[[mvc-ann-initbinder]]
= `@InitBinder`

[.small]#xref:web/webflux/controller/ann-initbinder.adoc[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
xref:web/webmvc/mvc-config/conversion.adoc[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:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
	@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.

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
	@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:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes"]
----
	@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.

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes"]
----
	@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.
======


[[mvc-ann-initbinder-model-design]]
NOTE: For more guidance on model design, please see xref:web/webmvc/mvc-data-binding.adoc[Data Binding].