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:
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 {@link javax.el.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 {@link javax.el.ELContextListener}
so that applications and frameworks can ensure their own context objects
are attached to any newly created ELContext.
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 {@link javax.el.ValueExpression} such as
"${customer.name}" can be used either
as an rvalue (return the value associated with property name
of the model object customer) or as an lvalue
(set the value of the property name of the model object
customer).
A {@link javax.el.MethodExpression} such as
"${handler.process}" makes it possible to invoke a method
(process) on a specific model object (handler).
All expression classes extend the base class {@link javax.el.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 {@link javax.el.ELContext},
which provides the context required to evaluate the expression.
An expression is created through the {@link javax.el.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 {@link javax.el.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
{@link javax.el.FunctionMapper} and {@link javax.el.VariableMapper}
objects must be available within the ELContext so that EL functions and
EL variables are properly mapped.
Through the {@link javax.el.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 ({@link javax.el.ArrayELResolver}), JavaBeans ({@link javax.el.BeanELResolver}), Lists ({@link javax.el.ListELResolver}),
Maps ({@link javax.el.MapELResolver}), and ResourceBundles ({@link javax.el.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.
If an EL expression uses a function
(for example ${fn:toUpperCase(customer.name)}), then a
{@link javax.el.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.
Just like {@link javax.el.FunctionMapper} provides a flexible mechanism to add functions to the EL, {@link javax.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 {@link javax.el.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.