Document customization of method validation errors

Closes gh-30653

framework-docs/modules/ROOT/pages/core/validation/beanvalidation.adoc

@@ -329,6 +329,16 @@ xref:core/aop/proxying.adoc#aop-understanding-aop-proxies[Understanding AOP Prox
 to always use methods and accessors on proxied classes; direct field access will not work.
 ====
 
+NOTE: Spring MVC and WebFlux have built-in support for method validation, and therefore
+for web controller methods there is no need for a class level `@Validated` and an AOP proxy.
+See the Spring MVC xref:web/webmvc/mvc-controller/ann-validation.adoc[Validation] section,
+the WebFlux xref:web/webflux/controller/ann-validation.adoc[Validation] section,
+and the xref:web/webmvc/mvc-controller/ann-validation.adoc[Error Responses] section.
+
+
+[[validation-beanvalidation-spring-method-exceptions]]
+==== Method Validation Exceptions
+
 By default, `jakarta.validation.ConstraintViolationException` is raised with the set of
 ``ConstraintViolation``s returned by `jakarata.validation.Validator`. As an alternative,
 you can have `MethodValidationException` raised instead with ``ConstraintViolation``s
@@ -373,18 +383,86 @@ fields and properties, the `ParameterValidationResult` is `ParameterErrors` whic
 implements `org.springframework.validation.Errors` and exposes validation errors as
 ``FieldError``s.
 
+
+[[validation-beanvalidation-spring-method-i18n]]
+==== Customizing Validation Errors
+
 The adapted `MessageSourceResolvable` errors can be turned into error messages to
-display to users through the configured
-xref:core/beans/context-introduction.adoc#context-functionality-messagesource[`MessageSource`]
-based on locale and language specific resource bundles.
+display to users through the configured `MessageSource` with locale and language specific
+resource bundles. This section provides an example for illustration.
 
-NOTE: Spring MVC and WebFlux have built-in support for method validation, and therefore
-for web controller methods there is no need for a class level `@Validated` and an AOP proxy.
-See the Spring MVC xref:web/webmvc/mvc-controller/ann-validation.adoc[Validation] section,
-the WebFlux xref:web/webflux/controller/ann-validation.adoc[Validation] section,
-and the xref:web/webmvc/mvc-controller/ann-validation.adoc[Error Responses] section.
+Given the following class declarations:
+
+[tabs]
+======
+Java::
++
+[source,java,indent=0,subs="verbatim,quotes",role="primary"]
+----
+	record Person(@Size(min = 1, max = 10) String name) {
+	}
+
+	@Validated
+	public class MyService {
+
+		void addStudent(@Valid Person person, @Max(2) int degrees) {
+			// ...
+		}
+	}
+----
+
+Kotlin::
++
+[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
+----
+	@JvmRecord
+	internal data class Person(@Size(min = 1, max = 10) val name: String)
+
+	@Validated
+	class MyService {
+
+		fun addStudent(person: @Valid Person?, degrees: @Max(2) Int) {
+			// ...
+		}
+	}
+----
+======
+
+A `ConstraintViolation` on `Person.name()` is adapted to a `FieldErrro` with the following:
+
+- Error codes `"Size.student.name"`, `"Size.name"`, `"Size.java.lang.String"`, and `"Size"`
+- Message arguments `"name"`, `10`, and `1` (the field name and the constraint attributes)
+- Default message "size must be between 1 and 10"
+
+To customize the default message, you can add properties to
+xref:core/beans/context-introduction.adoc#context-functionality-messagesource[MessageSource]
+resource bundles using any of the above errors codes and message arguments. Note also that the
+message argument `"name"` is itself a `MessagreSourceResolvable` with error codes
+`"student.name"` and `"name"` and can customized too. For example:
 
+Properties::
++
+[source,properties,indent=0,subs="verbatim,quotes",role="secondary"]
+----
+Size.student.name=Please, provide a {0} that is between {2} and {1} characters long
+student.name=username
+----
 
+A `ConstraintViolation` on the `degrees` method parameter is adapted to a
+`MessageSourceResolvable` with the following:
+
+- Error codes `"Max.myService#addStudent.degrees"`, `"Max.degrees"`, `"Max.int"`, `"Max"`
+- Message arguments "degrees2 and 2 (the field name and the constraint attribute)
+- Default message "must be less than or equal to 2"
+
+To customize the above default message, you can add a property such as:
+
+Properties::
++
+[source,properties,indent=0,subs="verbatim,quotes",role="secondary"]
+----
+Max.degrees=You cannot provide more than {1} {0}
+----
 
 
 [[validation-beanvalidation-spring-other]]

framework-docs/modules/ROOT/pages/web/webflux/ann-rest-exceptions.adoc

@@ -136,6 +136,14 @@ Message codes and arguments for each error are also resolved via `MessageSource`
 
 |===
 
+NOTE: Unlike other exceptions, the message arguments for
+`WebExchangeBindException` and `HandlerMethodValidationException` are based on a list of
+`MessageSourceResolvable` errors that can also be customized through a
+xref:core/beans/context-introduction.adoc#context-functionality-messagesource[MessageSource]
+resource bundle. See
+xref:core/validation/beanvalidation.adoc#validation-beanvalidation-spring-method-i18n[Customizing Validation Errors]
+for more details.
+
 
 
 

framework-docs/modules/ROOT/pages/web/webmvc/mvc-ann-rest-exceptions.adoc

@@ -180,6 +180,13 @@ Message codes and arguments for each error are also resolved via `MessageSource`
 
 |===
 
+NOTE: Unlike other exceptions, the message arguments for
+`MethodArgumentValidException` and `HandlerMethodValidationException` are baed on a list of
+`MessageSourceResolvable` errors that can also be customized through a
+xref:core/beans/context-introduction.adoc#context-functionality-messagesource[MessageSource]
+resource bundle. See
+xref:core/validation/beanvalidation.adoc#validation-beanvalidation-spring-method-i18n[Customizing Validation Errors]
+for more details.