`@InitBinder`

See equivalent in the Reactive stack

@Controller or @ControllerAdvice classes can have @InitBinder methods to initialize WebDataBinder instances that in turn can:

Bind request parameters to a model object.
Convert request values from string to object property types.
Format model object properties as strings when rendering HTML forms.

In an @Controller, DataBinder customizations apply locally within the controller, or even to a specific model attribute referenced by name through the annotation. In an @ControllerAdvice customizations can apply to all or a subset of controllers.

You can register PropertyEditor, Converter, and Formatter components in the DataBinder for type conversion. Alternatively, you can use the MVC config to register Converter and Formatter components in a globally shared FormattingConversionService.

@InitBinder methods can have many of the same arguments that @RequestMapping methods have, with the notable exception of @ModelAttribute. Typically, such methods have a WebDataBinder argument (for registrations) and a void return value, for example:

Java
Kotlin

@Controller
public class FormController {

	@InitBinder (1)
	public void initBinder(WebDataBinder binder) {
		SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
		dateFormat.setLenient(false);
		binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
	}

	// ...
}

1	Defining an `@InitBinder` method.

@Controller
class FormController {

	@InitBinder (1)
	fun initBinder(binder: WebDataBinder) {
		val dateFormat = SimpleDateFormat("yyyy-MM-dd")
		dateFormat.isLenient = false
		binder.registerCustomEditor(Date::class.java, CustomDateEditor(dateFormat, false))
	}

	// ...
}

1	Defining an `@InitBinder` method.

Alternatively, when you use a Formatter-based setup through a shared FormattingConversionService, you can re-use the same approach and register controller-specific Formatter implementations, as the following example shows:

Java
Kotlin

@Controller
public class FormController {

	@InitBinder (1)
	protected void initBinder(WebDataBinder binder) {
		binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd"));
	}

	// ...
}

1	Defining an `@InitBinder` method on a custom formatter.

@Controller
class FormController {

	@InitBinder (1)
	protected fun initBinder(binder: WebDataBinder) {
		binder.addCustomFormatter(DateFormatter("yyyy-MM-dd"))
	}

	// ...
}

1	Defining an `@InitBinder` method on a custom formatter.

Model Design

See equivalent in the Reactive stack

Data binding for web requests involves binding request parameters to a model object. By default, request parameters can be bound to any public property of the model object, which means malicious clients can provide extra values for properties that exist in the model object graph, but are not expected to be set. This is why model object design requires careful consideration.

The model object, and its nested object graph is also sometimes referred to as a command object, form-backing object, or POJO (Plain Old Java Object).

A good practice is to use a dedicated model object rather than exposing your domain model such as JPA or Hibernate entities for web data binding. For example, on a form to change an email address, create a ChangeEmailForm model object that declares only the properties required for the input:

public class ChangeEmailForm {

	private String oldEmailAddress;
	private String newEmailAddress;

	public void setOldEmailAddress(String oldEmailAddress) {
		this.oldEmailAddress = oldEmailAddress;
	}

	public String getOldEmailAddress() {
		return this.oldEmailAddress;
	}

	public void setNewEmailAddress(String newEmailAddress) {
		this.newEmailAddress = newEmailAddress;
	}

	public String getNewEmailAddress() {
		return this.newEmailAddress;
	}

}

Another good practice is to apply constructor binding, which uses only the request parameters it needs for constructor arguments, and any other input is ignored. This is in contrast to property binding which by default binds every request parameter for which there is a matching property.

If neither a dedicated model object nor constructor binding is sufficient, and you must use property binding, we strongly recommend registering allowedFields patterns (case sensitive) on WebDataBinder in order to prevent unexpected properties from being set. For example:

@Controller
public class ChangeEmailController {

	@InitBinder
	void initBinder(WebDataBinder binder) {
		binder.setAllowedFields("oldEmailAddress", "newEmailAddress");
	}

	// @RequestMapping methods, etc.

}

You can also register disallowedFields patterns (case insensitive). However, "allowed" configuration is preferred over "disallowed" as it is more explicit and less prone to mistakes.

By default, constructor and property binding are both used. If you want to use constructor binding only, you can set the declarativeBinding flag on WebDataBinder through an @InitBinder method either locally within a controller or globally through an @ControllerAdvice. Turning this flag on ensures that only constructor binding is used and that property binding is not used unless allowedFields patterns are configured. For example:

@Controller
public class MyController {

	@InitBinder
	void initBinder(WebDataBinder binder) {
		binder.setDeclarativeBinding(true);
	}

	// @RequestMapping methods, etc.

}