Make JSON-B auto-config back off when no implementation is available

Previously, the JSON-B auto-configuration would be enabled when the JSON-B API was
on the classpath. This led to a failure if there was no implementation available.

The JSON-B API loads an implementation using the service loader. This commit updates
the auto-configuration to be conditional on the presence of a
META-INF/services/javax.json.bind.spi.JsonbProvider file on the classpath.

Closes gh-14675

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jsonb/JsonbAutoConfiguration.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -22,6 +22,7 @@ import javax.json.bind.JsonbBuilder;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnResource;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
@@ -33,6 +34,7 @@ import org.springframework.context.annotation.Configuration;
  */
 @Configuration
 @ConditionalOnClass(Jsonb.class)
+@ConditionalOnResource(resources = "classpath:META-INF/services/javax.json.bind.spi.JsonbProvider")
 public class JsonbAutoConfiguration {
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jsonb/JsonbAutoConfigurationWithNoProviderTests.java

+/*
+ * Copyright 2012-2018 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.jsonb;
+
+import javax.json.bind.Jsonb;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.boot.testsupport.runner.classpath.ClassPathExclusions;
+import org.springframework.boot.testsupport.runner.classpath.ModifiedClassPathRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link JsonbAutoConfiguration} when there is no provider available.
+ *
+ * @author Andy Wilkinson
+ */
+@RunWith(ModifiedClassPathRunner.class)
+@ClassPathExclusions("johnzon-jsonb-*.jar")
+public class JsonbAutoConfigurationWithNoProviderTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+			.withConfiguration(AutoConfigurations.of(JsonbAutoConfiguration.class));
+
+	@Test
+	public void jsonbBacksOffWhenThereIsNoProvider() {
+		this.contextRunner.run((context) -> {
+			assertThat(context).doesNotHaveBean(Jsonb.class);
+		});
+	}
+
+}