是否有可能在运行时访问Java 8类信息？ [英] Is it possible to access Java 8 type informations at runtime?

问题描述

假设我有一类下面的成员，这使得使用Java 8型注释：

 私人列表＆LT; @Email串＆GT;电子邮件地址;
 

是否可以读取使用反射在运行时String类型的使用给出的 @Email 标注？如果是这样，怎么会这样做？

更新：这是注释类型的定义：

  @Target（值= ElementType.TYPE_USE）
@Retention（RetentionPolicy.RUNTIME）
公共@interface电子邮件{}
 

解决方案

是的，它是可能的。反射式重presenting这种结构被称为<一href=\"http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedParameterizedType.html\"><$c$c>AnnotatedParameterizedType.这里是如何让你的注释的例子：

  //获取电子邮件字段
现场emailAddressField = MyClass.class.getDeclaredField（emailAddresses）;//字段的类型是两个参数和注释，
//将它转换为正确的类型再presentation
AnnotatedParameterizedType annotatedParameterizedType =
        （AnnotatedParameterizedType）emailAddressField.getAnnotatedType（）;//获取所有类型参数
AnnotatedType [] = annotatedActualTypeArguments
        annotatedParameterizedType.getAnnotatedActualTypeArguments（）;//其中包含注解的字符串参数
AnnotatedType stringParameterType = annotatedActualTypeArguments [0];//实际的注解
注释emailAnnotation = stringParameterType.getAnnotations（）[0];的System.out.println（emailAnnotation）; // @Email（）
 

Assuming I have the following member in a class which makes use of Java 8 type annotations:

private List<@Email String> emailAddresses;

Is it possible to read the @Email annotation given on the String type use at runtime using reflection? If so, how would this be done?

Update: That's the definition of the annotation type:

@Target(value=ElementType.TYPE_USE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Email {}

解决方案

Yes it is possible. The reflection type representing this kind of structure is called AnnotatedParameterizedType. Here is an example of how to get your annotation:

// get the email field 
Field emailAddressField = MyClass.class.getDeclaredField("emailAddresses");

// the field's type is both parameterized and annotated,
// cast it to the right type representation
AnnotatedParameterizedType annotatedParameterizedType =
        (AnnotatedParameterizedType) emailAddressField.getAnnotatedType();

// get all type parameters
AnnotatedType[] annotatedActualTypeArguments = 
        annotatedParameterizedType.getAnnotatedActualTypeArguments();

// the String parameter which contains the annotation
AnnotatedType stringParameterType = annotatedActualTypeArguments[0];

// The actual annotation
Annotation emailAnnotation = stringParameterType.getAnnotations()[0]; 

System.out.println(emailAnnotation);  // @Email()

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