[[webflux-ann-requestparam]]
= `@RequestParam`

[.small]#xref:web/webmvc/mvc-controller/ann-methods/requestparam.adoc[See equivalent in the Servlet stack]#

You can use the `@RequestParam` annotation to bind query parameters to a method argument in a
controller. The following code snippet shows the usage:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Controller
	@RequestMapping("/pets")
	public class EditPetForm {

		// ...

		@GetMapping
		public String setupForm(@RequestParam("petId") int petId, Model model) { <1>
			Pet pet = this.clinic.loadPet(petId);
			model.addAttribute("pet", pet);
			return "petForm";
		}

		// ...
	}
----
<1> Using `@RequestParam`.

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	import org.springframework.ui.set

	@Controller
	@RequestMapping("/pets")
	class EditPetForm {

		// ...

		@GetMapping
		fun setupForm(@RequestParam("petId") petId: Int, model: Model): String { // <1>
			val pet = clinic.loadPet(petId)
			model["pet"] = pet
			return "petForm"
		}

		// ...
	}
----
<1> Using `@RequestParam`.
======

TIP: The Servlet API "`request parameter`" concept conflates query parameters, form
data, and multiparts into one. However, in WebFlux, each is accessed individually through
`ServerWebExchange`. While `@RequestParam` binds to query parameters only, you can use
data binding to apply query parameters, form data, and multiparts to a
xref:web/webflux/controller/ann-methods/modelattrib-method-args.adoc[command object].

Method parameters that use the `@RequestParam` annotation are required by default, but
you can specify that a method parameter is optional by setting the required flag of a `@RequestParam`
to `false` or by declaring the argument with a `java.util.Optional`
wrapper.

Type conversion is applied automatically if the target method parameter type is not
`String`. See xref:web/webflux/controller/ann-methods/typeconversion.adoc[Type Conversion].

When a `@RequestParam` annotation is declared on a `Map<String, String>` or
`MultiValueMap<String, String>` argument, the map is populated with all query parameters.

Note that use of `@RequestParam` is optional -- for example, to set its attributes. By
default, any argument that is a simple value type (as determined by
{api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty])
and is not resolved by any other argument resolver is treated as if it were annotated
with `@RequestParam`.


