如何防止json4s呈现空值？

Question

How do I prevent json4s rendering null values when converting an object/JObject into a json string?

In Jackson you can do this by doing this:

mapper.setSerializationInclusion(Include.NON_NULL)

How can I do the same thing in json4s?

Example

import org.json4s.jackson.JsonMethods._
import org.json4s.jackson.Serialization
import org.json4s.{Extraction, NoTypeHints}

case class Book(title: String, author: String)

implicit val formats = Serialization.formats(NoTypeHints)

val bookJValue = Extraction.decompose(Book(null, "Arthur C. Clark"))
# JObject(List((title,JNull), (author,JString(Arthur C. Clark))))

val compacted = compact(render(bookJValue))
# {"title":null,"author":"Arthur C. Clark"}

I'd like the compacted json be this:

{"author":"Arthur C. Clark"}

Answer 1

case class Book(title: Option[String], author: String)
val b = Book(None, "Arthur C. Clark")
println(write(b))
res1:> {"author":"Arthur C. Clark"}

You can use Option to define variable. if it is none, it will not serialize this variable.

There is another way to do this by using removeField after you decompose your object , like:

bookJValue.removeFile {
   case (_, JNull) => true
   case _ => false
}

Answer 2

You can easily create your own custom serializer. Follow me.

First of all make small change in formats :

implicit val formats = DefaultFormats + new BookSerializer

After that build your own serializer/deserializer:

class BookSerializer extends CustomSerializer[Book](format => (
  {
    case JObject(JField("title", JString(t)) :: JField("author", JString(a)) ::Nil) =>
      new Book(t, a)
  },
  {
    case x @ Book(t: String, a: String) =>
      JObject(JField("title", JString(t)) ::
        JField("author", JString(a)) :: Nil)
    case Book(null, a: String) =>
      JObject(JField("author", JString(a)) :: Nil) // `title` == null
  }
  ))

The first part is deserializer (convert data from json to case class) and the second is serializer (conversion from case class to json). I've added case of title == null . You can easily add cases as many as you need.

The whole listing:

import org.json4s.jackson.JsonMethods._
import org.json4s.{DefaultFormats, Extraction}
import org.json4s._


case class Book(title: String, author: String)

implicit val formats = DefaultFormats + new BookSerializer
class BookSerializer extends CustomSerializer[Book](format => (
  {
    case JObject(JField("title", JString(t)) :: JField("author", JString(a)) ::Nil) =>
      new Book(t, a)
  },
  {
    case x @ Book(t: String, a: String) =>
      JObject(JField("title", JString(t)) ::
        JField("author",   JString(a)) :: Nil)
    case Book(null, a: String) =>
      JObject(JField("author",   JString(a)) :: Nil) // `title` == null
  }
  ))

val bookJValue = Extraction.decompose(Book(null, "Arthur C. Clark"))
val compacted = compact(render(bookJValue))

Output:

compacted: String = {"author":"Arthur C. Clark"}

You can find additional information on the page of json4s project .