<?xml version="1.0" encoding="UTF-8"?>
<chapter id="el">
	<title>Expression Language (EL)</title>
	<sect1 id="el-introduction">
		<title>Introduction</title>
		<para>
			Web Flow uses EL to access its data model and invoke actions.
			This chapter will familiarize you with the EL syntax, and special EL variables you can reference from your flow definition.
		</para>
	</sect1>
	<sect1 id="el-language-choices">
		<title>Supported EL implementations</title>
		<sect2 id="el-unified-el">
			<title>Unified EL</title>
			<para>
				Web Flow attempts to use the <ulink url="http://en.wikipedia.org/wiki/Unified_Expression_Language">Unified EL</ulink> by default. 
				<code>jboss-el</code> is currently the default EL implementation.
				When found in your classpath along with the <code>el-api</code>, it will be used automatically.
				<note>
					The <code>el-api</code> dependency is typically a <emphasis>provided</emphasis> by your web container.  Tomcat 6 includes it, for example.
				</note>
			</para>
		</sect2>
		<sect2 id="el-ognl">
			<title>OGNL</title>
			<para>
				<ulink url="http://www.ognl.org">OGNL</ulink> is the other EL supported by Web Flow 2.
				OGNL is the EL most familiar to Web Flow version 1.0 users.
				To use ognl, simply include <code>ognl</code> in your classpath instead of <code>jboss-el</code>.
				Please refer to the <ulink url="http://www.ognl.org/2.6.9/Documentation/html/LanguageGuide/index.html">OGNL language guide</ulink> for specifics on its EL syntax.
			</para>
		</sect2>
	</sect1>
	<sect1 id="el-portability">
		<title>EL portability</title>
		<para>
			In general, you will find the Unified EL and OGNL have a very similar syntax.
			For basic variable resolution, property access, and method invocation the syntax is identical.
			We recommend adhering to Unified EL syntax whenever possible, and only relying on proprietary EL features when needed.
		</para>
	</sect1>
	<sect1 id="el-usage">
		<title>EL usage</title>
		<para>
			EL is used for many things within a flow, including:
		</para>
		<orderedlist>
			<listitem><para>Accessing data provided by the client, such as flow input attributes and request parameters.</para></listitem>
			<listitem><para>Accessing internal data structures such as <code>flowScope.</code></para></listitem>
			<listitem><para>Invoking methods on Spring beans.</para></listitem>
			<listitem><para>Resolving constructs such as state transition criteria, subflow ids, and view names.</para></listitem>
		</orderedlist>
		<para>
			Views rendered by flows typically access flow data structures using EL as well.
		</para>
		<sect2 id="el-types">
			<title>Expression types</title>
			<para>
				There are basically two types of expressions in Web Flow.
			</para>
			<sect3 id="el-types-eval">
				<title>Standard eval expressions</title>
				<para>
					The first, and most common type of expression, is the standard <emphasis>eval expression</emphasis>.
					Such expressions are dynamically evaluated by the EL and should not be enclosed in delimiters like <code>${}</code> or <code>#{}</code>.
					For example:
				</para>
				<programlisting type="xml"><![CDATA[
<evaluate expression="searchCriteria.nextPage()" />]]>
				</programlisting>
				<para>
					The expression above is a standard expression that invokes the <code>nextPage</code> method on the <code>searchCriteria</code> variable when evaluated.
					Attempting to enclose this expression in special eval delimiters like <code>${}</code> or <code>#{}</code> will result in an <code>IllegalArgumentException</code>.
					<note>
						We view use of special eval delimiters as redundant in this context, as the only acceptable value for the <code>expression</code> attribute is a single eval expression string.
					</note>
				</para>	
			</sect3>
			<sect3 id="el-types-template">
				<title>Template expressions</title>			
				<para>
					The second type of expression is a "template" expression.
					Such expressions allow a mixing of literal text with one or more eval blocks.
					Each eval block is explictly delimited with the <code>${}</code> delimiters.
					For example:
				</para>
				<programlisting type="xml"><![CDATA[
<view-state id="error" view="error-${externalContext.locale}.xhtml" />]]>
				</programlisting>
				<para>
					The expression above is a template expression.
					The result of evaluation will be a string that concatenates the literal text <code>error-</code> with the result of evaluating <code>externalContext.locale</code>.
					As you can see, explicit delimiters are necessary here to demarcate eval blocks within the template.
				</para>
			</sect3>
			<para>
				See the Web Flow XML schema for a complete listing of the XML attributes that accept standard expressions and template expressions.
			</para>
		</sect2>
	</sect1>
	<sect1 id="el-variables">
		<title>Special EL variables</title>
		<para>
			There are several implicit variables you may reference from within a flow.
			These variables are discussed in this section.
		</para>
		<sect2 id="el-variable-flowScope">
			<title>flowScope</title>
			<para>
				Use <code>flowScope</code> to assign a flow variable.
				Flow scope gets allocated when a flow starts and destroyed when the flow ends.
			</para>
			<programlisting type="xml"><![CDATA[
<evaluate expression="searchService.findHotel(hotelId)" result="flowScope.hotel" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-viewScope">
			<title>viewScope</title>
			<para>
				Use <code>viewScope</code> to assign a view variable.
				View scope gets allocated when a <code>view-state</code> enters and destroyed when the state exits.
				View scope is <emphasis>only</emphasis> referenceable from within a <code>view-state</code>.
			</para>
			<programlisting type="xml"><![CDATA[
<on-render>
    <evaluate expression="searchService.findHotels(searchCriteria)" result="viewScope.hotels"
              result-type="dataModel" />
</on-render>]]>
			</programlisting>
		</sect2>			
		<sect2 id="el-variable-requestScope">
			<title>requestScope</title>
			<para>
				Use <code>requestScope</code> to assign a request variable.
				Request scope gets allocated when a flow is called and destroyed when the flow returns.
			</para>
			<programlisting type="xml"><![CDATA[
<set name="requestScope.hotelId" value="requestParameters.id" type="long" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-flashScope">
			<title>flashScope</title>
			<para>
				Use <code>flashScope</code> to assign a flash variable.
				Flash scope gets allocated when a flow starts, cleared after every view render, and destroyed when the flow ends.
			</para>
			<programlisting type="xml"><![CDATA[
<set name="flashScope.statusMessage" value="'Booking confirmed'" />]]>				
			</programlisting>
		</sect2>
		<sect2 id="el-variable-conversationScope">
			<title>conversationScope</title>
			<para>
				Use <code>conversationScope</code> to assign a conversation variable.
				Conversation scope gets allocated when a top-level flow starts and destroyed when the top-level flow ends.
				Conversation scope is shared by a top-level flow and all of its subflows.
			</para>
			<programlisting type="xml"><![CDATA[
<evaluate expression="searchService.findHotel(hotelId)" result="conversationScope.hotel" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-requestParameters">
			<title>requestParameters</title>
			<para>
				Use <code>requestParameters</code> to access a client request parameter:
			</para>
			<programlisting type="xml"><![CDATA[
<set name="requestScope.hotelId" value="requestParameters.id" type="long" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-currentEvent">
			<title>currentEvent</title>
			<para>
				Use <code>currentEvent</code> to access attributes of the current <code>Event</code>:
			</para>
			<programlisting type="xml"><![CDATA[
<evaluate expression="booking.guests.add(currentEvent.guest)" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-currentUser">
			<title>currentUser</title>
			<para>
				Use <code>currentUser</code> to access the authenticated <code>Principal</code>:
			</para>
			<programlisting type="xml"><![CDATA[
<evaluate expression="bookingService.createBooking(hotelId, currentUser.name)" 
          result="flowScope.booking" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-messageContext">
			<title>messageContext</title>
			<para>
				Use <code>messageContext</code> to access a context for retrieving and creating flow execution messages, including error and success messages.
				See the <code>MessageContext</code> Javadocs for more information.
			</para>
			<programlisting type="xml"><![CDATA[
<evaluate expression="bookingValidator.validate(booking, messageContext)" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-resourceBundle">
			<title>resourceBundle</title>
			<para>
				Use <code>resourceBundle</code> to access a message resource.
			</para>
			<programlisting type="xml"><![CDATA[
<set name="flashScope.successMessage" value="resourceBundle.successMessage" />]]>
			</programlisting>
		</sect2>
		<sect2 id="el-variable-requestContext">
			<title>flowRequestContext</title>
			<para>
				Use <code>flowRequestContext</code> to access the <code>RequestContext</code> API, which is a representation of the current flow request.
				See the API Javadocs for more information.
			</para>
		</sect2>		
		<sect2 id="el-variable-flowExecutionContext">
			<title>flowExecutionContext</title>
			<para>
				Use <code>flowExecutionContext</code> to access the <code>FlowExecutionContext</code> API, which is a representation of the current flow state.
				See the API Javadocs for more information.
			</para>
		</sect2>		
		<sect2 id="el-variable-flowExecutionUrl">
			<title>flowExecutionUrl</title>
			<para>
				Use <code>flowExecutionUrl</code> to access the context-relative URI for the current flow execution view-state.
			</para>
		</sect2>		
		<sect2 id="el-variable-externalContext">
			<title>externalContext</title>
			<para>
				Use <code>externalContext</code> to access the client environment, including user session attributes.
				See the <code>ExternalContext</code> API JavaDocs for more information.
			</para>
			<programlisting type="xml"><![CDATA[
<evaluate expression="searchService.suggestHotels(externalContext.sessionMap.userProfile)" 
          result="viewScope.hotels" />]]>
			</programlisting>
		</sect2>
	</sect1>
	<sect1 id="el-scope-searching">
		<title>Scope searching algorithm</title>
		<para>
			When assigning a variable in one of the flow scopes, referencing that scope is required.
			For example:
		</para>
		<programlisting type="xml"><![CDATA[
<set name="requestScope.hotelId" value="requestParameters.id" type="long" />]]>
		</programlisting>
		<para>
			When simply accessing a variable in one of the scopes, referencing the scope is optional.
			For example:
		</para>
		<programlisting type="xml"><![CDATA[
<evaluate expression="entityManager.persist(booking)" />]]>
		</programlisting>
		<para>
			If no scope is specified, like in the use of <code>booking</code> above, a scope searching algorithm will be employed.
			The algorithm will look in request, flash, view, flow, and conversation scope for the variable.
			If no such variable is found, an <code>EvaluationException</code> will be thrown.
		</para>
	</sect1>
</chapter>