spring对Annotation的派生性应用可谓炉火纯青，在spring core：@Component的派生性讲过支持层次上派生性，而属性上派生的需求则借助了@AliasFor，它是从spring4.2中开始支持的。
@AliasFor注解用于声明注解元素的别名，应用于方法上（别忘了注解本质是接口）。Spring框架在内部使用大量的使用这个注解，例如，@Bean，@ComponentScan，@Scope等。
@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) @Documented public @interface AliasFor { /** * * 引用的注解别名.. */ @AliasFor("attribute") String value() default ""; /** * 引用的注解别名. * @see #value */ @AliasFor("value") String attribute() default ""; /**层次结构的父注解 */ Class<? extends Annotation> annotation() default Annotation.class; }
示例1
可通过AnnotatedElementUtils#getMergedAnnotationAttributes来读取。下面演示没有派生性的情况：
@Test public void metaTest() throws IOException { AnnotationAttributes aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home.class, AccessRole.class); System.out.println(aa);//{value=super-user, module=gui, accessType=super-user} } @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) public @interface AccessRole { @AliasFor("accessType") String value() default "visitor"; @AliasFor("value") String accessType() default "visitor"; String module() default "gui"; } @AccessRole("super-user") //@AccessRole(value = "super-user",accessType = "super")//error public class Home { }
注：alias references的默认值必须一致，使用时指定值也必须一致，否则会抛出AnnotationConfigurationException
下面演示有单层次及多层次派生性的情况：
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @AccessRole("admin") public @interface AdminAccess { @AliasFor(annotation = AccessRole.class, attribute = "module") String value() default "service"; } @AdminAccess public class Home2 { } @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @AdminAccess("supper") public @interface SupperAccess { String value() default "service3"; @AliasFor(annotation = AccessRole.class, attribute = "module") String module() default "service3"; } @SupperAccess public class Home3 { } @Test public void metaTest() throws IOException { AnnotationAttributes aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home2.class, AdminAccess.class); System.out.println(aa);//{value=service} aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home2.class, AccessRole.class); System.out.println(aa);//{value=admin, accessType=admin, module=service} aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home3.class, AccessRole.class); System.out.println(aa);//{value=admin, module=service3, accessType=admin} }
原理
AnnotatedElementUtils处理也不是特别复杂，很好理解
public abstract class AnnotatedElementUtils { @Nullable public static AnnotationAttributes getMergedAnnotationAttributes( AnnotatedElement element, Class<? extends Annotation> annotationType) { AnnotationAttributes attributes = searchWithGetSemantics(element, annotationType, null, new MergedAnnotationAttributesProcessor()); AnnotationUtils.postProcessAnnotationAttributes(element, attributes, false, false); return attributes; } //第一步：searchWithGetSemantics真正调用的方法，递归获取所有的注解 @Nullable private static <T> T searchWithGetSemantics(AnnotatedElement element, Set<Class<? extends Annotation>> annotationTypes, @Nullable String annotationName, @Nullable Class<? extends Annotation> containerType, Processor<T> processor, Set<AnnotatedElement> visited, int metaDepth) { if (visited.add(element)) { try { // Start searching within locally declared annotations List<Annotation> declaredAnnotations = Arrays.asList(AnnotationUtils.getDeclaredAnnotations(element)); T result = searchWithGetSemanticsInAnnotations(element, declaredAnnotations, annotationTypes, annotationName, containerType, processor, visited, metaDepth); if (result != null) { return result; } if (element instanceof Class) { // otherwise getAnnotations doesn't return anything new Class<?> superclass = ((Class<?>) element).getSuperclass(); if (superclass != null && superclass != Object.class) { List<Annotation> inheritedAnnotations = new LinkedList<>(); for (Annotation annotation : element.getAnnotations()) { if (!declaredAnnotations.contains(annotation)) { inheritedAnnotations.add(annotation); } } // Continue searching within inherited annotations result = searchWithGetSemanticsInAnnotations(element, inheritedAnnotations, annotationTypes, annotationName, containerType, processor, visited, metaDepth); if (result != null) { return result; } } } } catch (Throwable ex) { AnnotationUtils.handleIntrospectionFailure(element, ex); } } return null; } } public abstract class AnnotationUtils { //第二步：获取@AliasFor的标记，处理其值 static void postProcessAnnotationAttributes(@Nullable Object annotatedElement, @Nullable AnnotationAttributes attributes, boolean classValuesAsString, boolean nestedAnnotationsAsMap) { if (attributes == null) { return; } Class<? extends Annotation> annotationType = attributes.annotationType(); // Track which attribute values have already been replaced so that we can short // circuit the search algorithms. Set<String> valuesAlreadyReplaced = new HashSet<>(); if (!attributes.validated) { // Validate @AliasFor configuration Map<String, List<String>> aliasMap = getAttributeAliasMap(annotationType); aliasMap.forEach((attributeName, aliasedAttributeNames) -> { if (valuesAlreadyReplaced.contains(attributeName)) { return; } Object value = attributes.get(attributeName); boolean valuePresent = (value != null && !(value instanceof DefaultValueHolder)); for (String aliasedAttributeName : aliasedAttributeNames) { if (valuesAlreadyReplaced.contains(aliasedAttributeName)) { continue; } Object aliasedValue = attributes.get(aliasedAttributeName); boolean aliasPresent = (aliasedValue != null && !(aliasedValue instanceof DefaultValueHolder)); // Something to validate or replace with an alias? if (valuePresent || aliasPresent) { if (valuePresent && aliasPresent) { // Since annotation attributes can be arrays, we must use ObjectUtils.nullSafeEquals(). if (!ObjectUtils.nullSafeEquals(value, aliasedValue)) { String elementAsString = (annotatedElement != null ? annotatedElement.toString() : "unknown element"); throw new AnnotationConfigurationException(String.format( "In AnnotationAttributes for annotation [%s] declared on %s, " + "attribute '%s' and its alias '%s' are declared with values of [%s] and [%s], " + "but only one is permitted.", attributes.displayName, elementAsString, attributeName, aliasedAttributeName, ObjectUtils.nullSafeToString(value), ObjectUtils.nullSafeToString(aliasedValue))); } } else if (aliasPresent) { // Replace value with aliasedValue attributes.put(attributeName, adaptValue(annotatedElement, aliasedValue, classValuesAsString, nestedAnnotationsAsMap)); valuesAlreadyReplaced.add(attributeName); } else { // Replace aliasedValue with value attributes.put(aliasedAttributeName, adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap)); valuesAlreadyReplaced.add(aliasedAttributeName); } } } }); attributes.validated = true; } // Replace any remaining placeholders with actual default values for (Map.Entry<String, Object> attributeEntry : attributes.entrySet()) { String attributeName = attributeEntry.getKey(); if (valuesAlreadyReplaced.contains(attributeName)) { continue; } Object value = attributeEntry.getValue(); if (value instanceof DefaultValueHolder) { value = ((DefaultValueHolder) value).defaultValue; attributes.put(attributeName, adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap)); } } } }
待续
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