How to extract data from XML nodes in Scala
By Alvin Alexander. Last updated: June 3 2016
Problem: In a Scala application,you want to extract information from XML you receive,so you can use the data in your application. Solution Use the methods of theScala Commonly used methods of the Elem class Method Description ------ ----------- x "div" Searches the XML literal x for elements of type <div>. Only searches immediate child nodes (no grandchild or “descendant” nodes). x "div" Searches the XML literal x for elements of type <div>. Returns matching elements from child nodes at any depth of the XML tree. x.attribute("class") Returns the value of the given attribute in the current node. <a x="10" y="20">foo</a>.attribute("x") // returns Some(10). x.attributes Returns all attributes of the current node,prefixed and unprefixed,in no particular order. scala> <a x="10" y="20">foo</a>.attributes res0: scala.xml.MetaData = x="10" y="20" x.child Returns the children of the current node. <a><b>foo</b></a>.child // returns <b>foo</b>. x.copy(...) Returns a copy of the element,letting you replace data during the copy process. x.label The name of the current element. <a><b>foo</b></a>.label // returns a. x.text Returns a concatenation of text(n) for each child n. x.toString Emits the XML literal as a String. Use scala.xml.PrettyPrinter to format the output,if desired. ExamplesThe following examples demonstrate most of the methods just shown. Given this XML literal: scala> val x = <div class="content"><p>Hello</p><p>world</p></div> x: scala.xml.Elem = <div class="content"><p>Hello</p><p>world</p></div> you can search for and extract subelements with the scala> x "p" res0: scala.xml.NodeSeq = NodeSeq(<p>Hello</p>,<p>world</p>) scala> x "p" res1: scala.xml.NodeSeq = NodeSeq(<p>Hello</p>,<p>world</p>) These methods will be demonstrated more in subsequent recipes. The label method returns the name of the current element. A scala> x.label res2: String = div scala> <name>Joe</name>.label res3: String = name The scala> x.text res4: String = Helloworld Later examples will demonstrate how to improve on this result. Element attributes are extracted with the scala> x.attribute("class") res5: Option[Seq[scala.xml.Node]] = Some(content) scala> x.attributes("class") res6: Seq[scala.xml.Node] = content scala> x.attributes.get("class") res7: Option[Seq[scala.xml.Node]] = Some(content) The following examples demonstrate how those same method calls behave when you search for an attribute that doesn’t exist: scala> x.attribute("foo") res8: Option[Seq[scala.xml.Node]] = None scala> x.attributes("foo") res9: Seq[scala.xml.Node] = null scala> x.attributes.get("foo") res10: Option[Seq[scala.xml.Node]] = None scala> x.attributes.get("foo").getOrElse("N/A") res11: Object = N/A To demonstrate more ways to work with element attributes,let’s create a new element: scala> val w = <forecast day="Thu" date="10 Nov 2011" low="37" high="58" /> w: scala.xml.Elem = <forecast day="Thu" date="10 Nov 2011" low="37" high="58" /> These examples show how scala> w.attribute("day") res0: Option[Seq[scala.xml.Node]] = Some(Thu) scala> w.attributes("day") res1: Seq[scala.xml.Node] = Thu scala> w.attributes res2: scala.xml.MetaData = day="Thu" date="10 Nov 2011" low="37" high="58" These examples show how to iterate over a set of attributes: scala> for (a <- w.attributes) println(s"key: ${a.key},value: ${a.value}") key: day,value: Thu key: date,value: 10 Nov 2011 key: low,value: 37 key: high,value: 58 scala> w.attributes.asAttrMap res3: Map[String,String] = Map(low -> 37,date -> 10 Nov 2011,day -> Thu,high -> 58) Child elementschildmethod returns all child nodes of the current element. To demonstrate this,let’s create a new XML variable: scala> val p = <person><name>Ken</name><age>23</age></person> p: scala.xml.Elem = <person><name>Ken</name><age>23</age></person> childmethod returns immediate child nodes: scala> p.child res0: Seq[scala.xml.Node] = ArrayBuffer(<name>Ken</name>,<age>23</age>) You can use scala> for (n <- p.child) println(n) <name>Ken</name> <age>23</age> Because scala> p.child(0) res1: scala.xml.Node = <name>Ken</name> scala> p.child(0).label res2: String = name scala> p.child(0).text res3: String = Ken scala> p.child(1) res4: scala.xml.Node = <age>23</age> scala> p.child(1).text.toInt res5: Int = 23 Text and stringstoStringmethod returns the XML structure as a scala> p.toString res6: String = <person><name>Ken</name><age>23</age></person> You can improve this result withthe This approach shows another way to extract the text from the elements: scala> for (n <- p.child) yield n.text res7: Seq[String] = ArrayBuffer(Ken,23) There are more ways to tackle these problems using XPath methods,which will be shown in subsequent chapters. As a word of caution,be careful with the scala> <div><p>Hello,world,<br/>it's me.</p></div>.text res0: String = Hello,it's me. scala> <div><p>Hello,<br/>it's me.</p></div>.text res1: String = Hello,it's me. In the next examples the same XML,formatted in different ways,yields different results: scala> <div><p>Is 2 > 1?</p><p>Why do you ask?</p></div>.text res2: String = Is 2 > 1?Why do you ask? scala> <div> | <p>Is 2 > 1?</p> | <p>Why do you ask?</p> | </div>.text res3: String = " Is 2 > 1? Why do you ask? " If you need to extract text in this manner,a workaround is to extract the text components individually into a sequence,and then re-combine the sequence as desired. The following example demonstrates how to accomplish this with the val xml = <div><p>Is 2 > 1?</p><p>Why do you ask?</p></div> the scala> xml.child res0: Seq[scala.xml.Node] = ArrayBuffer(<p>Is 2 > 1?</p>,<p>Why do you ask?</p>) This lets you write the following code,which creates a sequence of strings from the val strings = for { e <- xml.child if e.label == "p" } yield e.text The REPL shows that the resulting variable strings has the following type and data: strings: Seq[String] = ArrayBuffer(Is 2 > 1?,Why do you ask?) In the XPath recipes in this chapter you’ll see how to accomplish some of the same tasks using the
Example data sets and REPL memory errorsIf you want to test these commands against large data sets,this URL maintains a nice collection of sample XML data:
The NASA data set is 23 MB,and causes the Scala REPL to crash with a Java heap space error: scala> val xml = scala.xml.XML.loadFile("nasa.xml") java.lang.OutOfMemoryError: Java heap space ... To get around this problem,you can allocate more heap space when starting the REPL with this command: $ scala -J-Xms256m -J-Xmx512m or this command: $ env JAVA_OPTS="-Xms256m -Xmx512m" scala (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |