INT-3140: Add #xpath() SpEL Function Support

JIRA: https://jira.springsource.org/browse/INT-3140

* Introduce `XPathUtils`
* Add `#xpath()` tests
* Add documentation

INT-3140: Polishing according PR comments

INT-3140 Polishing

- Doc and javadoc polishing
- Fix some compiler warnings (not all xpath)

src/reference/docbook/spel.xml

@@ -176,13 +176,11 @@
 						</note>
 						For more information regarding JSON see 'JSON Transformers' in <xref linkend="transformer"/>.
 				</listitem>
+				<listitem>
+					<emphasis role="bold">#xpath</emphasis> - to evaluate an 'xpath' on some provided object.
+					For more information regarding xml and xpath see <xref linkend="xml"/>.
+				</listitem>
 				<!--<listitem>
-					<emphasis>#xpath</emphasis> - TBD
-				</listitem>
-				<listitem>
-					<emphasis>#request</emphasis> - TBD
-				</listitem>
-				<listitem>
 					<emphasis>#auth</emphasis> - TBD
 				</listitem>-->
 			</itemizedlist>

src/reference/docbook/transformer.xml

@@ -361,6 +361,13 @@ public class Foo {
       In addition to JSON Transformers, Spring Integration provides a built-in <emphasis>#jsonPath</emphasis>
       SpEL function for use in expressions. For more information see <xref linkend="spel"/>.
     </para>
+    <para>
+        <emphasis role="bold">#xpath SpEL Function</emphasis>
+    </para>
+	<para>
+	 Since version <emphasis>3.0</emphasis>, Spring Integration also provides a built-in <emphasis>#xpath</emphasis>
+	 SpEL function for use in expressions. For more information see <xref linkend="xpath-spel-function"/>.
+	</para>
   </section>
 
   <section id="transformer-annotation">

src/reference/docbook/whats-new.xml

@@ -117,8 +117,9 @@
 			<title>SpEL Functions Support</title>
 			<para>
 				To customize the SpEL <interfacename>EvaluationContext</interfacename> with static
-				<classname>Method</classname> functions the new <code>&lt;spel-function/&gt;</code>
-				component is introduced. For more information see <xref linkend="spel-functions" />.
+				<classname>Method</classname> functions, the new <code>&lt;spel-function/&gt;</code>
+				component is introduced. Two built-in functions are also provided (<code>#jsonPath</code>
+				and <code>#xpath</code>). For more information see <xref linkend="spel-functions" />.
 			</para>
 		</section>
 		<section id="3.0-spel-property-accessors">

src/reference/docbook/xml.xml

@@ -34,6 +34,9 @@
         <listitem>
             <para><emphasis><link linkend='xml-xpath-filter'>XPath Filter</link></emphasis></para>
         </listitem>
+        <listitem>
+            <para><emphasis><link linkend='xpath-spel-function'>#xpath SpEL Function</link></emphasis></para>
+        </listitem>
         <listitem>
             <para><emphasis><link linkend='xml-validating-filter'>Validating Filter</link></emphasis></para>
         </listitem>
@@ -1253,6 +1256,39 @@
 	</calloutlist></para>
   </section>
 
+	<section id="xpath-spel-function">
+		<title>#xpath SpEL Function</title>
+		<para>
+			Spring Integration, since version <emphasis>3.0</emphasis>, provides the <code>#xpath</code>
+			built-in SpEL function, which invokes the static method <code>XPathUtils.evaluate(...)</code>.
+			This method delegates to an <interfacename>org.springframework.xml.xpath.XPathExpression</interfacename>.
+			The following shows some usage examples:
+			<programlisting language="xml"><![CDATA[<transformer expression="#xpath(payload, '/name')"/>
+
+<filter expression="#xpath(payload, headers.xpath, 'boolean')"/>
+
+<splitter expression="#xpath(payload, '//book', 'document_list')"/>
+
+<router expression="#xpath(payload, '/person/@age', 'number')">
+	<mapping channel="output1" value="16"/>
+	<mapping channel="output2" value="45"/>
+</router>]]></programlisting>
+			<code>#xpath</code> also supports a third optional parameter for converting the result of the xpath evaluation.
+			It can be
+			one of the String constants <code>'string'</code>, <code>'boolean'</code>, <code>'number'</code>,
+			<code>'node'</code>, <code>'node_list'</code> and <code>'document_list'</code> or an
+			<interfacename>org.springframework.xml.xpath.NodeMapper</interfacename> instance.
+			By default the <code>#xpath</code> SpEL function returns a String representation of the xpath evaluation.
+		</para>
+		<note>
+			To enable the <code>#xpath</code> SpEL function, simply add the <code>spring-integration-xml.jar</code>
+			to the CLASSPATH; there is no need to declare any component(s) from the Spring Integration Xml Namespace.
+		</note>
+		<para>
+			For more information see <xref linkend="spel"/>.
+		</para>
+	</section>
+
   <section id="xml-validating-filter">
     <title>XML Validating Filter</title>
     <para>