Refine validator and MVC validator configuration

Update `ValidationAutoConfiguration` and `WebMvcAutoConfiguration` to
ensure as much as possible that only a single Validator bean of each
type is registered.

Validation auto-configuration now does the following:
- If no validator is found: Registers a `LocalValidatorFactoryBean`
  (providing both Spring and JSR validation)
- If the user defines a Spring & JSR validator: Backs off
- If the user defines only a JSR validator: Adapts it to a Spring
  validator (without exposing another JSR implementation)

WebMvcAutoConfiguration auto-configuration has been updated to make
MVC validation follow common Spring Boot patterns:
- If not validator beans are found (due to the user excluding
  ValidationAutoConfiguration) a new `mvcValidator` bean will be
  registered.
- If a single validator bean is found it will be used for MVC
  validation.
- If multiple validator beans are defined it will either use the one
  named `mvcValidator` or it will register a new `mvcValidator` bean

Any automatically registered `mvcValidator` bean will not implement
the JSR validator interface.

Finally, it is no longer possible to provide an MVC validator via a
`WebMvcConfigurer`.

Fixes gh-8495

Author: philwebb

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/EndpointMvcIntegrationTests.java

@@ -41,6 +41,7 @@
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration;
 import org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration;
+import org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration;
@@ -156,9 +157,9 @@ protected TestInterceptor interceptor() {
 	@Documented
 	@Import({ EmbeddedServletContainerAutoConfiguration.class,
 			ServerPropertiesAutoConfiguration.class,
-			DispatcherServletAutoConfiguration.class, WebMvcAutoConfiguration.class,
-			JacksonAutoConfiguration.class, ErrorMvcAutoConfiguration.class,
-			PropertyPlaceholderAutoConfiguration.class })
+			DispatcherServletAutoConfiguration.class, ValidationAutoConfiguration.class,
+			WebMvcAutoConfiguration.class, JacksonAutoConfiguration.class,
+			ErrorMvcAutoConfiguration.class, PropertyPlaceholderAutoConfiguration.class })
 	protected @interface MinimalWebConfiguration {
 
 	}

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

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2012-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure.validation;
+
+import org.springframework.beans.factory.config.BeanDefinition;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.validation.MessageInterpolatorFactory;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Role;
+import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean;
+
+/**
+ * Default validator configuration imported by {@link ValidationAutoConfiguration}.
+ *
+ * @author Stephane Nicoll
+ * @author Phillip Webb
+ */
+@Configuration
+class DefaultValidatorConfiguration {
+
+	@Bean
+	@ConditionalOnMissingBean(type = { "javax.validation.Validator",
+			"org.springframework.validation.Validator" })
+	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
+	public static LocalValidatorFactoryBean defaultValidator() {
+		LocalValidatorFactoryBean factoryBean = new LocalValidatorFactoryBean();
+		MessageInterpolatorFactory interpolatorFactory = new MessageInterpolatorFactory();
+		factoryBean.setMessageInterpolator(interpolatorFactory.getObject());
+		return factoryBean;
+	}
+
+}

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

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2012-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure.validation;
+
+import org.springframework.util.Assert;
+import org.springframework.validation.Errors;
+import org.springframework.validation.SmartValidator;
+import org.springframework.validation.Validator;
+import org.springframework.validation.beanvalidation.SpringValidatorAdapter;
+
+/**
+ * {@link Validator} implementation that delegates calls to another {@link Validator}.
+ * This {@link Validator} implements Spring's {@link SmartValidator} interface but does
+ * not implement the JSR-303 {@code javax.validator.Validator} interface.
+ *
+ * @author Phillip Webb
+ * @since 1.5.3
+ */
+public class DelegatingValidator implements SmartValidator {
+
+	private final Validator delegate;
+
+	/**
+	 * Create a new {@link DelegatingValidator} instance.
+	 * @param targetValidator the target JSR validator
+	 */
+	public DelegatingValidator(javax.validation.Validator targetValidator) {
+		this.delegate = new SpringValidatorAdapter(targetValidator);
+	}
+
+	/**
+	 * Create a new {@link DelegatingValidator} instance.
+	 * @param targetValidator the target validator
+	 */
+	public DelegatingValidator(Validator targetValidator) {
+		Assert.notNull(targetValidator, "Target Validator must not be null");
+		this.delegate = targetValidator;
+	}
+
+	@Override
+	public boolean supports(Class<?> clazz) {
+		return this.delegate.supports(clazz);
+	}
+
+	@Override
+	public void validate(Object target, Errors errors) {
+		this.delegate.validate(target, errors);
+	}
+
+	@Override
+	public void validate(Object target, Errors errors, Object... validationHints) {
+		if (this.delegate instanceof SmartValidator) {
+			((SmartValidator) this.delegate).validate(target, errors, validationHints);
+		}
+		else {
+			this.delegate.validate(target, errors);
+		}
+	}
+
+	protected final Validator getDelegate() {
+		return this.delegate;
+	}
+
+}

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/validation/Jsr303ValidatorAdapterConfiguration.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2012-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure.validation;
+
+import javax.validation.Validator;
+
+import org.springframework.beans.factory.config.BeanDefinition;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnSingleCandidate;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Role;
+import org.springframework.validation.SmartValidator;
+
+/**
+ * JSR 303 adapter configration imported by {@link ValidationAutoConfiguration}.
+ *
+ * @author Stephane Nicoll
+ * @author Phillip Webb
+ */
+@Configuration
+class Jsr303ValidatorAdapterConfiguration {
+
+	@Bean
+	@ConditionalOnSingleCandidate(Validator.class)
+	@ConditionalOnMissingBean(org.springframework.validation.Validator.class)
+	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
+	public SmartValidator jsr303ValidatorAdapter(Validator validator) {
+		return new DelegatingValidator(validator);
+	}
+
+}

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

@@ -19,18 +19,16 @@
 import javax.validation.Validator;
 import javax.validation.executable.ExecutableValidator;
 
-import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 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.bind.RelaxedPropertyResolver;
-import org.springframework.boot.validation.MessageInterpolatorFactory;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
-import org.springframework.context.annotation.Role;
+import org.springframework.context.annotation.Import;
 import org.springframework.core.env.Environment;
-import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean;
 import org.springframework.validation.beanvalidation.MethodValidationPostProcessor;
 
 /**
@@ -43,19 +41,12 @@
 @Configuration
 @ConditionalOnClass(ExecutableValidator.class)
 @ConditionalOnResource(resources = "classpath:META-INF/services/javax.validation.spi.ValidationProvider")
+@Import({ DefaultValidatorConfiguration.class,
+		Jsr303ValidatorAdapterConfiguration.class })
 public class ValidationAutoConfiguration {
 
 	@Bean
-	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
-	@ConditionalOnMissingBean
-	public static Validator jsr303Validator() {
-		LocalValidatorFactoryBean factoryBean = new LocalValidatorFactoryBean();
-		MessageInterpolatorFactory interpolatorFactory = new MessageInterpolatorFactory();
-		factoryBean.setMessageInterpolator(interpolatorFactory.getObject());
-		return factoryBean;
-	}
-
-	@Bean
+	@ConditionalOnBean(Validator.class)
 	@ConditionalOnMissingBean
 	public static MethodValidationPostProcessor methodValidationPostProcessor(
 			Environment environment, Validator validator) {