How to convert Row to json in Spark 2 Scala

问题:

Is there a simple way to converting a given Row object to json?

Found this about converting a whole Dataframe to json output: Spark Row to JSON

But I just want to convert a one Row to json. Here is pseudo code for what I am trying to do.

More precisely I am reading json as input in a Dataframe. I am producing a new output that is mainly based on columns, but with one json field for all the info that does not fit into the columns.

My question what is the easiest way to write this function: convertRowToJson()

def convertRowToJson(row: Row): String = ???

def transformVenueTry(row: Row): Try[Venue] = {
  Try({
    val name = row.getString(row.fieldIndex("name"))
    val metadataRow = row.getStruct(row.fieldIndex("meta"))
    val score: Double = calcScore(row)
    val combinedRow: Row = metadataRow ++ ("score" -> score)
    val jsonString: String = convertRowToJson(combinedRow)
    Venue(name = name, json = jsonString)
  })
}

Psidom's Solutions:

def convertRowToJSON(row: Row): String = {
    val m = row.getValuesMap(row.schema.fieldNames)
    JSONObject(m).toString()
}

only works if the Row only has one level not with nested Row. This is the schema:

StructType(
    StructField(indicator,StringType,true),   
    StructField(range,
    StructType(
        StructField(currency_code,StringType,true),
        StructField(maxrate,LongType,true), 
        StructField(minrate,LongType,true)),true))

Also tried Artem suggestion, but that did not compile:

def row2DataFrame(row: Row, sqlContext: SQLContext): DataFrame = {
  val sparkContext = sqlContext.sparkContext
  import sparkContext._
  import sqlContext.implicits._
  import sqlContext._
  val rowRDD: RDD[Row] = sqlContext.sparkContext.makeRDD(row :: Nil)
  val dataFrame = rowRDD.toDF() //XXX does not compile
  dataFrame
}

回答1:

You can use getValuesMap to convert the row object to a Map and then convert it JSON:

import scala.util.parsing.json.JSONObject
import org.apache.spark.sql._

val df = Seq((1,2,3),(2,3,4)).toDF("A", "B", "C")    
val row = df.first()          // this is an example row object

def convertRowToJSON(row: Row): String = {
    val m = row.getValuesMap(row.schema.fieldNames)
    JSONObject(m).toString()
}

convertRowToJSON(row)
// res46: String = {"A" : 1, "B" : 2, "C" : 3}

回答2:

I need to read json input and produce json output. Most fields are handled individually, but a few json sub objects need to just be preserved.

When Spark reads a dataframe it turns a record into a Row. The Row is a json like structure. That can be transformed and written out to json.

But I need to take some sub json structures out to a string to use as a new field.

This can be done like this:

dataFrameWithJsonField = dataFrame.withColumn("address_json", to_json($"location.address"))

location.address is the path to the sub json object of the incoming json based dataframe. address_json is the column name of that object converted to a string version of the json.

to_json is implemented in Spark 2.1.

If generating it output json using json4s address_json should be parsed to an AST representation otherwise the output json will have the address_json part escaped.

回答3:

Essentially, you can have a dataframe which contains just one row. Thus, you can try to filter your initial dataframe and then parse it to json.

回答4:

JSon has schema but Row doesn't have a schema, so you need to apply schema on Row & convert to JSon. Here is how you can do it.

import org.apache.spark.sql.Row
import org.apache.spark.sql.types._

def convertRowToJson(row: Row): String = {

  val schema = StructType(
      StructField("name", StringType, true) ::
      StructField("meta", StringType, false) ::  Nil)

      return sqlContext.applySchema(row, schema).toJSON
}

回答5:

Pay attention scala class scala.util.parsing.json.JSONObject is deprecated and not support null values.

@deprecated("This class will be removed.", "2.11.0")

"JSONFormat.defaultFormat doesn't handle null values"

https://issues.scala-lang.org/browse/SI-5092

回答6:

I combining the suggestion from: Artem, KiranM and Psidom. Did a lot of trails and error and came up with this solutions that I tested for nested structures:

def row2Json(row: Row, sqlContext: SQLContext): String = {
  import sqlContext.implicits
  val rowRDD: RDD[Row] = sqlContext.sparkContext.makeRDD(row :: Nil)
  val dataframe = sqlContext.createDataFrame(rowRDD, row.schema)
  dataframe.toJSON.first
}

This solution worked, but only while running in driver mode.

回答7:

I had the same issue, I had parquet files with canonical schema (no arrays), and I only want to get json events. I did as follows, and it seems to work just fine (Spark 2.1):

import org.apache.spark.sql.types.StructType
import org.apache.spark.sql.{DataFrame, Dataset, Row}
import scala.util.parsing.json.JSONFormat.ValueFormatter
import scala.util.parsing.json.{JSONArray, JSONFormat, JSONObject}

def getValuesMap[T](row: Row, schema: StructType): Map[String,Any] = {
  schema.fields.map {
    field =>
      try{
        if (field.dataType.typeName.equals("struct")){
          field.name -> getValuesMap(row.getAs[Row](field.name),   field.dataType.asInstanceOf[StructType]) 
        }else{
          field.name -> row.getAs[T](field.name)
        }
      }catch {case e : Exception =>{field.name -> null.asInstanceOf[T]}}
  }.filter(xy => xy._2 != null).toMap
}

def convertRowToJSON(row: Row, schema: StructType): JSONObject = {
  val m: Map[String, Any] = getValuesMap(row, schema)
  JSONObject(m)
}
//I guess since I am using Any and not nothing the regular ValueFormatter is not working, and I had to add case jmap : Map[String,Any] => JSONObject(jmap).toString(defaultFormatter)
val defaultFormatter : ValueFormatter = (x : Any) => x match {
  case s : String => "\"" + JSONFormat.quoteString(s) + "\""
  case jo : JSONObject => jo.toString(defaultFormatter)
  case jmap : Map[String,Any] => JSONObject(jmap).toString(defaultFormatter)
  case ja : JSONArray => ja.toString(defaultFormatter)
  case other => other.toString
}

val someFile = "s3a://bucket/file"
val df: DataFrame = sqlContext.read.load(someFile)
val schema: StructType = df.schema
val jsons: Dataset[JSONObject] = df.map(row => convertRowToJSON(row, schema))