如何在 Scala 中使用 Circe 解码 JSON 列表/数组
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How to Use Circe for Decoding JSON Lists/Arrays in Scala
提问by mattmar10
I have the code snippet
我有代码片段
cursor.downField("params").downField("playlist").downField("items").as[List[Clip]]
Where Clip is a simple case class of strings and numbers. The incoming Json should contain a json object "playlist" with an array of "items" where each item is a clip. So the json should look like
其中 Clip 是字符串和数字的简单 case 类。传入的 Json 应该包含一个 json 对象“播放列表”，其中包含一个“项目”数组，其中每个项目都是一个剪辑。所以json应该看起来像
{
"playlist": {
"name": "Sample Playlist",
"items": [
{
"clipId":"xyz",
"name":"abc"
},
{
"clipId":"pqr",
"name":"def"
}
]
}
}
With the code snippet above, I'm getting the compile error:
使用上面的代码片段，我收到编译错误：
Error:(147, 81) could not find implicit value for parameter d:
io.circe.Decoder[List[com.packagename.model.Clip]]
cursor.downField("params").downField("playlist").downField("items").as[List[Clip]]
What am I doing wrong? How do you setup decoding for a list/array of simple items using circe?
我究竟做错了什么？您如何使用 circe 为简单项目的列表/数组设置解码？
回答by Travis Brown
For the sake of completeness, instead of navigating into the JSON value and then decoding the clips, you could create a custom decoder that includes the navigation in its processing:
为了完整起见，您可以创建一个自定义解码器，在其处理中包含导航，而不是导航到 JSON 值然后解码剪辑：
import io.circe.Decoder, io.circe.generic.auto._
case class Clip(clipId: String, name: String)
val decodeClipsParam = Decoder[List[Clip]].prepare(
_.downField("params").downField("playlist").downField("items")
)
And then if you've got this:
然后如果你有这个：
val json = """{ "params": {
"playlist": {
"name": "Sample Playlist",
"items": [
{
"clipId":"xyz",
"name":"abc"
},
{
"clipId":"pqr",
"name":"def"
}
]
}
}}"""
You can use the decoder like this:
您可以像这样使用解码器：
scala> io.circe.parser.decode(json)(decodeClipsParam)
res3: Either[io.circe.Error,List[Clip]] = Right(List(Clip(xyz,abc), Clip(pqr,def)))
I'd probably go a step further and use a custom case class:
我可能会更进一步并使用自定义案例类：
import io.circe.generic.auto._
import io.circe.generic.semiauto.deriveDecoder
case class Clip(clipId: String, name: String)
case class PlaylistParam(name: String, items: List[Clip])
object PlaylistParam {
implicit val decodePlaylistParam: Decoder[PlaylistParam] =
deriveDecoder[PlaylistParam].prepare(
_.downField("params").downField("playlist")
)
}
Now you can just write this:
现在你可以这样写：
scala> io.circe.parser.decode[PlaylistParam](json).foreach(println)
PlaylistParam(Sample Playlist,List(Clip(xyz,abc), Clip(pqr,def)))
How you want to split up the navigation and decoding is mostly a matter of taste, though.
但是，您希望如何拆分导航和解码主要是一个品味问题。
回答by mattmar10
Thanks for the help. I was able to figure it out after stepping away for awhile and coming back with fresh eyes.
谢谢您的帮助。在离开一段时间并以全新的眼光回来后，我能够弄清楚。
I think I was going wrong by using the downArray function.
我想我使用 downArray 函数出错了。
My solution was to do the following:
我的解决方案是执行以下操作：
override def main(args: Array[String]): Unit = {
import ClipCodec.decodeClip
val json = parse(Source.fromFile("playlist.json").mkString).right.get
val clips = json.hcursor.downField("params").downField("playlist")
.downField("items").as[Seq[Clip]]
}
回答by acidghost
Circe is looking for an implicitly declared decoder for List[Clip]and cannot find it.
Circe 正在寻找隐式声明的解码器，List[Clip]但找不到它。
I suspect that you have not defined a decoder either manually or (semi)automatically. You can do both by following the official docs https://circe.github.io/circe/codec.html.
我怀疑您没有手动或（半）自动定义解码器。您可以按照官方文档https://circe.github.io/circe/codec.html进行这两种操作。
Unfortunately I cannot provide more detail than this because the question is rather vague. I will update my answer when more details are given.
不幸的是，我无法提供比这更多的细节，因为这个问题相当模糊。当提供更多详细信息时，我会更新我的答案。
