如何从parquet文件中读取和编写自定义类

Question

我正在尝试使用DataFrame/datassets为特定类类型编写parquet读/写类

类模式:

class A {
  long count;
  List<B> listOfValues;
}
class B {
  String id;
  long count;
}

代码:

  String path = "some path";
  List<A> entries = somerandomAentries();
  JavaRDD<A> rdd = sc.parallelize(entries, 1);
  DataFrame df = sqlContext.createDataFrame(rdd, A.class);
  df.write().parquet(path);
  DataFrame newDataDF = sqlContext.read().parquet(path);
  newDataDF.show();

当我尝试运行它时，它抛出了一个错误。我错过了什么?在创建数据框架时，我需要为整个类提供模式吗错误:

    Caused by: scala.MatchError: B(Id=abc, count=0) (of class B)
    at org.apache.spark.sql.catalyst.CatalystTypeConverters$StructConverter.toCatalystImpl(CatalystTypeConverters.scala:255)
    at org.apache.spark.sql.catalyst.CatalystTypeConverters$StructConverter.toCatalystImpl(CatalystTypeConverters.scala:250)
    at org.apache.spark.sql.catalyst.CatalystTypeConverters$CatalystTypeConverter.toCatalyst(CatalystTypeConverters.scala:102)
    at org.apache.spark.sql.catalyst.CatalystTypeConverters$ArrayConverter.toCatalystImpl(CatalystTypeConverters.scala:169)
    at org.apache.spark.sql.catalyst.CatalystTypeConverters$ArrayConverter.toCatalystImpl(CatalystTypeConverters.scala:153)
    at org.apache.spark.sql.catalyst.CatalystTypeConverters$CatalystTypeConverter.toCatalyst(CatalystTypeConverters.scala:102)
    at org.apache.spark.sql.catalyst.CatalystTypeConverters$$anonfun$createToCatalystConverter$2.apply(CatalystTypeConverters.scala:401)
    at org.apache.spark.sql.SQLContext$$anonfun$org$apache$spark$sql$SQLContext$$beansToRows$1$$anonfun$apply$1.apply(SQLContext.scala:1358)
    at org.apache.spark.sql.SQLContext$$anonfun$org$apache$spark$sql$SQLContext$$beansToRows$1$$anonfun$apply$1.apply(SQLContext.scala:1358)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
    at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
    at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:108)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
    at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:108)
    at org.apache.spark.sql.SQLContext$$anonfun$org$apache$spark$sql$SQLContext$$beansToRows$1.apply(SQLContext.scala:1358)
    at org.apache.spark.sql.SQLContext$$anonfun$org$apache$spark$sql$SQLContext$$beansToRows$1.apply(SQLContext.scala:1356)
    at scala.collection.Iterator$$anon$11.next(Iterator.scala:328)
    at org.apache.spark.sql.execution.datasources.DefaultWriterContainer.writeRows(WriterContainer.scala:263)
    ... 8 more

Answer 1

你得到错误，因为嵌套JavaBeans不支持Spark 1.6版本。请参阅https://spark.apache.org/docs/1.6.0/sql-programming-guide.html#inferring-the-schema-using-reflection

目前，Spark SQL不支持包含嵌套或包含复杂类型(如列表或数组)的JavaBeans。