Spark中json字符串和DataFrame相互转换

本文介绍基于Spark（2.0+）的Json字符串和DataFrame相互转换。

json字符串转DataFrame

spark提供了将json字符串解析为DF的接口，如果不指定生成的DF的schema，默认spark会先扫码一遍给的json字符串，然后推断生成DF的schema：

• 若列数据全为null会用String类型
• 整数默认会用Long类型
• 浮点数默认会用Double类型

val json1 = """{"a":null, "b": 23.1, "c": 1}"""
val json2 = """{"a":null, "b": "hello", "d": 1.2}"""

val ds = spark.createDataset(Seq(json1, json2))
val df = spark.read.json(ds)
df.show
df.printSchema

+----+-----+----+----+
|   a|    b|   c|   d|
+----+-----+----+----+
|null| 23.1|   1|null|
|null|hello|null| 1.2|
+----+-----+----+----+

root
 |-- a: string (nullable = true)
 |-- b: string (nullable = true)
 |-- c: long (nullable = true)
 |-- d: double (nullable = true)

若指定schema会按照schema生成DF：

• schema中不存在的列会被忽略
• 可以用两种方法指定schema，StructType和String，具体对应关系看后面
• 若数据无法匹配schema中类型：若schema中列允许为null会转为null；若不允许为null会转为相应类型的空值（如Double类型为0.0值），若无法转换为值会抛出异常

val schema = StructType(List(
        StructField("a", ByteType, true),
        StructField("b", FloatType, false),
        StructField("c", ShortType, true)
    ))
//或 val schema = "b float, c short"  
val df = spark.read.schema(schema).json(ds)
df.show
df.printSchema

+----+----+----+
|   a|   b|   c|
+----+----+----+
|null|23.1|   1|
|null|   0|null|
+----+----+----+

root
 |-- a: byte (nullable = true)
 |-- b: float (nullable = true)
 |-- c: short (nullable = true)

json解析相关配置参数

primitivesAsString (default false): 把所有列看作string类型
prefersDecimal(default false): 将小数看作decimal，如果不匹配decimal,就看做doubles.
allowComments (default false): 忽略json字符串中Java/C++风格的注释
allowUnquotedFieldNames (default false): 允许不加引号的列名
allowSingleQuotes (default true): 除双引号外，还允许用单引号
allowNumericLeadingZeros (default false): 允许数字中额外的前导0（如0012）
allowBackslashEscapingAnyCharacter (default false): 允许反斜杠机制接受所有字符
allowUnquotedControlChars (default false): 允许JSON字符串包含未加引号的控制字符（值小于32的ASCII字符，包括制表符和换行字符）。

mode (default PERMISSIVE): 允许在解析期间处理损坏记录的模式。

PERMISSIVE :当遇到损坏的记录时，将其他字段设置为null，并将格式错误的字符串放入由columnNameOfCorruptRecord配置的字段中。若指定schema，在schema中设置名为columnNameOfCorruptRecord的字符串类型字段。 如果schema中不具有该字段，则会在分析过程中删除损坏的记录。若不指定schema（推断模式），它会在输出模式中隐式添加一个columnNameOfCorruptRecord字段。
DROPMALFORMED : 忽略整条损害记录
FAILFAST : 遇到损坏记录throws an exception
columnNameOfCorruptRecord (默认值为spark.sql.columnNameOfCorruptRecord的值):允许PERMISSIVE mode添加的新字段，会重写spark.sql.columnNameOfCorruptRecord

dateFormat (default yyyy-MM-dd): 自定义日期格式，遵循java.text.SimpleDateFormat格式. 只有日期部分（无详细时间）
timestampFormat (default yyyy-MM-dd’T’HH:mm:ss.SSSXXX): 自定义日期格式，遵循java.text.SimpleDateFormat格式. 可以有详细时间部分（到微秒）
multiLine (default false): 解析一个记录，该记录可能跨越多行，每个文件

以上参数可用option方法配置：

val stringDF = spark.read.option("primitivesAsString", "true").json(ds)
stringDF.show
stringDF.printSchema

+----+-----+----+----+
|   a|    b|   c|   d|
+----+-----+----+----+
|null| 23.1|   1|null|
|null|hello|null| 1.2|
+----+-----+----+----+

root
 |-- a: string (nullable = true)
 |-- b: string (nullable = true)
 |-- c: string (nullable = true)
 |-- d: string (nullable = true)

二进制类型会自动用base64编码方式表示

‘Man’（ascci） base64编码后为：”TWFu”

val byteArr = Array('M'.toByte, 'a'.toByte, 'n'.toByte)
val binaryDs = spark.createDataset(Seq(byteArr))
val dsWithB64 = binaryDs.withColumn("b64", base64(col("value")))

dsWithB64.show(false)
dsWithB64.printSchema

+----------+----+
|value     |b64 |
+----------+----+
|[4D 61 6E]|TWFu|
+----------+----+

root
 |-- value: binary (nullable = true)
 |-- b64: string (nullable = true)

//=================================================

dsWithB64.toJSON.show(false)
+-----------------------------+
|value                        |
+-----------------------------+
|{"value":"TWFu","b64":"TWFu"}|
+-----------------------------+

//=================================================

val json = """{"value":"TWFu"}"""
val jsonDs = spark.createDataset(Seq(json))
val binaryDF = spark.read.schema("value binary").json(jsonDs )

binaryDF.show
binaryDF.printSchema

+----------+
|     value|
+----------+
|[4D 61 6E]|
+----------+

root
 |-- value: binary (nullable = true)

指定schema示例：

以下是Spark SQL支持的所有基本类型：

val json = """{"stringc":"abc", "shortc":1, "integerc":null, "longc":3, "floatc":4.5, "doublec":6.7, "decimalc":8.90, "booleanc":true, "bytec":23, "binaryc":"TWFu", "datec":"2010-01-01", "timestampc":"2012-12-12 11:22:22.123123"}"""
val ds = spark.createDataset(Seq(json))
val schema = "stringc string, shortc short, integerc int, longc long, floatc float, doublec double, decimalc decimal(10, 3), booleanc boolean, bytec byte, binaryc binary, datec date, timestampc timestamp"
val df = spark.read.schema(schema).json(ds)
df.show(false)
df.printSchema

+-------+------+--------+-----+------+-------+--------+--------+-----+----------+----------+-----------------------+
|stringc|shortc|integerc|longc|floatc|doublec|decimalc|booleanc|bytec|binaryc   |datec     |timestampc             |
+-------+------+--------+-----+------+-------+--------+--------+-----+----------+----------+-----------------------+
|abc    |1     |null    |3    |4.5   |6.7    |8.900   |true    |23   |[4D 61 6E]|2010-01-01|2012-12-12 11:22:22.123|
+-------+------+--------+-----+------+-------+--------+--------+-----+----------+----------+-----------------------+

root
 |-- stringc: string (nullable = true)
 |-- shortc: short (nullable = true)
 |-- integerc: integer (nullable = true)
 |-- longc: long (nullable = true)
 |-- floatc: float (nullable = true)
 |-- doublec: double (nullable = true)
 |-- decimalc: decimal(10,3) (nullable = true)
 |-- booleanc: boolean (nullable = true)
 |-- bytec: byte (nullable = true)
 |-- binaryc: binary (nullable = true)
 |-- datec: date (nullable = true)
 |-- timestampc: timestamp (nullable = true)

复合类型：

val json = """
{
  "arrayc" : [ 1, 2, 3 ],
  "structc" : {
    "strc" : "efg",
    "decimalc" : 1.1
  },
  "mapc" : {
    "key1" : 1.2,
    "key2" : 1.1
  }
}
"""
val ds = spark.createDataset(Seq(json))
val schema = "arrayc array<short>, structc struct<strc:string, decimalc:decimal>, mapc map<string, float>"
val df = spark.read.schema(schema).json(ds)
df.show(false)
df.printSchema

+---------+--------+--------------------------+
|arrayc   |structc |mapc                      |
+---------+--------+--------------------------+
|[1, 2, 3]|[efg, 1]|[key1 -> 1.2, key2 -> 1.1]|
+---------+--------+--------------------------+

root
 |-- arrayc: array (nullable = true)
 |    |-- element: short (containsNull = true)
 |-- structc: struct (nullable = true)
 |    |-- strc: string (nullable = true)
 |    |-- decimalc: decimal(10,0) (nullable = true)
 |-- mapc: map (nullable = true)
 |    |-- key: string
 |    |-- value: float (valueContainsNull = true)

SparkSQL数据类型

基本类型：

三个复合类型：