Special case for YAML ParseException caused by bad document boundary

YAML documents can be text literals, so some common typos can be misleading
and not generate the expected exception (Boot fails to cast the
document to a Map, rather than reporting the underlying exception).

Fixes gh-235

spring-boot/src/main/java/org/springframework/boot/config/YamlProcessor.java

@@ -167,6 +167,11 @@ public class YamlProcessor {
 	private Map<String, Object> asMap(Object object) {
 		// YAML can have numbers as keys
 		Map<String, Object> result = new LinkedHashMap<String, Object>();
+		if (!(object instanceof Map)) {
+			// A document can be a text literal
+			result.put("document", object);
+			return result;
+		}
 		Map<Object, Object> map = (Map<Object, Object>) object;
 		for (Entry<Object, Object> entry : map.entrySet()) {
 			Object value = entry.getValue();

spring-boot/src/main/java/org/springframework/boot/context/listener/ConfigFileApplicationListener.java

@@ -328,21 +328,22 @@ public class ConfigFileApplicationListener implements
 
 	private PropertySource<?> getPropertySource(String name, Resource resource,
 			String profile, List<PropertySourceLoader> loaders) {
+		if (resource == null || !resource.exists()) {
+			return null;
+		}
 		String key = resource.getDescription() + (profile == null ? "" : "#" + profile);
 		if (this.cached.containsKey(key)) {
 			return this.cached.get(key);
 		}
 		boolean satisfied = true;
 		for (PropertySourceLoader loader : loaders) {
-			if (resource != null && resource.exists()) {
-				if (loader.supports(resource)) {
-					PropertySource<?> propertySource = loader.load(name, resource);
-					this.cached.put(key, propertySource);
-					return propertySource;
-				}
-				else {
-					satisfied = false;
-				}
+			if (loader.supports(resource)) {
+				PropertySource<?> propertySource = loader.load(name, resource);
+				this.cached.put(key, propertySource);
+				return propertySource;
+			}
+			else {
+				satisfied = false;
 			}
 		}
 		if (!satisfied) {

spring-boot/src/test/java/org/springframework/boot/config/YamlProcessorTests.java

@@ -24,6 +24,7 @@ import org.junit.rules.ExpectedException;
 import org.springframework.boot.config.YamlProcessor.MatchCallback;
 import org.springframework.core.io.ByteArrayResource;
 import org.springframework.core.io.Resource;
+import org.yaml.snakeyaml.parser.ParserException;
 import org.yaml.snakeyaml.scanner.ScannerException;
 
 import static org.junit.Assert.assertEquals;
@@ -55,6 +56,31 @@ public class YamlProcessorTests {
 		});
 	}
 
+	@Test
+	public void testStringResource() throws Exception {
+		this.processor.setResources(new Resource[] { new ByteArrayResource(
+				"foo # a document that is a literal".getBytes()) });
+		this.processor.process(new MatchCallback() {
+			@Override
+			public void process(Properties properties, Map<String, Object> map) {
+				assertEquals("foo", map.get("document"));
+			}
+		});
+	}
+
+	@Test
+	public void testBadDocumentStart() throws Exception {
+		this.processor.setResources(new Resource[] { new ByteArrayResource(
+				"foo # a document\nbar: baz".getBytes()) });
+		this.exception.expect(ParserException.class);
+		this.exception.expectMessage("line 2, column 1");
+		this.processor.process(new MatchCallback() {
+			@Override
+			public void process(Properties properties, Map<String, Object> map) {
+			}
+		});
+	}
+
 	@Test
 	public void testBadResource() throws Exception {
 		this.processor.setResources(new Resource[] { new ByteArrayResource(