Use a ThreadLocal to allow reusing parser instances

Currently scala-xml creates a new SAXParser instance every time. This
is not very good for performance, but explained by the fact that
SAXParser is not thread safe. We can greatly increase performance by
using a ThreadLocal, giving each thread their own instance.

Author: fgoepel

shared/src/main/scala/scala/xml/factory/XMLLoader.scala

@@ -23,13 +23,18 @@ trait XMLLoader[T <: Node] {
   import scala.xml.Source._
   def adapter: FactoryAdapter = new NoBindingFactoryAdapter()
 
-  /* Override this to use a different SAXParser. */
-  def parser: SAXParser = {
-    val f = SAXParserFactory.newInstance()
-    f.setNamespaceAware(false)
-    f.newSAXParser()
+  private lazy val parserInstance = new ThreadLocal[SAXParser] {
+    override def initialValue = {
+      val parser = SAXParserFactory.newInstance()
+
+      parser.setNamespaceAware(false)
+      parser.newSAXParser()
+    }
   }
 
+  /* Override this to use a different SAXParser. */
+  def parser: SAXParser = parserInstance.get
+
   /**
    * Loads XML from the given InputSource, using the supplied parser.
    *  The methods available in scala.xml.XML use the XML parser in the JDK.
@@ -58,4 +63,4 @@ trait XMLLoader[T <: Node] {
 
   /** Loads XML from the given String. */
   def loadString(string: String): T = loadXML(fromString(string), parser)
-}
+}