How to convert the datasets of Spark Row into string?

问题

I have written the code to access the Hive table using SparkSQL. Here is the code:

SparkSession spark = SparkSession
        .builder()
        .appName("Java Spark Hive Example")
        .master("local[*]")
        .config("hive.metastore.uris", "thrift://localhost:9083")
        .enableHiveSupport()
        .getOrCreate();
Dataset<Row> df =  spark.sql("select survey_response_value from health").toDF();
df.show();

I would like to know how I can convert the complete output to String or String array? As I am trying to work with another module where only I can pass String or String type Array values.
I have tried other methods like .toString or typecast to String values. But did not worked for me.
Kindly let me know how I can convert the DataSet values to String?

回答1:

Here is the sample code in Java.

public class SparkSample {
    public static void main(String[] args) {
        SparkSession spark = SparkSession
            .builder()
            .appName("SparkSample")
            .master("local[*]")
            .getOrCreate();
    //create df
    List<String> myList = Arrays.asList("one", "two", "three", "four", "five");
    Dataset<Row> df = spark.createDataset(myList, Encoders.STRING()).toDF();
    df.show();
    //using df.as
    List<String> listOne = df.as(Encoders.STRING()).collectAsList();
    System.out.println(listOne);
    //using df.map
    List<String> listTwo = df.map(row -> row.mkString(), Encoders.STRING()).collectAsList();
    System.out.println(listTwo);
  }
}

"row" is java 8 lambda parameter. Please check developer.com/java/start-using-java-lambda-expressions.html

回答2:

You can use the map function to convert every row into a string, e.g.:

df.map(row => row.mkString())

Instead of just mkString you can of course do more sophisticated work

The collect method then can retreive the whole thing into an array

val strings = df.map(row => row.mkString()).collect

(This is the Scala syntax, I think in Java it's quite similar)

来源：https://stackoverflow.com/questions/42389203/how-to-convert-the-datasets-of-spark-row-into-string