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. Web Flow attempts to use the Unified EL by default. jboss-el is currently the default EL implementation. When found in your classpath along with the el-api, it will be used automatically. The el-api dependency is typically a provided by your web container. Tomcat 6 includes it, for example. OGNL 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 ognl in your classpath instead of jboss-el. Please refer to the OGNL language guide for specifics on its EL syntax. 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. EL is used for many things within a flow, including: Accessing data provided by the client, such as flow input attributes and request parameters. Accessing internal data structures such as flowScope. Invoking methods on Spring beans. Resolving constructs such as state transition criteria, subflow ids, and view names. Views rendered by flows typically access flow data structures using EL as well. There are basically two types of expressions in Web Flow. The first, and most common type of expression, is the standard eval expression. Such expressions are dynamically evaluated by the EL and should not be enclosed in delimiters like ${} or #{}. For example: ]]> The expression above is a standard expression that invokes the nextPage method on the searchCriteria variable when evaluated. Attempting to enclose this expression in special eval delimiters like ${} or #{} will result in an IllegalArgumentException. We view use of special eval delimiters as redundant in this context, as the only acceptable value for the expression attribute is a single eval expression string. 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 ${} delimiters. For example: ]]> The expression above is a template expression. The result of evaluation will be a string that concatenates the literal text error- with the result of evaluating externalContext.locale. As you can see, explicit delimiters are necessary here to demarcate eval blocks within the template. See the Web Flow XML schema for a complete listing of the XML attributes that accept standard expressions and template expressions. There are several implicit variables you may reference from within a flow. These variables are discussed in this section. Use flowScope to assign a flow variable. Flow scope gets allocated when a flow starts and destroyed when the flow ends. ]]> Use viewScope to assign a view variable. View scope gets allocated when a view-state enters and destroyed when the state exits. View scope is only referenceable from within a view-state. ]]> Use requestScope to assign a request variable. Request scope gets allocated when a flow is called and destroyed when the flow returns. ]]> Use flashScope to assign a flash variable. Flash scope gets allocated when a flow starts, cleared after every view render, and destroyed when the flow ends. ]]> Use conversationScope 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. ]]> Use requestParameters to access a client request parameter: ]]> Use currentEvent to access attributes of the current Event: ]]> Use currentUser to access the authenticated Principal: ]]> Use messageContext to access a context for retrieving and creating flow execution messages, including error and success messages. See the MessageContext Javadocs for more information. ]]> Use resourceBundle to access a message resource. ]]> Use flowRequestContext to access the RequestContext API, which is a representation of the current flow request. See the API Javadocs for more information. Use flowExecutionContext to access the FlowExecutionContext API, which is a representation of the current flow state. See the API Javadocs for more information. Use flowExecutionUrl to access the context-relative URI for the current flow execution view-state. Use externalContext to access the client environment, including user session attributes. See the ExternalContext API JavaDocs for more information. ]]> When assigning a variable in one of the flow scopes, referencing that scope is required. For example: ]]> When simply accessing a variable in one of the scopes, referencing the scope is optional. For example: ]]> If no scope is specified, like in the use of booking 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 EvaluationException will be thrown.