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。
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@Retention(RetentionPolicy.RUNTIME)
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@Target(ElementType.METHOD)
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@Documented
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public
@interface AliasFor {
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/**
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* * 引用的注解别名..
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*/
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@AliasFor(
"attribute")
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String value() default "";
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/**
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* 引用的注解别名.
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* @see #value
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*/
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@AliasFor(
"value")
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String attribute() default "";
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/**层次结构的父注解
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*/
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Class<? extends Annotation> annotation()
default Annotation.class;
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}
示例1
可通过AnnotatedElementUtils#getMergedAnnotationAttributes来读取。下面演示没有派生性的情况：
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@Test
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public void metaTest() throws IOException {
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AnnotationAttributes aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home.class, AccessRole.class);
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System.out.println(aa);
//{value=super-user, module=gui, accessType=super-user}
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}
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@Target(ElementType.TYPE)
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@Retention(RetentionPolicy.RUNTIME)
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public
@interface AccessRole {
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@AliasFor(
"accessType")
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String value() default "visitor";
-
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@AliasFor(
"value")
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String accessType() default "visitor";
-
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String module() default "gui";
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}
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@AccessRole(
"super-user")
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//@AccessRole(value = "super-user",accessType = "super")//error
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public
class Home {
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}
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：
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@Target(ElementType.TYPE)
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@Retention(RetentionPolicy.RUNTIME)
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@AccessRole(
"admin")
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public
@interface AdminAccess {
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@AliasFor(annotation = AccessRole.class, attribute =
"module")
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String value() default "service";
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}
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@AdminAccess
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public
class Home2 {
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}
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@Target(ElementType.TYPE)
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@Retention(RetentionPolicy.RUNTIME)
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@AdminAccess(
"supper")
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public
@interface SupperAccess {
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String value() default "service3";
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@AliasFor(annotation = AccessRole.class, attribute =
"module")
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String module() default "service3";
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}
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@SupperAccess
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public
class Home3 {
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}
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@Test
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public void metaTest() throws IOException {
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AnnotationAttributes aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home2.class, AdminAccess.class);
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System.out.println(aa);
//{value=service}
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aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home2.class, AccessRole.class);
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System.out.println(aa);
//{value=admin, accessType=admin, module=service}
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aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home3.class, AccessRole.class);
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System.out.println(aa);
//{value=admin, module=service3, accessType=admin}
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}
原理
AnnotatedElementUtils处理也不是特别复杂，很好理解
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public
abstract
class AnnotatedElementUtils {
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@Nullable
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public static AnnotationAttributes getMergedAnnotationAttributes(
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AnnotatedElement element, Class<? extends Annotation> annotationType) {
-
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AnnotationAttributes attributes = searchWithGetSemantics(element, annotationType,
null,
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new MergedAnnotationAttributesProcessor());
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AnnotationUtils.postProcessAnnotationAttributes(element, attributes,
false,
false);
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return attributes;
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}
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//第一步：searchWithGetSemantics真正调用的方法，递归获取所有的注解
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@Nullable
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private
static <T>
T searchWithGetSemantics(AnnotatedElement element,
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Set<Class<? extends Annotation>> annotationTypes, @Nullable String annotationName,
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@Nullable Class<? extends Annotation> containerType, Processor<T> processor,
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Set<AnnotatedElement> visited, int metaDepth) {
-
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if (visited.add(element)) {
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try {
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// Start searching within locally declared annotations
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List<Annotation> declaredAnnotations = Arrays.asList(AnnotationUtils.getDeclaredAnnotations(element));
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T result = searchWithGetSemanticsInAnnotations(element, declaredAnnotations,
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annotationTypes, annotationName, containerType, processor, visited, metaDepth);
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if (result !=
null) {
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return result;
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}
-
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if (element
instanceof Class) {
// otherwise getAnnotations doesn't return anything new
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Class<?> superclass = ((Class<?>) element).getSuperclass();
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if (superclass !=
null && superclass != Object.class) {
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List<Annotation> inheritedAnnotations =
new LinkedList<>();
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for (Annotation annotation : element.getAnnotations()) {
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if (!declaredAnnotations.contains(annotation)) {
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inheritedAnnotations.add(annotation);
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}
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}
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// Continue searching within inherited annotations
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result = searchWithGetSemanticsInAnnotations(element, inheritedAnnotations,
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annotationTypes, annotationName, containerType, processor, visited, metaDepth);
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if (result !=
null) {
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return result;
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}
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}
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}
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}
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catch (Throwable ex) {
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AnnotationUtils.handleIntrospectionFailure(element, ex);
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}
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}
-
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return
null;
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}
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}
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public
abstract
class AnnotationUtils {
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//第二步：获取@AliasFor的标记，处理其值
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static void postProcessAnnotationAttributes(@Nullable Object annotatedElement,
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@Nullable AnnotationAttributes attributes, boolean classValuesAsString, boolean nestedAnnotationsAsMap) {
-
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if (attributes ==
null) {
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return;
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}
-
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Class<? extends Annotation> annotationType = attributes.annotationType();
-
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// Track which attribute values have already been replaced so that we can short
-
// circuit the search algorithms.
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Set<String> valuesAlreadyReplaced =
new HashSet<>();
-
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if (!attributes.validated) {
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// Validate @AliasFor configuration
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Map<String, List<String>> aliasMap = getAttributeAliasMap(annotationType);
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aliasMap.forEach((attributeName, aliasedAttributeNames) -> {
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if (valuesAlreadyReplaced.contains(attributeName)) {
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return;
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}
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Object value = attributes.get(attributeName);
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boolean valuePresent = (value !=
null && !(value
instanceof DefaultValueHolder));
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for (String aliasedAttributeName : aliasedAttributeNames) {
-
if (valuesAlreadyReplaced.contains(aliasedAttributeName)) {
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continue;
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}
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Object aliasedValue = attributes.get(aliasedAttributeName);
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boolean aliasPresent = (aliasedValue !=
null && !(aliasedValue
instanceof DefaultValueHolder));
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// Something to validate or replace with an alias?
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if (valuePresent || aliasPresent) {
-
if (valuePresent && aliasPresent) {
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// Since annotation attributes can be arrays, we must use ObjectUtils.nullSafeEquals().
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if (!ObjectUtils.nullSafeEquals(value, aliasedValue)) {
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String elementAsString =
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(annotatedElement !=
null ? annotatedElement.toString() :
"unknown element");
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throw
new AnnotationConfigurationException(String.format(
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"In AnnotationAttributes for annotation [%s] declared on %s, " +
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"attribute '%s' and its alias '%s' are declared with values of [%s] and [%s], " +
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"but only one is permitted.", attributes.displayName, elementAsString,
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attributeName, aliasedAttributeName, ObjectUtils.nullSafeToString(value),
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ObjectUtils.nullSafeToString(aliasedValue)));
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}
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}
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else
if (aliasPresent) {
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// Replace value with aliasedValue
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attributes.put(attributeName,
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adaptValue(annotatedElement, aliasedValue, classValuesAsString, nestedAnnotationsAsMap));
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valuesAlreadyReplaced.add(attributeName);
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}
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else {
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// Replace aliasedValue with value
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attributes.put(aliasedAttributeName,
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adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap));
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valuesAlreadyReplaced.add(aliasedAttributeName);
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}
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}
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}
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});
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attributes.validated =
true;
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}
-
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// Replace any remaining placeholders with actual default values
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for (Map.Entry<String, Object> attributeEntry : attributes.entrySet()) {
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String attributeName = attributeEntry.getKey();
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if (valuesAlreadyReplaced.contains(attributeName)) {
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continue;
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}
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Object value = attributeEntry.getValue();
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if (value
instanceof DefaultValueHolder) {
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value = ((DefaultValueHolder) value).defaultValue;
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attributes.put(attributeName,
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adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap));
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}
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}
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}
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}
待续
转载：https://blog.csdn.net/itsoftchenfei/article/details/103787806
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