Tightened StringValueResolver contract

Issue: SPR-14842

spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanFactory.java

@@ -370,8 +370,8 @@ public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport imp
 			}
 			catch (TypeMismatchException ex) {
 				if (logger.isDebugEnabled()) {
-					logger.debug("Failed to convert bean '" + name + "' to required type [" +
-							ClassUtils.getQualifiedName(requiredType) + "]", ex);
+					logger.debug("Failed to convert bean '" + name + "' to required type '" +
+							ClassUtils.getQualifiedName(requiredType) + "'", ex);
 				}
 				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
 			}
@@ -803,12 +803,15 @@ public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport imp
 
 	@Override
 	public String resolveEmbeddedValue(String value) {
+		if (value == null) {
+			return null;
+		}
 		String result = value;
 		for (StringValueResolver resolver : this.embeddedValueResolvers) {
+			result = resolver.resolveStringValue(result);
 			if (result == null) {
 				return null;
 			}
-			result = resolver.resolveStringValue(result);
 		}
 		return result;
 	}

spring-core/src/main/java/org/springframework/util/StringValueResolver.java

@@ -31,8 +31,11 @@ public interface StringValueResolver {
 
 	/**
 	 * Resolve the given String value, for example parsing placeholders.
-	 * @param strVal the original String value
-	 * @return the resolved String value
+	 * @param strVal the original String value (never {@code null})
+	 * @return the resolved String value (may be {@code null} when resolved to a null
+	 * value), possibly the original String value itself (in case of no placeholders
+	 * to resolve or when ignoring unresolvable placeholders)
+	 * @throws IllegalArgumentException in case of an unresolvable String value
 	 */
 	String resolveStringValue(String strVal);