Defining conditional symbols

Within the <symbolDefinitions> tag, you can define a conditional symbol by using the <conditionalSymbol> tag. A conditional symbol represents a value that is acquired by evaluation a series of condition expressions. If any expression returns true, request mapper uses the member to calculate the return value. When a member meets the condition expression, request mapper evaluates a corresponding return expression, and return the result. After the request mapper finds a result to return, it does not evaluate any further expressions.

Within the <conditionalSymbol> tag, use the following tags.

<name>: The name of the symbol. It is a string and must start with the $ character.

<type>: The type of the value that the symbol returns. Specify this value as a fully qualified Java™ class name, or a Java primitive. Specifying the type for the symbol is optional. If it is not defined, the request mapper attempts to establish the field type based on the expression. If the request mapper is unable to determine the symbol type before it evaluates the expression, performance is affected. Therefore, for best performance, it is better to specify the type.

<args>: The arguments for the symbol. This tag is optional; if it is specified, arguments must be supplied for evaluating the symbol. For more information, see Defining symbol arguments.

<if condition="expression">

The condition attribute defines a condition expression to evaluate. The expression must yield a Boolean value. If the value is true, request mapper uses the contents of the <if> tag to try to determine the return value. The <if> tag must contain either, but not both, of the following contents:

A <return> tag. This tag contains an expression. If the condition expression is true, request mapper evaluates the expression and returns the result.
Any number of <if> tags, nested within this <if> tag. If the condition expression is true, request mapper processes the nested <if> tags in the same way as a top-level <if> tag. That is, it evaluates the expression in the condition attribute, and if the expression is true, uses the contents of the tag to try and determine the return value.

Important: If a return value is determined, request mapper does not evaluate any further expressions. However, if a condition expression in an <if> tag is true, but it contains nested <if> tags and none of their condition expressions are true, no value is determined. In this case, request mapper continues to evaluate subsequent expressions.

<defaultValue>: Optional. If request mapper has evaluated all condition expressions, but none of the condition expression has returned true, request mapper evaluates the expression in the <defaultValue> tag. The conditional symbol returns the value that the expression produces. If this tag is not present, the default value is null.

Example

<symbol>
  <name>$GET</name>
  <eval>"GET"</eval>
</symbol>
<symbol>
  <name>$PUT</name>
  <eval>"PUT"</eval>
</symbol>
<conditionalSymbol>
  <name>$sessionAttribute</name>
  <if condition="$httpServletRequest.getSession(false) != null>
     <if condition="$httpServletRequest.getSession(false).getAttribute($GET) != null">
       <return>$httpServletRequest.getSession(false).getAttribute($GET)</return>
     </if>
     <if condition="true">
       <return>$httpServletRequest.getSession(false).getAttribute($PUT)</return>
      </if>
  </if>
</conditionalSymbol>

This symbol is assumed to be a part of the servlet request mapper. First, it checks whether an HTTP session exists for the servlet; if not, the symbol returns null. If a session is present, the symbol checks whether the servlet has a GET attribute, it returns the value of that attribute. Otherwise, it returns the value of the PUT attribute. The second condition expression is true; this value is used as an else clause. If the first condition is true, the request mapper does not evaluate any further expressions; otherwise, it continues to the second expression.