运行时注释通过反射进行检查

Java 的 Reflection API 允许程序员在运行时对类字段，方法和注释执行各种检查和操作。但是，为了使注释在运行时完全可见，必须将 RetentionPolicy 更改为 RUNTIME，如下例所示：

@interface MyDefaultAnnotation {

}

@Retention(RetentionPolicy.RUNTIME)
@interface MyRuntimeVisibleAnnotation {

}

public class AnnotationAtRuntimeTest {

    @MyDefaultAnnotation
    static class RuntimeCheck1 {
    }
    
    @MyRuntimeVisibleAnnotation
    static class RuntimeCheck2 {
    }
    
    public static void main(String[] args) {
        Annotation[] annotationsByType = RuntimeCheck1.class.getAnnotations();
        Annotation[] annotationsByType2 = RuntimeCheck2.class.getAnnotations();

        System.out.println("default retention: " + Arrays.toString(annotationsByType));
        System.out.println("runtime retention: " + Arrays.toString(annotationsByType2));
    }
}