使用 json4s 将 [String,Any] 映射到压缩 json 字符串 [英] Map[String,Any] to compact json string using json4s

问题描述

我目前正在从不同的数据源中提取一些指标并将它们存储在 Map[String,Any] 类型的映射中，其中键对应于指标名称，值对应于指标值.我需要它或多或少是通用的，这意味着值类型可以是原始类型或原始类型列表.

I am currently extracting some metrics from different data sources and storing them in a map of type Map[String,Any] where the key corresponds to the metric name and the value corresponds to the metric value. I need this to be more or less generic, which means that values types can be primitive types or lists of primitive types.

我想将此映射序列化为 JSON 格式的字符串，为此我使用了 json4s 库.问题是这似乎不可能，我看不到可能的解决方案.我希望像下面这样的东西开箱即用:)

I would like to serialize this map to a JSON-formatted string and for that I am using json4s library. The thing is that it does not seem possible and I don't see a possible solution for that. I would expect something like the following to work out of the box :)

val myMap: Map[String,Any] = ...    // extract metrics
val json = myMap.reduceLeft(_ ~ _)  // create JSON of metrics

浏览源代码我已经看到 json4s 提供隐式转换，以便将原始类型转换为 JValue 并转换 Traversable[A]/Map[String,A]/Option[A] 到 JValue 的(在从 A 到 JValue 的隐式转换可用的限制下，我理解它实际上意味着 A 是一种原始类型).~ 运算符提供了一种从 JField 构造 JObject 的好方法，它只是 (字符串，JValue).

Navigating through source code I've seen json4s provides implicit conversions in order to transform primitive types to JValue's and also to convert Traversable[A]/Map[String,A]/Option[A] to JValue's (under the restriction of being available an implicit conversion from A to JValue, which I understand it actually means A is a primitive type). The ~ operator offers a nice way of constructing JObject's out of JField's, which is just a type alias for (String, JValue).

在这种情况下，映射值类型是 Any，因此不会发生隐式转换，因此编译器会抛出以下错误:

In this case, map values type is Any, so implicit conversions don't take place and hence the compiler throws the following error:

                    value ~ is not a member of (String, Any)
[error]             val json = r.reduceLeft(_ ~ _)

是否有针对我想要完成的任务的解决方案?

Is there a solution for what I want to accomplish?

推荐答案

由于你实际上只是在寻找 myMap 的 JSON 字符串表示，你可以使用 Serialization直接对象.这是一个小例子(如果使用原生版本的 json4s 将导入更改为 org.json4s.native.Serialization):

Since you are actually only looking for the JSON string representation of myMap, you can use the Serialization object directly. Here is a small example (if using the native version of json4s change the import to org.json4s.native.Serialization):

EDIT:添加了格式隐式

 import org.json4s.jackson.Serialization

 implicit val formats = org.json4s.DefaultFormats

 val m: Map[String, Any] = Map(
   "name "-> "joe",
   "children" -> List(
     Map("name" -> "Mary", "age" -> 5),
     Map("name" -> "Mazy", "age" -> 3)
   )
 )
 // prints {"name ":"joe","children":[{"name":"Mary","age":5},{"name":"Mazy","age":3}]}
 println(Serialization.write(m)) 

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