api/src/main/java/jakarta/el/package.html

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Provides the API for <strong>Jakarta Expression Language 6.1</strong>

<p>Jakarta Expression Language is a simple language originally designed to
satisfy the specific needs of web application developers. It has evolved
into its own specification intended for general use inside and outside of the
web containers.</p>

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

<ul>
  <li><a href="#Context">Jakarta Expression Language Context</a></li>
  <li><a href="#ExpressionObjects">Expression Objects</a></li>
  <li><a href="#ExpressionCreation">Creation of Expressions</a></li>
  <li><a href="#ExpressionEvaluation">Evaluation of Expressions</a></li>
  <li><a href="#EvaluationListener">Evaluation Listeners</a></li>
  <li><a href="#Resolver">Resolution of Model Objects and their Properties</a></li>
  <li><a href="#Functions">Jakarta Expression Language Functions</a></li>
  <li><a href="#Variables">Jakarta Expression Language Variables</a></li>
  <li><a href="#Standalone">Jakarta Expression Language in Stand-alone environment</a></li>
</ul>

<h3><a name="Context">Jakarta Expression Language Context</a></h3>

<p>An important goal of Jakarta Expression Language 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.</p>

<p>Class {@link jakarta.el.ELContext} is what links
the Jakarta Expression Language 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.
</p>

<p>When Jakarta Expression Language is used in a web container, the creation of <code>ELContext
</code> objects is controlled through the underlying technology.
For example, in Jakarta Server Pages, the
 <code>JspContext.getELContext()</code> factory method is used.  In an
stand-alone environment, a default {@link jakarta.el.StandardELContext} is
provided.</p>
 
 <p>Some technologies provide the ability to add an {@link jakarta.el.ELContextListener}
 so that applications and frameworks can ensure their own context objects
 are attached to any newly created <code>ELContext</code>.</p>
 
<h3><a name="ExpressionObjects">Expression Objects</a></h3>

<p>At the core of the Expression Language is the notion of an <i>expression</i>
that gets parsed according to the grammar defined by the Expression Language.</p>

<p>There are two types of expressions defined by Jakarta Expression Language: <i>value expressions</i>
and <i>method expressions</i>. A {@link jakarta.el.ValueExpression} such as
<code>"${customer.name}"</code> can be used either
as an <i>rvalue</i> (return the value associated with property <code>name</code>
of the model object <code>customer</code>) or as an <i>lvalue</i> 
(set the value of the property <code>name</code> of the model object
<code>customer</code>).</p>

<p>A {@link jakarta.el.MethodExpression} such as
<code>"${handler.process}"</code> makes it possible to invoke a method 
(<code>process</code>) on a specific model object (<code>handler</code>).</p>

<p>In version 2.2 and later, either type of Jakarta Expression Language expression can represent a method
invocation, such as <code>${trader.buy("JAVA")}</code>, where the arguments to
the method invocation are specified in the expression.</p>

<p>All expression classes extend the base class {@link jakarta.el.Expression}, making them
serializable and forcing them to implement <code>equals()</code> and 
<code>hashCode()</code>. Moreover, each method on these expression classes 
that actually evaluates an expression receives a parameter
of class {@link jakarta.el.ELContext},
which provides the context required to evaluate the expression.</p>

<h3><a name="ExpressionCreation">Creation of Expressions</a></h3>

<p>An expression is created through the {@link jakarta.el.ExpressionFactory} class.
The factory provides two creation methods; one for each type of expression
 supported by Jakarta Expression Language.</p>

<p>To create an expression, one must provide an {@link jakarta.el.ELContext},
a string representing 
the expression, and the expected type (<code>ValueExpression</code>) or signature 
(<code>MethodExpression</code>). 

The <code>ELContext</code> provides the context necessary to parse an expression.
Specifically, if the expression uses a Jakarta Expression Language function 
(for example <code>${fn:toUpperCase(customer.name)}</code>) or a Jakarta Expression Language 
variable, then {@link jakarta.el.FunctionMapper} and {@link jakarta.el.VariableMapper}
objects must be available within the <code>ELContext</code> so that Jakarta Expression Language 
functions and Jakarta Expression Language variables are properly mapped.

<h3><a name="ExpressionEvaluation">Evaluation of Expressions</a></h3>
<p>The creation and the evaluation of an expression are done in two separate
steps.  At the evaluation of an expression, the {@link jakarta.el.ELContext}
provides the context necessary to support property and method resolution
for modal objects.</p>

<p>A deferred expression is one that is created but not immediately evaluated.
In a Jakarta Faces request processing life cycle, Jakarta Expression Language 
expressions are typically created in the tree building phase and evaluated in the 
rendering phrase.</p>

<p>Adding parameters to a <code>ValueExpression</code> further enhances the
power of deferred expressions.  The {@link jakarta.el.LambdaExpression}
encapsulates such a construct.  A <code>LambdaExpression</code> can be
invoked by supplying the actual parameters at evaluation.  It plays
an important role in the support for collections operators.</p>

<h3><a name="EvaluationListener">Evaluation Listeners</a></h3>
<p>By registering {@link jakarta.el.EvaluationListener}s in ELContext, a user can
receive notifications during the Jakarta Expression Language expression evaluations.  
There are three events that trigger the notification:
<ul>
  <li>Before evaluation</li>
  <li>After evaluation</li>
  <li>When (base, property) is resolved</li>
</ul></p>

<h3><a name="Resolver">Resolution of Model Objects and their Properties</a></h3>

<p>Through the {@link jakarta.el.ELResolver} base class, Jakarta Expression Language
features a pluggable mechanism to resolve model object references as well as properties 
and method invocations of these objects.</p>

<p>The Jakarta Expression Language API provides implementations of <code>ELResolver</code> supporting 
property resolution for common data types which include arrays 
({@link jakarta.el.ArrayELResolver}), JavaBeans ({@link jakarta.el.BeanELResolver}),
<code>List</code>s ({@link jakarta.el.ListELResolver}),
<code>Map</code>s ({@link jakarta.el.MapELResolver}), and
<code>ResourceBundle</code>s ({@link jakarta.el.ResourceBundleELResolver}).</p>

<h3><a name="Functions">Jakarta Expression Language Functions</a></h3>

<p>If a Jakarta Expression Language expression uses a function 
(for example <code>${fn:toUpperCase(customer.name)}</code>), then a 
{@link jakarta.el.FunctionMapper} object must also be specified within
the <code>ELContext</code>. 

The <code>FunctionMapper</code> is responsible to map
 <code>${prefix:name()}</code> style functions to 
static methods that can execute the specified functions. 
</p>

<h3><a name="Variables">Jakarta Expression Language Variables</a></h3>

<p>Just like {@link jakarta.el.FunctionMapper} provides
a flexible mechanism to add functions to Jakarta Expression Language, 
{@link jakarta.el.VariableMapper} provides a flexible mechanism to support the notion of
<strong>Jakarta Expression Language variables</strong>.
</p>
 
<p>
A Jakarta Expression Language variable does not directly refer to a model object that can then
be resolved by an <code>ELResolver</code>. Instead, it refers to a Jakarta Expression Language
expression. The evaluation of that Jakarta Expression Language expression gives the
Jakarta Expression Language variable its value.
</p>

<p>
For example, in the following code snippet
<blockquote>
  <code>&lt;h:inputText value="#{handler.customer.name}"/&gt;</code>
</blockquote>

<code>handler</code> refers to a model object that can be resolved by a Jakarta Expression Language Resolver.
</p>
<p>
However, in this other example:
<blockquote>
<pre>
&lt;c:forEach var="item" items="#{model.list}"&gt;
   &lt;h:inputText value="#{item.name}"/&gt;
&lt;/c:forEach&gt;
</pre>
</blockquote>

<code>item</code> is a Jakarta Expression Language variable because it does not refer directly to a model
object.  Instead, it refers to another Jakarta Expression Language expression, namely a
specific item in the collection referred to by the Jakarta Expression Language expression
<code>#{model.list}</code>.
</p>

<p>
Assuming that there are three elements in <code>${model.list}</code>, this means
that for each invocation of <code>&lt;h:inputText&gt;</code>, the following information 
about <code>item</code> must be preserved in the {@link jakarta.el.VariableMapper}:
<blockquote>
    first invocation: <code>item</code> maps to first element in <code>${model.list}</code><br>
    second invocation: <code>item</code> maps to second element in <code>${model.list}</code><br>
    third invocation: <code>item</code> maps to third element in <code>${model.list}</code><br>
</blockquote>
<p>
<code>VariableMapper</code> provides the mechanisms required to allow the mapping
of a Jakarta Expression Language variable to the Jakarta Expression Language expression from which it gets its value.
</p>

<h3><a name="Standalone">Jakarta Expression Language in Stand-alone environment</a></h3>
<p>Jakarta Expression Language 6.1 includes APIs for using Jakarta Expression Language in a stand-alone environment.</p>
<p>{@link jakarta.el.ELProcessor} provides simple APIs for the direct
evaluations of expressions. It also makes it easy to define functions,
set variables, and define beans locally.</p>

<p>{@link jakarta.el.ELManager} provides lower level APIs for managing the Jakarta Expression Language
parsing and evaluation environment.  It contains a default ELContext {@link jakarta.el.StandardELContext}.</p>

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