Fix method validation in child contexts

Bean post-processors only apply to the context in which they're
registered. ValidationAutoConfiguration will only auto-configure the
MethodValidationPostProcessor if the post-processor is missing from
the current context and any of its ancestors. If an ancestor context
contains the post-processor it will not be auto-configured and the
descendant context will not have method validation configured.

This commit updates the auto-configuration to limit the search for
an existing MethodValidationPostProcessor bean to the current
context.

Fixes gh-27890

Author: wilkinsona

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/validation/ValidationAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnResource;
+import org.springframework.boot.autoconfigure.condition.SearchStrategy;
 import org.springframework.boot.validation.MessageInterpolatorFactory;
 import org.springframework.boot.validation.beanvalidation.FilteredMethodValidationPostProcessor;
 import org.springframework.boot.validation.beanvalidation.MethodValidationExcludeFilter;
@@ -62,7 +63,7 @@ public static LocalValidatorFactoryBean defaultValidator() {
 	}
 
 	@Bean
-	@ConditionalOnMissingBean
+	@ConditionalOnMissingBean(search = SearchStrategy.CURRENT)
 	public static MethodValidationPostProcessor methodValidationPostProcessor(Environment environment,
 			@Lazy Validator validator, ObjectProvider<MethodValidationExcludeFilter> excludeFilters) {
 		FilteredMethodValidationPostProcessor processor = new FilteredMethodValidationPostProcessor(

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/validation/ValidationAutoConfigurationTests.java

@@ -34,7 +34,6 @@
 import org.springframework.boot.test.context.assertj.AssertableApplicationContext;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.validation.beanvalidation.MethodValidationExcludeFilter;
-import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Primary;
@@ -58,9 +57,8 @@
  */
 class ValidationAutoConfigurationTests {
 
-	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner(
-			AnnotationConfigApplicationContext::new)
-					.withConfiguration(AutoConfigurations.of(ValidationAutoConfiguration.class));
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+			.withConfiguration(AutoConfigurations.of(ValidationAutoConfiguration.class));
 
 	@Test
 	void validationAutoConfigurationShouldConfigureDefaultValidator() {
@@ -224,6 +222,20 @@ void methodValidationPostProcessorValidatorDependencyDoesNotTriggerEarlyInitiali
 						.contains("someService"));
 	}
 
+	@Test
+	void validationIsEnabledInChildContext() {
+		this.contextRunner.run((parent) -> new ApplicationContextRunner()
+				.withConfiguration(AutoConfigurations.of(ValidationAutoConfiguration.class))
+				.withUserConfiguration(SampleService.class).withParent(parent).run((context) -> {
+					assertThat(context.getBeansOfType(Validator.class)).hasSize(0);
+					assertThat(parent.getBeansOfType(Validator.class)).hasSize(1);
+					SampleService service = context.getBean(SampleService.class);
+					service.doSomething("Valid");
+					assertThatExceptionOfType(ConstraintViolationException.class)
+							.isThrownBy(() -> service.doSomething("KO"));
+				}));
+	}
+
 	private boolean isPrimaryBean(AssertableApplicationContext context, String beanName) {
 		return ((BeanDefinitionRegistry) context.getSourceApplicationContext()).getBeanDefinition(beanName).isPrimary();
 	}