Package javax.el

Provides the API for the Unified Expression Language shared by the JSP 2.1 and JSF 1.2 technologies.


Interface Summary
ELContextListener The listener interface for receiving notification when an ELContext is created.

Class Summary
ArrayELResolver Defines property resolution behavior on arrays.
BeanELResolver Defines property resolution behavior on objects using the JavaBeans component architecture.
CompositeELResolver Maintains an ordered composite list of child ELResolvers.
ELContext Context information for expression evaluation.
ELContextEvent An event which indicates that an ELContext has been created.
ELResolver Enables customization of variable and property resolution behavior for EL expression evaluation.
ELUtil Utility methods for this portion of the EL implementation
Expression Base class for the expression subclasses ValueExpression and MethodExpression, implementing characterstics common to both.
ExpressionFactory Parses a String into a ValueExpression or MethodExpression instance for later evaluation.
FunctionMapper The interface to a map between EL function names and methods.
ListELResolver Defines property resolution behavior on instances of List.
MapELResolver Defines property resolution behavior on instances of Map.
MethodExpression An Expression that refers to a method on an object.
MethodInfo Holds information about a method that a MethodExpression evaluated to.
ResourceBundleELResolver Defines property resolution behavior on instances of ResourceBundle.
ValueExpression An Expression that can get or set a value.
VariableMapper The interface to a map between EL variables and the EL expressions they are associated with.

Exception Summary
ELException Represents any of the exception conditions that can arise during expression evaluation.
MethodNotFoundException Thrown when a method could not be found while evaluating a MethodExpression.
PropertyNotFoundException Thrown when a property could not be found while evaluating a ValueExpression or MethodExpression.
PropertyNotWritableException Thrown when a property could not be written to while setting the value on a ValueExpression.

Package javax.el Description

Provides the API for the Unified Expression Language shared by the JSP 2.1 and JSF 1.2 technologies.

The Expression Language (EL) is a simple language designed to satisfy the specific needs of web application developers. It is currently defined in its own specification document within the JavaServer Pages (tm) (JSP) 2.1 specification, but does not have any dependencies on any portion of the JSP 2.1 specification. It is intended for general use outside of the JSP and JSF specifications as well.

This package contains the classes and interfaces that describe and define the programmatic access to the Expression Language engine. The API is logically partitioned as follows:

EL Context

An important goal of the EL is to ensure it can be used in a variety of environments. It must therefore provide enough flexibility to adapt to the specific requirements of the environment where it is being used.

Class ELContext is what links the EL with the specific environment where it is being used. It provides the mechanism through which all relevant context for creating or evaluating an expression is specified.

Creation of ELContext objects is controlled through the underlying technology. For example, in JSP, the JspContext.getELContext() factory method is used.

Some technologies provide the ability to add an ELContextListener so that applications and frameworks can ensure their own context objects are attached to any newly created ELContext.

Expression Objects

At the core of the Expression Language is the notion of an expression that gets parsed according to the grammar defined by the Expression Language.

There are two types of expressions defined by the EL: value expressions and method expressions. A ValueExpression such as "${}" can be used either as an rvalue (return the value associated with property name of the model object customer) or as an lvalue (set the value of the property name of the model object customer).

A MethodExpression such as "${handler.process}" makes it possible to invoke a method (process) on a specific model object (handler).

All expression classes extend the base class Expression, making them serializable and forcing them to implement equals() and hashCode(). Morevover, each method on these expression classes that actually evaluates an expression receives a parameter of class ELContext, which provides the context required to evaluate the expression.

Creation of Expressions

An expression is created through the ExpressionFactory class. The factory provides two creation methods; one for each type of expression supported by the EL.

To create an expression, one must provide an ELContext, a string representing the expression, and the expected type (ValueExpression) or signature (MethodExpression). The ELContext provides the context necessary to parse an expression. Specifically, if the expression uses an EL function (for example ${fn:toUpperCase(}) or an EL variable, then FunctionMapper and VariableMapper objects must be available within the ELContext so that EL functions and EL variables are properly mapped.

Resolution of Model Objects and their Properties

Through the ELResolver base class, the EL features a pluggable mechanism to resolve model object references as well as properties of these objects.

The EL API provides implementations of ELResolver supporting property resolution for common data types which include arrays (ArrayELResolver), JavaBeans (BeanELResolver), Lists (ListELResolver), Maps (MapELResolver), and ResourceBundles (ResourceBundleELResolver).

Tools can easily obtain more information about resolvable model objects and their resolvable properties by calling method getFeatureDescriptors on the ELResolver. This method exposes objects of type java.beans.FeatureDescriptor, providing all information of interest on top-level model objects as well as their properties.

EL Functions

If an EL expression uses a function (for example ${fn:toUpperCase(}), then a FunctionMapper object must also be specified within the ELContext. The FunctionMapper is responsible to map ${prefix:name()} style functions to static methods that can execute the specified functions.

EL Variables

Just like FunctionMapper provides a flexible mechanism to add functions to the EL, VariableMapper provides a flexible mechanism to support the notion of EL variables.

An EL variable does not directly refer to a model object that can then be resolved by an ELResolver. Instead, it refers to an EL expression. The evaluation of that EL expression gives the EL variable its value.

For example, in the following code snippet

<h:inputText value="#{}"/>
handler refers to a model object that can be resolved by an EL Resolver.

However, in this other example:

<c:forEach var="item" items="#{model.list}">
   <h:inputText value="#{}"/>
item is an EL variable because it does not refer directly to a model object. Instead, it refers to another EL expression, namely a specific item in the collection referred to by the EL expression #{model.list}.

Assuming that there are three elements in ${model.list}, this means that for each invocation of <h:inputText>, the following information about item must be preserved in the VariableMapper:

first invocation: item maps to first element in ${model.list}
second invocation: item maps to second element in ${model.list}
third invocation: item maps to third element in ${model.list}

VariableMapper provides the mechanisms required to allow the mapping of an EL variable to the EL expression from which it gets its value.

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