Fixed QualifierAnnotationAutowireCandidateResolver's detection of custom qualifier annotations

Issue: SPR-10107

spring-beans/src/main/java/org/springframework/beans/factory/annotation/QualifierAnnotationAutowireCandidateResolver.java

@@ -36,6 +36,7 @@ import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
+import org.springframework.util.StringUtils;
 
 /**
  * {@link AutowireCandidateResolver} implementation that matches bean definition qualifiers
@@ -192,7 +193,7 @@ public class QualifierAnnotationAutowireCandidateResolver implements AutowireCan
 						foundMeta = true;
 						// Only accept fallback match if @Qualifier annotation has a value...
 						// Otherwise it is just a marker for a custom qualifier annotation.
-						if ((fallbackToMeta && AnnotationUtils.getValue(metaAnn) == null) ||
+						if ((fallbackToMeta && StringUtils.isEmpty(AnnotationUtils.getValue(metaAnn))) ||
 								!checkQualifier(bdHolder, metaAnn, typeConverter)) {
 							return false;
 						}

spring-context/src/test/java/org/springframework/beans/factory/support/QualifierAnnotationAutowireContextTests.java

@@ -167,7 +167,7 @@ public class QualifierAnnotationAutowireContextTests {
 		RootBeanDefinition person = new RootBeanDefinition(QualifiedPerson.class, cavs, null);
 		ConstructorArgumentValues cavs2 = new ConstructorArgumentValues();
 		cavs2.addGenericArgumentValue(MARK);
-		RootBeanDefinition person2 = new RootBeanDefinition(Person.class, cavs2, null);
+		RootBeanDefinition person2 = new RootBeanDefinition(DefaultValueQualifiedPerson.class, cavs2, null);
 		context.registerBeanDefinition(JUERGEN, person);
 		context.registerBeanDefinition(MARK, person2);
 		context.registerBeanDefinition("autowired",
@@ -745,13 +745,25 @@ public class QualifierAnnotationAutowireContextTests {
 	}
 
 
+	@TestQualifierWithDefaultValue
+	private static class DefaultValueQualifiedPerson extends Person {
+
+		public DefaultValueQualifiedPerson() {
+			super(null);
+		}
+
+		public DefaultValueQualifiedPerson(String name) {
+			super(name);
+		}
+	}
+
+
 	@Retention(RetentionPolicy.RUNTIME)
 	@Qualifier
 	public static @interface TestQualifier {
 	}
 
 
-	@Target(ElementType.FIELD)
 	@Retention(RetentionPolicy.RUNTIME)
 	@Qualifier
 	public static @interface TestQualifierWithDefaultValue {

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

@@ -69,6 +69,21 @@ public abstract class StringUtils {
 	// General convenience methods for working with Strings
 	//---------------------------------------------------------------------
 
+	/**
+	 * Check whether the given String is empty.
+	 * <p>This method accepts any Object as an argument, comparing it to
+	 * <code>null</code> and the empty String. As a consequence, this method
+	 * will never return <code>true</code> for a non-null non-String object.
+	 * <p>The Object signature is useful for general attribute handling code
+	 * that commonly deals with Strings but generally has to iterate over
+	 * Objects since attributes may e.g. be primitive value objects as well.
+	 * @param str the candidate String
+	 * @since 3.2.1
+	 */
+	public static boolean isEmpty(Object str) {
+		return (str == null || "".equals(str));
+	}
+
 	/**
 	 * Check that the given CharSequence is neither <code>null</code> nor of length 0.
 	 * Note: Will return <code>true</code> for a CharSequence that purely consists of whitespace.