where necessary. For more on data binding (and validation), see Validation. For more on customizing data binding, see DataBinder.

Data binding can result in errors. By default, a BindException is raised. However, to check for such errors in the controller method, you can add a BindingResult argument immediately next to the @ModelAttribute, as the following example shows:

Java
Kotlin

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) { (1)
	if (result.hasErrors()) {
		return "petForm";
	}
	// ...
}

1	Adding a `BindingResult` next to the `@ModelAttribute`.

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
fun processSubmit(@ModelAttribute("pet") pet: Pet, result: BindingResult): String { (1)
	if (result.hasErrors()) {
		return "petForm"
	}
	// ...
}

1	Adding a `BindingResult` next to the `@ModelAttribute`.

In some cases, you may want access to a model attribute without data binding. For such cases, you can inject the Model into the controller and access it directly or, alternatively, set @ModelAttribute(binding=false), as the following example shows:

Java
Kotlin

@ModelAttribute
public AccountForm setUpForm() {
	return new AccountForm();
}

@ModelAttribute
public Account findAccount(@PathVariable String accountId) {
	return accountRepository.findOne(accountId);
}

@PostMapping("update")
public String update(@Valid AccountForm form, BindingResult result,
		@ModelAttribute(binding=false) Account account) { (1)
	// ...
}

1	Setting `@ModelAttribute(binding=false)`.

@ModelAttribute
fun setUpForm(): AccountForm {
	return AccountForm()
}

@ModelAttribute
fun findAccount(@PathVariable accountId: String): Account {
	return accountRepository.findOne(accountId)
}

@PostMapping("update")
fun update(@Valid form: AccountForm, result: BindingResult,
		   @ModelAttribute(binding = false) account: Account): String { (1)
	// ...
}

1	Setting `@ModelAttribute(binding=false)`.

You can automatically apply validation after data binding by adding the jakarta.validation.Valid annotation or Spring’s @Validated annotation (Bean Validation and Spring validation). The following example shows how to do so:

Java
Kotlin

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@Valid @ModelAttribute("pet") Pet pet, BindingResult result) { (1)
	if (result.hasErrors()) {
		return "petForm";
	}
	// ...
}

1	Validate the `Pet` instance.

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
fun processSubmit(@Valid @ModelAttribute("pet") pet: Pet, result: BindingResult): String { (1)
	if (result.hasErrors()) {
		return "petForm"
	}
	// ...
}

1	Validate the `Pet` instance.

Note that using @ModelAttribute is optional (for example, to set its attributes). By default, any argument that is not a simple value type (as determined by BeanUtils#isSimpleProperty) and is not resolved by any other argument resolver is treated as if it were annotated with @ModelAttribute.

When compiling to a native image with GraalVM, the implicit @ModelAttribute support described above does not allow proper ahead-of-time inference of related data binding reflection hints. As a consequence, it is recommended to explicitly annotate method parameters with @ModelAttribute for use in a GraalVM native image.