Apache Spark：Scala XML 转换不可序列化？

Question

我有一堆带有 DTD 的 xml header，我正在尝试加载所有这些文件，忽略 DTD。

val input = sc.wholeTextFiles("""\path\*.nxml""")

val saxfac = SAXParserFactory.newInstance();
saxfac.setValidating(false);
saxfac.setFeature("http://xml.org/sax/features/validation", false);
saxfac.setFeature("http://apache.org/xml/features/nonvalidating/load-dtd-grammar", false);
saxfac.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
saxfac.setFeature("http://xml.org/sax/features/external-general-entities", false);
saxfac.setFeature("http://xml.org/sax/features/external-parameter-entities", false);

val loadnode = input.map { case (k,v) => xml.XML.withSAXParser(saxfac.newSAXParser()).loadString(v)}
println(loadnode.count())

我遇到了一个奇怪的错误....（由于 SAX 解析器）我做错了什么？

Exception in thread "main" org.apache.spark.SparkException: Task not serializable
    at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:166)
    at org.apache.spark.util.ClosureCleaner$.clean(ClosureCleaner.scala:158)
    at org.apache.spark.SparkContext.clean(SparkContext.scala:1435)
    at org.apache.spark.rdd.RDD.map(RDD.scala:271)
    at graphXtutorial.PubMedMainApp$.main(PubMedMainApp.scala:59)
    at graphXtutorial.PubMedMainApp.main(PubMedMainApp.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
Caused by: java.io.NotSerializableException: scala.xml.XML$$anon
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
    at org.apache.spark.serializer.JavaSerializationStream.writeObject(JavaSerializer.scala:42)
    at org.apache.spark.serializer.JavaSerializerInstance.serialize(JavaSerializer.scala:73)
    at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:164)
    ... 10 more

Answer 1

Spark 任务必须是 java 可序列化的，以便它们可以发送到其他集群节点到运行上。尝试在 map 内构造解析器，这样您就不会尝试在每个集群节点上使用单个共享解析器实例（或者，更好的是，使用类似 mapPartitions 的东西，这样您就可以构造一个解析器实例对于每个分区 - 为每一行构造一个分区可能会有很多开销）。

Answer 2

我知道我迟到了很多年，但我在挣扎中遇到了这个 post 所以想分享我的解决方案

class XMLParser extends Serializable { @transient lazy val parseXml = (xmlString: String) => {
if(null != xmlString && xmlString.startsWith("<")) {
  val parsedElem = scala.xml.XML.loadString(xmlString)
  val fields = parsedElem \ "field"
  fields.map(node =>
  Field((node \ "name").text,(node \ "key").text,(node \ "description").text,
  (node \ "fullPathKey").text,(node \ "value").text))
}else{
  Nil
}}}

解决任务不可序列化问题的一般方法是将不可序列化的代码标记为@transient lazy val，然后将其封装在可序列化的class 中。这样 Spark 将不会序列化变量，但只会为每个执行程序加载一次

Apache Spark：Scala XML 转换不可序列化？

Apache Spark: Scala XML transformation not serializable?

xml

scala

serialization

apache-spark