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import org.json4s.JsonAST.{JNull, JString}
import org.json4s.{DefaultFormats, Extraction, Formats}
import org.json4s.jackson.JsonMethods.render
import org.json4s.jackson.JsonMethods.pretty
case class Person(name: String, age: Int)
implicit val formats: Formats = DefaultFormats
val person = Person("iteblog", 110)
val jvalue = Extraction.decompose(person)
println(pretty(render(jvalue)))
输出
{
"name" : "iteblog",
"age" : 110
}
同时我们也可以直接将一个 json 对象转换成类对象：
val person = Extraction.extract[Person](jvalue) println(person) 输出 Person(iteblog,110)
我们可以看到上面的例子使用起来都很简单的。但是如果碰到元素的类型在 json4s 中没有事先定义，结果会怎么样呢？比如在 json4s 中并没有定义对 java.sql.Date 类型的解析，那如果我们用到了这个类型，会出现什么问题呢？具体如下：
case class Person(name: String, age: Int, birthday: Date)
implicit val formats: Formats = DefaultFormats
val person = Person("iteblog", 110, Date.valueOf("2019-07-01"))
val jvalue = Extraction.decompose(person)
println(pretty(render(jvalue)))
输出
{
"name" : "iteblog",
"age" : 110,
"birthday" : { }
}
可以从上面的结果看出，json4s 并没有正确的解析出 birthday 字段的值。 如果我们将 json 解析到类中，会出现什么问题呢？
val jvalue1 = parse("""{"name" : "iteblog", "age" : 110, "birthday" : "2019-07-01"}""")
val person = Extraction.extract[Person](jvalue1)
println(person)
结果
Exception in thread "main" org.json4s.package$MappingException: No usable value for birthday Parsed JSON values do not match with class constructor args= arg types= executable=Executable(Constructor(public java.sql.Date(int,int,int))) cause=wrong number of arguments types comparison result=MISSING(int),MISSING(int),MISSING(int) at org.json4s.reflect.package$.fail(package.scala:95) at org.json4s.Extraction$ClassInstanceBuilder.org$json4s$Extraction$ClassInstanceBuilder$$buildCtorArg(Extraction.scala:526) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$15.apply(Extraction.scala:546) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$15.apply(Extraction.scala:546) at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234) at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234) at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59) at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48) at scala.collection.TraversableLike$class.map(TraversableLike.scala:234) at scala.collection.AbstractTraversable.map(Traversable.scala:104) at org.json4s.Extraction$ClassInstanceBuilder.instantiate(Extraction.scala:546) at org.json4s.Extraction$ClassInstanceBuilder.result(Extraction.scala:597) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:400) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:392) at org.json4s.Extraction$.customOrElse(Extraction.scala:606) at org.json4s.Extraction$.extract(Extraction.scala:392) at org.json4s.Extraction$.extract(Extraction.scala:39) at com.iteblog.Test$.main(Test.scala:32) at com.iteblog.Test.main(Test.scala) Caused by: org.json4s.package$MappingException: Parsed JSON values do not match with class constructor args= arg types= executable=Executable(Constructor(public java.sql.Date(int,int,int))) cause=wrong number of arguments types comparison result=MISSING(int),MISSING(int),MISSING(int) at org.json4s.reflect.package$.fail(package.scala:95) at org.json4s.Extraction$ClassInstanceBuilder.instantiate(Extraction.scala:573) at org.json4s.Extraction$ClassInstanceBuilder.result(Extraction.scala:597) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:400) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:392) at org.json4s.Extraction$.customOrElse(Extraction.scala:606) at org.json4s.Extraction$.extract(Extraction.scala:392) at org.json4s.Extraction$ClassInstanceBuilder.org$json4s$Extraction$ClassInstanceBuilder$$buildCtorArg(Extraction.scala:514) ... 17 more
可以看出，出现无法解析出 birthday 字段，因为 json4s 并没有提供将字符串解析到 java.sql.Date。那怎么办呢？json4s 为我们提供了自定义解析类型的方法，那就是 CustomSerializer，我们只需要继承这个类，并实现对自定义类型的序列化和反序列化的方法即可。那对我们的例子可以实现如下：
package com.iteblog
import java.sql.Date
import org.json4s.JsonAST.{JNull, JString}
import org.json4s.{CustomSerializer, DefaultFormats, Extraction, Formats}
import org.json4s.jackson.JsonMethods.render
import org.json4s.jackson.JsonMethods.pretty
import org.json4s.jackson.JsonMethods.parse
object Iteblog {
case class Person(name: String, age: Int, birthday: Date)
case object DateSerializer extends CustomSerializer[Date](_ => ( {
case JString(s) => Date.valueOf(s)
case JNull => null
}, {
case d: Date => JString(d.toString)
}))
def main(args: Array[String]): Unit = {
implicit val formats: Formats = DefaultFormats + DateSerializer
val person = Person("iteblog", 110, Date.valueOf("2019-07-01"))
val jvalue = Extraction.decompose(person)
println(pretty(render(jvalue)))
val jvalue1 = parse("""{"name" : "iteblog", "age" : 110, "birthday" : "2019-07-01"}""")
val r = Extraction.extract[Person](jvalue1)
println(r)
}
}
输出
{
"name" : "iteblog",
"age" : 110,
"birthday" : "2019-07-01"
}
Person(iteblog,110,2019-07-01)
可见，通过自定义的 DateSerializer 我们可以解析 java.sql.Date 类型了。
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