【小家Spring】Spring贡献的多个注解相关的工具类：AnnotationUtils、AnnotatedElementUtils、AnnotationConfigUtils...(上)

@MyAnno
interface Eat {
}
class Parent implements Eat {
}
class Child extends Parent {
}
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@RequestMapping // 特意注解上放一个注解，方面测试看结果
@Inherited
@interface MyAnno {
}
  // 现在我们来获取类上面的注解如下
    public static void main(String[] args) {
        MyAnno anno1 = Eat.class.getAnnotation(MyAnno.class);
        MyAnno anno2 = Parent.class.getAnnotation(MyAnno.class);
        MyAnno anno3 = Child.class.getAnnotation(MyAnno.class);
        System.out.println(anno1); //@com.fsx.maintest.MyAnno()
        System.out.println(anno2); //null
        System.out.println(anno3); //null
    }

  static <A extends Annotation> A synthesizeAnnotation(A annotation) {
    return synthesizeAnnotation(annotation, null);
  }
  public static <A extends Annotation> A synthesizeAnnotation(
      A annotation, @Nullable AnnotatedElement annotatedElement) {
    return synthesizeAnnotation(annotation, (Object) annotatedElement);
  }
  static <A extends Annotation> A synthesizeAnnotation(A annotation, @Nullable Object annotatedElement) {
    if (annotation instanceof SynthesizedAnnotation) {
      return annotation;
    }
    Class<? extends Annotation> annotationType = annotation.annotationType();
    if (!isSynthesizable(annotationType)) {
      return annotation;
    }
    DefaultAnnotationAttributeExtractor attributeExtractor =
        new DefaultAnnotationAttributeExtractor(annotation, annotatedElement);
    InvocationHandler handler = new SynthesizedAnnotationInvocationHandler(attributeExtractor);
    // Can always expose Spring's SynthesizedAnnotation marker since we explicitly check for a
    // synthesizable annotation before (which needs to declare @AliasFor from the same package)
    Class<?>[] exposedInterfaces = new Class<?>[] {annotationType, SynthesizedAnnotation.class};
    return (A) Proxy.newProxyInstance(annotation.getClass().getClassLoader(), exposedInterfaces, handler);
  }

    public static void main(String[] args) {
        MyAnno anno1 = Eat.class.getAnnotation(MyAnno.class);
        // 注解交给这么一处理  相当于就会被Spring代理了  这就是优势
        MyAnno sAnno1 = AnnotationUtils.getAnnotation(anno1, MyAnno.class);
        System.out.println(sAnno1); //@com.fsx.maintest.MyAnno()
        // 这样前后类型不一致的话，会把这个注解上面的注解给获取出来
        RequestMapping annotation = AnnotationUtils.getAnnotation(anno1, RequestMapping.class);
        System.out.println(annotation); //@org.springframework.web.bind.annotation.RequestMapping
    }

    public static void main(String[] args) {
        MyAnno anno1 = Eat.class.getAnnotation(MyAnno.class);
        // 注意这两种写法的区别：
        // 这个相当于是获取Child.class的它上面的所有注解， 所以只有继承过来的一个@MyAnno
        Annotation[] sAnno1 = AnnotationUtils.getAnnotations(Child.class);
        // 而这里传入的为anno1.annotationType，所以相当于获取该注解上面的注解  所以使用的时候需要注意
        Annotation[] sAnno2 = AnnotationUtils.getAnnotations(anno1.annotationType());
        System.out.println(sAnno1);
        System.out.println(sAnno2);
    }

@MyAnno
interface Eat {
}
class Parent implements Eat {
}
//本来，接口上的注解我们无论如何都继承不了了，但用了Spring的，你就可以
    public static void main(String[] args) {
        MyAnno annotation = AnnotationUtils.findAnnotation(Child.class, MyAnno.class);
        System.out.println(annotation);
    }
//备注：哪怕@MyAnno上没有标注@Inherited，也是能找出来的（这是后面讲解@RequestMapping为何能被子类继承的重要原因）

    public static void main(String[] args) {
        Class<?> annotationDeclaringClass = AnnotationUtils.findAnnotationDeclaringClass(MyAnno.class, Child.class);
        System.out.println(annotationDeclaringClass);
    }

    public static void main(String[] args) {
        System.out.println(AnnotationUtils.isAnnotationInherited(MyAnno.class, Parent.class)); //false
        // 说明一下：clazz.isAnnotationPresent(annotationType) JDK的。表示注解存在就行，不管你是自己的还是继承来的
        System.out.println(AnnotationUtils.isAnnotationInherited(MyAnno.class, Child.class)); //true 很显然，Child的这个注解是继承来的
    // Child的MyAnno注解是父类的，但这里还是会返回true
        System.out.println(Child.class.isAnnotationPresent(MyAnno.class)); //true
        System.out.println(Child.class.getAnnotation(MyAnno.class)); // @com.fsx.maintest.MyAnno(c...
    }

    public static void main(String[] args) {
        System.out.println(AnnotationUtils.isAnnotationMetaPresent(MyAnno.class, RequestMapping.class)); //true
        System.out.println(AnnotationUtils.isAnnotationMetaPresent(MyAnno.class, Component.class)); //false
    }