Fail hard if snakeyaml not present when loading yaml

Closes gh-8900

spring-boot/src/main/java/org/springframework/boot/env/YamlPropertySourceLoader.java

@@ -40,13 +40,14 @@ public class YamlPropertySourceLoader implements PropertySourceLoader {
 	@Override
 	public PropertySource<?> load(String name, Resource resource, String profile)
 			throws IOException {
-		if (ClassUtils.isPresent("org.yaml.snakeyaml.Yaml", null)) {
+		if (!ClassUtils.isPresent("org.yaml.snakeyaml.Yaml", null)) {
+			throw new IllegalStateException("Attempted to load " + name + " but snakeyaml was not found on the classpath");
+		}
 		Map<String, Object> source = new OriginTrackedYamlLoader(resource, profile)
 				.load();
 		if (!source.isEmpty()) {
 			return new OriginTrackedMapPropertySource(name, source);
 		}
-		}
 		return null;
 	}
 

spring-boot/src/test/java/org/springframework/boot/env/NoSnakeYamlPropertySourceLoaderTests.java

@@ -0,0 +1,52 @@
+/*
+ * 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.env;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.junit.runner.classpath.ClassPathExclusions;
+import org.springframework.boot.junit.runner.classpath.ModifiedClassPathRunner;
+import org.springframework.core.io.ByteArrayResource;
+
+/**
+ * Tests for {@link YamlPropertySourceLoader} when snakeyaml is not available.
+ *
+ * @author Madhura Bhave
+ */
+@RunWith(ModifiedClassPathRunner.class)
+@ClassPathExclusions("snakeyaml-*.jar")
+public class NoSnakeYamlPropertySourceLoaderTests {
+
+	@Rule
+	public ExpectedException thrown = ExpectedException.none();
+
+	private YamlPropertySourceLoader loader = new YamlPropertySourceLoader();
+
+	@Test
+	public void load() throws Exception {
+		this.thrown.expect(IllegalStateException.class);
+		this.thrown.expectMessage("Attempted to load resource but snakeyaml was not found on the classpath");
+		ByteArrayResource resource = new ByteArrayResource(
+				"foo:\n  bar: spam".getBytes());
+		this.loader.load("resource", resource, null);
+	}
+
+}
+