Refine validation auto-configuration

Checking for the presence of a `ValidationProvider` resource is
unfortunately not enough to trigger the validation auto-configuration.

If `hibernate-validator` is added on the classpath without a `javax.el`
implementation, the former will blow up on startup. So far this was a
pilot error so we didn't have to care about it. Now that we have an
auto-configuration that may lead to this error, we need to be extra
careful and check that scenario ourselves.

This commit adds an extra condition that runs as late as possible and
attempt to actually initialize a `Validator`. If that fails for whatever
reason, the auto-configuration will now back off.

Several additional tests have been added to exercise this scenario.

Closes gh-6228

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

@@ -16,13 +16,22 @@
 
 package org.springframework.boot.autoconfigure.validation;
 
+import javax.validation.Validation;
 import javax.validation.Validator;
 
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.autoconfigure.condition.ConditionMessage;
+import org.springframework.boot.autoconfigure.condition.ConditionOutcome;
 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.SpringBootCondition;
 import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.ConditionContext;
+import org.springframework.context.annotation.Conditional;
+import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
+import org.springframework.core.type.AnnotatedTypeMetadata;
 import org.springframework.validation.beanvalidation.MethodValidationPostProcessor;
 
 /**
@@ -37,9 +46,29 @@ public class ValidationAutoConfiguration {
 
 	@Bean
 	@ConditionalOnResource(resources = "META-INF/services/javax.validation.spi.ValidationProvider")
+	@Conditional(OnValidatorAvailableCondition.class)
 	@ConditionalOnMissingBean
 	public MethodValidationPostProcessor methodValidationPostProcessor() {
 		return new MethodValidationPostProcessor();
 	}
 
+	@Order(Ordered.LOWEST_PRECEDENCE)
+	static class OnValidatorAvailableCondition extends SpringBootCondition {
+
+		@Override
+		public ConditionOutcome getMatchOutcome(ConditionContext context,
+				AnnotatedTypeMetadata metadata) {
+			ConditionMessage.Builder message = ConditionMessage.forCondition(
+					getClass().getName());
+			try {
+				Validation.buildDefaultValidatorFactory().getValidator();
+				return ConditionOutcome.match(message.available("JSR-303 provider"));
+			}
+			catch (Exception ex) {
+				return ConditionOutcome.noMatch(message.notAvailable("JSR-303 provider"));
+			}
+		}
+
+	}
+
 }

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

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2012-2016 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.junit.After;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.junit.runner.classpath.ClassPathExclusions;
+import org.springframework.boot.junit.runner.classpath.ModifiedClassPathRunner;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.validation.beanvalidation.MethodValidationPostProcessor;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Test for {{@link ValidationAutoConfiguration} when Hibernate validator is present
+ * but no EL implementation is available.
+ *
+ * @author Stephane Nicoll
+ */
+@RunWith(ModifiedClassPathRunner.class)
+@ClassPathExclusions("tomcat-embed-el-*.jar")
+public class ValidationAutoConfigurationWithHibernateValidatorMissingElImplTests {
+
+	private AnnotationConfigApplicationContext context;
+
+	@After
+	public void close() {
+		if (this.context != null) {
+			this.context.close();
+		}
+	}
+
+	@Test
+	public void validationIsDisabled() {
+		this.context = new AnnotationConfigApplicationContext(
+				ValidationAutoConfiguration.class);
+		assertThat(this.context.getBeansOfType(MethodValidationPostProcessor.class))
+				.isEmpty();
+	}
+
+}

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

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2012-2016 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.junit.After;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.junit.runner.classpath.ClassPathExclusions;
+import org.springframework.boot.junit.runner.classpath.ModifiedClassPathRunner;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.validation.beanvalidation.MethodValidationPostProcessor;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Test for {{@link ValidationAutoConfiguration} when no JSR-303 provider is available.
+ *
+ * @author Stephane Nicoll
+ */
+@RunWith(ModifiedClassPathRunner.class)
+@ClassPathExclusions("hibernate-validator-*.jar")
+public class ValidationAutoConfigurationWithoutValidatorTests {
+
+	private AnnotationConfigApplicationContext context;
+
+	@After
+	public void close() {
+		if (this.context != null) {
+			this.context.close();
+		}
+	}
+
+	@Test
+	public void validationIsDisabled() {
+		this.context = new AnnotationConfigApplicationContext(
+				ValidationAutoConfiguration.class);
+		assertThat(this.context.getBeansOfType(MethodValidationPostProcessor.class))
+				.isEmpty();
+	}
+
+}