我使用下面的代码来提取sparksql中需要的字符串。但是现在我正在spark hadoop中处理更多的数据，我想提取字符串。我尝试了相同的代码，但不起作用。

val sparkConf = new SparkConf().setAppName("myapp").setMaster("local[*]")
val sc = new SparkContext(sparkConf)

val sqlContext = new org.apache.spark.sql.SQLContext(sc)
import sqlContext.implicits._
import org.apache.spark.sql.functions.{col, udf}
import java.util.regex.Pattern

//User Defined function to extract
def toExtract(str: String) = {      
  val pattern = Pattern.compile("@\\w+")
  val tmplst = scala.collection.mutable.ListBuffer.empty[String]
  val matcher = pattern.matcher(str)
  while (matcher.find()) {
    tmplst += matcher.group()
  }
  tmplst.mkString(",")
}

val Extract = udf(toExtract _)
val values = List("@always_nidhi @YouTube no i dnt understand bt i loved the music nd their dance awesome all the song of this mve is rocking")
val df = sc.parallelize(values).toDF("words")
df.select(Extract(col("words"))).show()

如何解决这个问题？