如何使用反射获取注解类名、属性值 [英] How to get annotation class name, attribute values using reflection

问题描述

我知道如果我们知道注解类，我们可以很容易地得到具体的注解并访问它的属性.例如:

I know if we know the annotation class, we can easily get the specific annotation and access its attribute. For example:

field.getAnnotation(Class<T> annotationClass) 

这将返回特定注释接口的引用，因此您可以轻松访问其值.

Which will return a reference of specific annotation interface, so you can easily access its values.

我的问题是我是否对特定的注释类没有预先了解.我只想使用反射在运行时获取所有注释类名称及其属性，以便将类信息转储为 JSON 文件.我怎样才能以简单的方式做到这一点.

My question is if I have no pre knowledge about the particular annotations class. I just want to use reflection to get all the annotation class name and their attributes at run-time for the purpose of dumping the class information for example as a JSON file. How can I do it in an easy way.

Annotation[] field.getAnnotations();

这个方法只会返回注解接口的动态代理.

This method will only return dynamic proxies of the annotation interfaces.

推荐答案

与人们的预期相反，注解的元素不是属性 - 它们实际上是返回提供值或默认值的方法.

Contrary to what one might expect, the elements of an annotation are not attributes - they are actually methods that return the provided value or a default value.

您必须遍历注释的方法并调用它们以获取值.使用annotationType()获取注解的类，getClass()返回的对象只是一个代理.

You have to iterate through the annotations' methods and invoke them to get the values. Use annotationType() to get the annotation's class, the object returned by getClass() is just a proxy.

这是一个打印 @Resource 一个类的注解:

Here is an example which prints all elements and their values of the @Resource annotation of a class:

@Resource(name = "foo", description = "bar")
public class Test {

    public static void main(String[] args) throws Exception {

        for (Annotation annotation : Test.class.getAnnotations()) {
            Class<? extends Annotation> type = annotation.annotationType();
            System.out.println("Values of " + type.getName());

            for (Method method : type.getDeclaredMethods()) {
                Object value = method.invoke(annotation, (Object[])null);
                System.out.println(" " + method.getName() + ": " + value);
            }
        }

    }
}

输出:

Values of javax.annotation.Resource
 name: foo
 type: class java.lang.Object
 lookup: 
 description: bar
 authenticationType: CONTAINER
 mappedName: 
 shareable: true

感谢 Aaron 指出您需要强制转换 null避免警告的论据.

Thanks to Aaron for pointing out the you need to cast the null argument to avoid warnings.

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