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Package javax.el
Provides the API for the Unified Expression Language 3.0
See: Description
Interface Summary Interface Description
ELContextListener
The listener interface for receiving notification when an ELContext is created.
Class Summary Class Description
ArrayELResolver
Defines property resolution behavior on arrays.
BeanELResolver
Defines property resolution behavior on objects using the JavaBeans component architecture.
BeanNameELResolver
An ELResolver for resolving user or container managed beans.
BeanNameResolver
Resolves a bean by its known name.
CompositeELResolver
Maintains an ordered composite list of child ELResolvers.
ELClass
A runtime representation of a Class in the EL expressions.
ELContext
Context information for expression parsing and evaluation.
ELContextEvent
An event which indicates that an ELContext has been created.
ELManager
Manages EL parsing and evaluation environment.
ELProcessor
Provides an API for using EL in a stand-alone environment.
ELResolver
Enables customization of variable, property, method call, and type conversion resolution behavior for EL expression evaluation.
EvaluationListener
The listener interface for receiving notification when an EL expression is evaluated.
Expression
Base class for the expression subclasses ValueExpression and MethodExpression, implementing characteristics common to both.
ExpressionFactory
Provides an implementation for creating and evaluating EL expressions.
FunctionMapper
The interface to a map between EL function names and methods.
ImportHandler
Handles imports of class names and package names.
LambdaExpression
Encapsulates a parameterized ValueExpression.
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.
StandardELContext
A standard ELContext suitable for use in a stand alone environment.
StaticFieldELResolver
An ELResolver for resolving static fields, enum constants and static methods.
TypeConverter
A convenient class for writing an ELResolver to do custom type conversions.
ValueExpression
An Expression that can get or set a value.
ValueReference
This encapsulates a base model object and one of its properties.
VariableMapper
The interface to a map between EL variables and the EL expressions they are associated with.
Exception Summary Exception Description
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 3.0
The Expression Language (EL) is a simple language originally designed to satisfy the specific needs of web application developers. It has evloved into its own specification intended for general use inside and outside of the web containers.
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
Expression Objects
Creation of Expressions
Evaluation of Expressions
Evaluation Listeners
Resolution of Model Objects and their Properties
EL Functions
EL Variables
EL in Stand-alone environment
THE 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.
When EL used in a web container, the creation of ELContext objects is controlled through the underlying technology. For example, in JSP, the JspContext.getELContext() factory method is used. In an stand-alone environment, a default StandardELContext is provided.
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 “${customer.name}” can be used either as an rvalue (return the value associated with property name of the model object customer) or as an lvalue (set the value of the property name of the model object customer).
A MethodExpression such as “${handler.process}” makes it possible to invoke a method (process) on a specific model object (handler).
In version 2.2 and later, either type of EL expression can represent a method invocation, such as ${trader.buy(“JAVA”)}, where the arugments to the method invocation are specified in the expression.
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(customer.name)}) 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.
Evaluation of Expressions
The creation and the evaluation of an expression are done in two separate steps. At the evaluation of an expression, the ELContext provides the context necessary to support property and method resolution for modal objects.
A deferred expression is one that is created but not immediately evaluated. In a JSF request processing life cycle, EL expressions are typically created in the tree building phase and evaluated in the rendering phrase.
Adding parameters to a ValueExpression further enhances the power of deferred expressions. The LambdaExpression encapsulates such a construct. A LambdaExpression can be invoked by supplying the actual parameters at evaluation. It plays an important role in the support for collections operators.
Evaluation Listeners
By registering EvaluationListeners in ELContext, a user can receive notifications during the EL expression evaluations. There are three events that trigger the notification:
Before evaluation
After evaluation
When (base, property) is resolved
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 and method invocations 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(customer.name)}), 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.customer.name}"/>
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.name}"/>
</c:forEach>
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.
EL in Stand-alone environment
EL 3.0 includes APIs for using EL in a stand-alone environment.
ELProcessor provides simple APIs for the direct evaluations of expressions. It also makes it easy to define functions, set variables, and define a beans locally.
ELManager provides a lower level APIs for managing the EL parsing and evaluation environment. It contains a default ELContext StandardELContext.
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