在 Kotlin 中注释属性时，注释的默认目标是什么? [英] What is the default target for an annotation when annotating property in Kotlin?

问题描述

Kotlin 中的注释可以有不同的使用站点目标，如下所述:https://kotlinlang.org/docs/reference/annotations.html#annotation-use-site-targets

我的问题是:当未明确定义 use-site 时，在如下示例中注释类中的属性时的默认目标是什么?

class 测试 {@SomeAnnotationvar someProperty: 字符串?= 空}

背景

我正在 Kotlin 中尝试将 Jongo 作为 MongoDB 客户端，但在注释 id 字段时遇到问题.当像这样注释时，Jongo 没有正确映射 id 属性:

@MongoId @MongoObjectId var id: String?= 空

有问题的注释只是 Jackson 的元注释.但是，当我像这样注释属性时，它似乎可以工作，表明使用站点问题:

@field:[MongoId MongoObjectId]var id:字符串?= 空

我希望 @field 是默认使用站点，但似乎不是.

解决方案

参考 说:

<块引用>

如果不指定使用站点目标，则根据目标选择目标到正在使用的注释的 @Target 注释.如果有多个适用目标，第一个适用目标使用以下列表:

• param(构造函数参数)
• property(带有此目标的注释对 Java 不可见)
• 字段

因此，如果您的注解具有 @Target({ElementType.FIELD})，则 Kotlin 中的注解将针对 @field:.

如果它没有指定 @Target，它可以用于任何程序元素:@property: 目标也适用并且默认选择.>

Annotations in Kotlin can have different use-site targets as explained here: https://kotlinlang.org/docs/reference/annotations.html#annotation-use-site-targets

My question is: When use-site is not explicitly defined, what is the default target when annotating property in a class like in the example below?

class Test {
  @SomeAnnotation
  var someProperty: String? = null
}

Background

I'm trying Jongo as MongoDB client in Kotlin and have problems annotating id field. Jongo does not map id property correctly when it's annotated like this:

@MongoId @MongoObjectId var id: String? = null

The annotations in question are just meta-annotations for Jackson. However it seems to work when I annotate the property like this, indicating use-site problem:

@field:[MongoId MongoObjectId]
var id: String? = null

I would expect that @field is default use-site, but it seems that it isn't.

解决方案

The reference says:

If you don’t specify a use-site target, the target is chosen according to the @Target annotation of the annotation being used. If there are multiple applicable targets, the first applicable target from the following list is used:

• param (constructor parameter)
• property (annotations with this target are not visible to Java)
• field

So if your annotation has @Target({ElementType.FIELD}), the annotation in Kotlin will target @field:.

If it has no @Target specified, it may be used on any program element: the @property: target is also applicable and is chosen by default.

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