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 SE enumerated types

• Implicit objects

See Implicit Objects for more detail on the implicit objects available with JSP technology.

To refer to these objects, you write an expression using a variable name with which you created the object. The following expression references a JavaBeans component called customer.

${customer}

The web container evaluates a 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. A variable that matches one of the implicit objects described in Implicit Objects will return that implicit object instead of the variable’s value.

You can alter the way variables are resolved with a custom EL resolver, which is a new feature of the unified EL. 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. However, you cannot override implicit objects in this way. See EL Resolvers for more information on EL resolvers.

You can set the variable name, customer, when you declare the bean. See Creating and Using a JavaBeans Component for information on how to declare a JavaBeans component for use in your JSP pages.

To declare beans in JavaServer Faces applications, you use the managed bean facility. See Backing Beans for information on how to declare beans for use in JavaServer Faces applications.

When referencing an enum constant with an expression, you 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 a Suit.hearts before it is compared to the instance.

${mySuit == "hearts"}