Improve attribute alias support in @ComponentScan

Prior to this commit, @ComponentScan already had a value/basePackages
alias pair; however, the semantics were not properly enforced.

This commit addresses this issue by refactoring
ComponentScanAnnotationParser to ensure that it is not possible to
declare both of the aliased attributes. In addition, the 'value' and
'basePackages' attributes are now annotated with @AliasFor in order to
make the semantics clearer.

Issue: SPR-11393

spring-context/src/main/java/org/springframework/context/annotation/ComponentScan.java

@@ -23,6 +23,7 @@ import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 import org.springframework.beans.factory.support.BeanNameGenerator;
+import org.springframework.core.annotation.AliasFor;
 import org.springframework.core.type.filter.TypeFilter;
 
 /**
@@ -46,6 +47,7 @@ import org.springframework.core.type.filter.TypeFilter;
  *
  * @author Chris Beams
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 3.1
  * @see Configuration
  */
@@ -60,6 +62,7 @@ public @interface ComponentScan {
 	 * are needed — for example, {@code @ComponentScan("org.my.pkg")}
 	 * instead of {@code @ComponentScan(basePackages = "org.my.pkg")}.
 	 */
+	@AliasFor(attribute = "basePackages")
 	String[] value() default {};
 
 	/**
@@ -69,6 +72,7 @@ public @interface ComponentScan {
 	 * <p>Use {@link #basePackageClasses} for a type-safe alternative to
 	 * String-based package names.
 	 */
+	@AliasFor(attribute = "value")
 	String[] basePackages() default {};
 
 	/**

spring-context/src/main/java/org/springframework/context/annotation/ComponentScanAnnotationParser.java

@@ -30,6 +30,7 @@ import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.BeanNameGenerator;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.annotation.AnnotationAttributes;
+import org.springframework.core.annotation.AnnotationConfigurationException;
 import org.springframework.core.env.Environment;
 import org.springframework.core.io.ResourceLoader;
 import org.springframework.core.type.filter.AbstractTypeHierarchyTraversingFilter;
@@ -40,6 +41,7 @@ import org.springframework.core.type.filter.RegexPatternTypeFilter;
 import org.springframework.core.type.filter.TypeFilter;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 /**
@@ -115,11 +117,7 @@ class ComponentScanAnnotationParser {
 		}
 
 		Set<String> basePackages = new LinkedHashSet<String>();
-		Set<String> specifiedPackages = new LinkedHashSet<String>();
+		for (String pkg : getBasePackages(componentScan, declaringClass)) {
-		specifiedPackages.addAll(Arrays.asList(componentScan.getStringArray("value")));
-		specifiedPackages.addAll(Arrays.asList(componentScan.getStringArray("basePackages")));
-
-		for (String pkg : specifiedPackages) {
 			String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),
 					ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
 			basePackages.addAll(Arrays.asList(tokenized));
@@ -140,6 +138,25 @@ class ComponentScanAnnotationParser {
 		return scanner.doScan(StringUtils.toStringArray(basePackages));
 	}
 
+	private String[] getBasePackages(AnnotationAttributes componentScan, String declaringClass) {
+		String[] value = componentScan.getStringArray("value");
+		String[] basePackages = componentScan.getStringArray("basePackages");
+		boolean valueDeclared = !ObjectUtils.isEmpty(value);
+		boolean basePackagesDeclared = !ObjectUtils.isEmpty(basePackages);
+
+		if (valueDeclared && basePackagesDeclared && !ObjectUtils.nullSafeEquals(value, basePackages)) {
+			String msg = String.format("In @ComponentScan declared on [%s], attribute [value] "
+					+ "and its alias [basePackages] are present with values of [%s] and [%s], "
+					+ "but only one is permitted.", declaringClass, ObjectUtils.nullSafeToString(value),
+				ObjectUtils.nullSafeToString(basePackages));
+			throw new AnnotationConfigurationException(msg);
+		}
+		if (!basePackagesDeclared) {
+			basePackages = value;
+		}
+		return basePackages;
+	}
+
 	private List<TypeFilter> typeFiltersFor(AnnotationAttributes filterAttributes) {
 		List<TypeFilter> typeFilters = new ArrayList<TypeFilter>();
 		FilterType filterType = filterAttributes.getEnum("type");