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Lift Framework example source code file (Xml.scala)

This example Lift Framework source code file (Xml.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lift Framework tags/keywords

jobject, list, list, nil, nil, node, node, nodeseq, string, xelem, xelem, xleaf, xmlelem, xvalue

The Lift Framework Xml.scala source code

/*
 * Copyright 2009-2010 WorldWide Conferencing, LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package net.liftweb
package json

/** Functions to convert between JSON and XML.
 */
object Xml {
  import scala.xml._

  /** Convert given XML to JSON.
   * <p>
   * Following rules are used in conversion.
   * <ul>
   *   <li>XML leaf element is converted to JSON string
   *   <li>XML parent element is converted to JSON object and its children to JSON fields
   *   <li>XML elements with same name at same level are converted to JSON array
   *   <li>XML attributes are converted to JSON fields
   * </ul>
   * <p>
   * Example:<pre>
   * scala> val xml =
   *     <users>
   *       <user>
   *         <id>1</id>
   *         <name>Harry</name>
   *       </user>
   *       <user>
   *         <id>2</id>
   *         <name>David</name>
   *       </user>
   *     </users>   
   *
   * scala> val json = toJson(xml)
   * scala> pretty(render(json))
   * 
   * {
   *   "users":{
   *     "user":[{
   *       "id":"1",
   *       "name":"Harry"
   *     },{
   *       "id":"2",
   *       "name":"David"
   *     }]
   *   }
   * }
   * </pre>
   *
   * Now, the above example has two problems. First, the id is converted to String while
   * we might want it as an Int. This is easy to fix by mapping JString(s) to JInt(s.toInt).
   * The second problem is more subtle. The conversion function decides to use JSON array
   * because there's more than one user-element in XML. Therefore a structurally equivalent
   * XML document which happens to have just one user-element will generate a JSON document
   * without JSON array. This is rarely a desired outcome. These both problems can be fixed
   * by following map function.
   * <pre>
   * json map {
   *   case JField("id", JString(s)) => JField("id", JInt(s.toInt))
   *   case JField("user", x: JObject) => JField("user", JArray(x :: Nil))
   *   case x => x 
   * }
   * </pre>
   */
  def toJson(xml: NodeSeq): JValue = {
    def empty_?(node: Node) = node.child.isEmpty

    /* Checks if given node is leaf element. For instance these are considered leafs:
     * <foo>bar, { doSomething() }, etc.
     */
    def leaf_?(node: Node) = {
      def descendant(n: Node): List[Node] = n match {
        case g: Group => g.nodes.toList.flatMap(x => x :: descendant(x))
        case _ => n.child.toList.flatMap { x => x :: descendant(x) }
      }

      !descendant(node).find(_.isInstanceOf[Elem]).isDefined
    }

    def array_?(nodeNames: Seq[String]) = nodeNames.size != 1 && nodeNames.toList.distinct.size == 1
    def directChildren(n: Node): NodeSeq = n.child.filter(c => c.isInstanceOf[Elem])
    def nameOf(n: Node) = (if (n.prefix ne null) n.prefix + ":" else "") + n.label
    def buildAttrs(n: Node) = n.attributes.map((a: MetaData) => (a.key, XValue(a.value.text))).toList

    sealed trait XElem
    case class XValue(value: String) extends XElem
    case class XLeaf(value: (String, XElem), attrs: List[(String, XValue)]) extends XElem
    case class XNode(fields: List[(String, XElem)]) extends XElem
    case class XArray(elems: List[XElem]) extends XElem

    def toJValue(x: XElem): JValue = x match {
      case XValue(s) => JString(s)
      case XLeaf((name, value), attrs) => (value, attrs) match {
        case (_, Nil) => toJValue(value)
        case (XValue(""), xs) => JObject(mkFields(xs))
        case (_, xs) => JObject(JField(name, toJValue(value)) :: mkFields(xs))
      }
      case XNode(xs) => JObject(mkFields(xs))
      case XArray(elems) => JArray(elems.map(toJValue))
    }

    def mkFields(xs: List[(String, XElem)]) = 
      xs.flatMap { case (name, value) => (value, toJValue(value)) match {
        // This special case is needed to flatten nested objects which resulted from
        // XML attributes. Flattening keeps transformation more predicatable.  
        // <a>x -> {"a":{"foo":{"foo":"x","id":"1"}}} vs
        // <a>x -> {"a":{"foo":"x","id":"1"}}
        case (XLeaf(v, x :: xs), o: JObject) => o.obj
        case (_, json) => JField(name, json) :: Nil }}

    def buildNodes(xml: NodeSeq): List[XElem] = xml match {
      case n: Node =>
        if (empty_?(n)) XLeaf((nameOf(n), XValue("")), buildAttrs(n)) :: Nil
        else if (leaf_?(n)) XLeaf((nameOf(n), XValue(n.text)), buildAttrs(n)) :: Nil
        else {
          val children = directChildren(n)
          XNode(buildAttrs(n) ::: children.map(nameOf).toList.zip(buildNodes(children))) :: Nil
        }
      case nodes: NodeSeq => 
        val allLabels = nodes.map(_.label)
        if (array_?(allLabels)) {
          val arr = XArray(nodes.toList.flatMap { n => 
            if (leaf_?(n) && n.attributes.length == 0) XValue(n.text) :: Nil
            else buildNodes(n)
          })
          XLeaf((allLabels(0), arr), Nil) :: Nil
        } else nodes.toList.flatMap(buildNodes)
    }
    
    buildNodes(xml) match {
      case List(x @ XLeaf(_, _ :: _)) => toJValue(x)
      case List(x) => JObject(JField(nameOf(xml.head), toJValue(x)) :: Nil)
      case x => JArray(x.map(toJValue))
    }
  }

  /** Convert given JSON to XML.
   * <p>
   * Following rules are used in conversion.
   * <ul>
   *   <li>JSON primitives are converted to XML leaf elements
   *   <li>JSON objects are converted to XML elements
   *   <li>JSON arrays are recursively converted to XML elements
   * </ul>
   * <p>
   * Use <code>map function to preprocess JSON before conversion to adjust
   * the end result. For instance a common conversion is to encode arrays as comma
   * separated Strings since XML does not have array type.
   * <p>
   * toXml(json map {
   *   case JField("nums",JArray(ns)) => JField("nums",JString(ns.map(_.values).mkString(",")))
   *   case x => x
   * })
   * </pre>
   */
  def toXml(json: JValue): NodeSeq = {
    def toXml(name: String, json: JValue): NodeSeq = json match {
      case JObject(fields) => new XmlNode(name, fields flatMap { f => toXml(f.name, f.value) })
      case JArray(xs) => xs flatMap { v => toXml(name, v) }
      case JField(n, v) => new XmlNode(name, toXml(n, v))
      case JInt(x) => new XmlElem(name, x.toString)
      case JDouble(x) => new XmlElem(name, x.toString)
      case JString(x) => new XmlElem(name, x)
      case JBool(x) => new XmlElem(name, x.toString)
      case JNull => new XmlElem(name, "null")
      case JNothing => Text("")
    }

    json match {
      case JField(n, v) => toXml(n, v)
      case JObject(fields) => fields flatMap { f => toXml(f.name, f.value) }
      case x => toXml("root", x)
    }
  }

  private[json] class XmlNode(name: String, children: Seq[Node]) extends Elem(null, name, xml.Null, TopScope, children :_*)

  private[json] class XmlElem(name: String, value: String) extends Elem(null, name, xml.Null, TopScope, Text(value))
}

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