在写spark项目时遇到scala对象与json对象之间转换的问题，花点时间总结一下。
Json4s介绍
Json4s下面有两个类型的包
- Lift Json
import org.json4s._
import org.json4s.native.JsonMethods._
native package具有lift-json的所有功能。
- Jackson
import org.json4s._
import org.josn4s.jackson.JsonMethods._
与native package不同的是，jackson包含了大部分jackson-module-scala的功能，也可以使用lift-json下的所有功能。在这个项目中我们使用jackson。
Json AST
lift-json library 的核心就是Json AST，它将Json对象转换成了语法树。
sealed abstract class JValue
case object JNothing extends JValue // 'zero' for JValue
case object JNull extends JValue
case class JString(s: String) extends JValue
case class JDouble(num: Double) extends JValue
case class JDecimal(num: BigDecimal) extends JValue
case class JInt(num: BigInt) extends JValue
case class JBool(value: Boolean) extends JValue
case class JObject(obj: List[JField]) extends JValue
case class JArray(arr: List[JValue]) extends JValue
type JField = (String, JValue)
转换的常见方式有：
- Json DSL(implicit)
- String parse
- case class
下面一一介绍。
Json4s安装
- SBT users
- native support
val json4sNative = "org.json4s" %% "json4s-native" % "{latestVersion}"
- jackson support
val json4sJackson = "org.json4s" %% "json4s-jackson" % "{latestVersion}"
- Maven users
- native support
<dependency>
<groupId>org.json4s</groupId>
<artifactId>json4s-native_${scala.version}</artifactId>
<version>{latestVersion}</version>
</dependency>
- jackson support
<dependency>
<groupId>org.json4s</groupId>
<artifactId>json4s-jackson_${scala.version}</artifactId>
<version>{latestVersion}</version>
</dependency>
JSON转换
这里我们都以jackson为例，native support请参考http://json4s.org/
一个有效的json对象可以转换成AST格式
String parse
import org.json4s._
import org.json4s.jackson.JsonMethods._
parse("""{ "numbers" : [1, 2, 3, 4] }""")
parse("""{"name":"Toy","price":35.35}""", useBigDecimalForDouble = true)
DSL Parse
任意scala原始对象与json原始对象之间的转换
#List to json
scala> val json = List(1, 2, 3)
scala> compact(render(json))
res0: String = [1,2,3]
#Map to json
scala> val json = ("name" -> "joe")
scala> compact(render(json))
res1: String = {"name":"joe"}
#当有多行数据时，用~符号连接
scala> val json = ("name" -> "joe") ~ ("age" -> 35)
scala> compact(render(json))
res2: String = {"name":"joe","age":35}
case class
当你有自定义的数据类型想进行转换时，可以使用下面的方法
type DslConversion = T => JValue
object JsonExample extends App {
import org.json4s._
import org.json4s.JsonDSL._
import org.json4s.jackson.JsonMethods._
#定义类 Winner, Lotto
case class Winner(id: Long, numbers: List[Int])
case class Lotto(id: Long, winningNumbers: List[Int], winners: List[Winner], drawDate: Option[java.util.Date])
val winners = List(Winner(23, List(2, 45, 34, 23, 3, 5)), Winner(54, List(52, 3, 12, 11, 18, 22)))
val lotto = Lotto(5, List(2, 45, 34, 23, 7, 5, 3), winners, None)
#转换json
val json =
("lotto" ->
("lotto-id" -> lotto.id) ~
("winning-numbers" -> lotto.winningNumbers) ~
("draw-date" -> lotto.drawDate.map(_.toString)) ~
("winners" ->
lotto.winners.map { w =>
(("winner-id" -> w.id) ~
("numbers" -> w.numbers))}))
println(compact(render(json)))
}
scala> JsonExample
{"lotto":{"lotto-id":5,"winning-numbers":[2,45,34,23,7,5,3],"winners":
[{"winner-id":23,"numbers":[2,45,34,23,3,5]},{"winner-id":54,"numbers":[52,3,12,11,18,22]}]}}
如果想将结果打印出来，可以使用
scala> pretty(render(JsonExample.json))
{
"lotto":{
"lotto-id":5,
"winning-numbers":[2,45,34,23,7,5,3],
"winners":[{
"winner-id":23,
"numbers":[2,45,34,23,3,5]
},{
"winner-id":54,
"numbers":[52,3,12,11,18,22]
}]
}
}
