Github user makagonov commented on a diff in the pull request: https://github.com/apache/spark/pull/20884#discussion_r176842732 --- Diff: sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/json/JacksonGeneratorSuite.scala --- @@ -56,7 +56,7 @@ class JacksonGeneratorSuite extends SparkFunSuite { val gen = new JacksonGenerator(dataType, writer, option) gen.write(input) gen.flush() - assert(writer.toString === """[{}]""") + assert(writer.toString === """[{"a":null}]""") --- End diff -- @HyukjinKwon actually, it looks like the result should be `[null]` rather than `[{}]`. Look at the following repro from spark-shell (downloaded binaries): ```scala scala> val df = sqlContext.sql(""" select array(cast(null as struct<k:string>)) as my_array""") df: org.apache.spark.sql.DataFrame = [my_array: array<struct<k:string>>] scala> df.printSchema root |-- my_array: array (nullable = false) | |-- element: struct (containsNull = true) | | |-- k: string (nullable = true) scala> df.toJSON.collect().foreach(println) {"my_array":[null]} scala> df.select(to_json($"my_array")).collect().foreach(x => println(x(0))) [null] ``` In older version of `JacksonGenerator`, we had a filter by element value, and if it was `null`, `gen.writeNull()` was called no matter what the type was ([old implementation](https://github.com/apache/spark/blob/3258f27a881dfeb5ab8bae90c338603fa4b6f9d8/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/json/JacksonGenerator.scala#L41)). But currently, we're calling `gen.writeStartObject()...gen.writeEndObject()` no matter if the value is null. I couldn't repro this with a query, but when `StructsToJson` is called from this unit test, it goes through `JacksonGenerator.arrElementWriter` which has lines ```scala case st: StructType => (arr: SpecializedGetters, i: Int) => { writeObject(writeFields(arr.getStruct(i, st.length), st, rootFieldWriters)) } ``` that makes it print json object even there is `null`. I'll look into this later and will try to find the easy workaround.
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