Referencing Objects Using Value Expressions

Both rvalue and lvalue expressions can refer to the following objects and their properties or attributes:

• JavaBeans components

• Collections

• Java^TM SE enumerated types

• Implicit objects

To refer to these objects, you write an expression using a variable which is the name of the object. The following expression references a backing bean (a JavaBeans component) called customer.

${customer}

The web container evaluates the variable that appears in an expression by looking up its value according to the behavior of PageContext.findAttribute(String), where the String argument is the name of the variable. For example, when evaluating the expression ${customer}, the container will look for customer in the page, request, session, and application scopes and will return its value. If customer is not found, null is returned.

You can alter the way variables are resolved with a custom EL resolver. For instance, you can provide an EL resolver that intercepts objects with the name customer, so that ${customer} returns a value in the EL resolver instead. Creation of custom EL resolvers is an advanced topic covered in Java EE 6 Tutorial, Volume II: Advanced Topics.

To reference an enum constant with an expression, use a String literal. For example, consider this Enum class:

public enum Suit {hearts, spades, diamonds, clubs}

To refer to the Suit constant, Suit.hearts with an expression, you use the String literal, "hearts". Depending on the context, the String literal is converted to the enum constant automatically. For example, in the following expression in which mySuit is an instance of Suit, "hearts" is first converted to Suit.hearts before it is compared to the instance.

${mySuit == "hearts"}