The Java EE 5 Tutorial (PDF) - Oracle Software Downloads

After declaring an enum constant called mySuit, you can write the following expression to test if
mySuit is spade:
${mySuit == "spade"}
When the EL resolving mechanism resolves this expression it will invoke the valueOf method
of the Enum class with the Suit class and the spade type, as shown here:
mySuit.valueOf(Suit.class, "spade"}
See “JavaBeans Components” on page 165 for more information on using expressions to
reference JavaBeans components and their properties.
WhereValue Expressions Can Be Used
Value expressions using the ${} delimiters can be used in the following places:
■ In static text
■ In any standard or custom tag attribute that can accept an expression
The value of an expression in static text is computed and inserted into the current output. Here
is an example of an expression embedded in static text:
some text ${expr} some text
If the static text appears in a tag body, note that an expression will not be evaluated if the body is
declared to be tagdependent (see “Tags with Attributes” on page 227).
Lvalue expressions can only be used in tag attributes that can accept lvalue expressions.
There are three ways to set a tag attribute value using either an rvalue or lvalue expression:
■ With a single expression construct:
These expressions are evaluated and the result is coerced to the attribute’s expected type.
■ With one or more expressions separated or surrounded by text:
Unified Expression Language
These kinds of expression are called a composite expressions. They are evaluated from left to
right. Each expression embedded in the composite expression is coerced to a String and
then concatenated with any intervening text. The resulting String is then coerced to the
attribute’s expected type.
Chapter 5 • JavaServer PagesTechnology 153