Provides the API for Jakarta Expression Language 4.0

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.

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:

- Jakarta Expression Language Context
- Expression Objects
- Creation of Expressions
- Evaluation of Expressions
- Evaluation Listeners
- Resolution of Model Objects and their Properties
- Jakarta Expression Language Functions
- Jakarta Expression Language Variables
- Jakarta Expression Language in Stand-alone environment

### Jakarta Expression Language Context

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.

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.

When Jakarta Expression Language is used in a web container, the creation of `ELContext` objects is controlled through the underlying technology. For example, in Jakarta Server Pages, the `JspContext.getELContext()` factory method is used. In an stand-alone environment, a default {@link jakarta.el.StandardELContext} is provided.

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 `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 Jakarta Expression Language: value expressions and method expressions. A {@link jakarta.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 jakarta.el.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 Jakarta Expression Language expression can represent a method invocation, such as `${trader.buy("JAVA")}`, where the arguments to the method invocation are specified in the expression.

All expression classes extend the base class {@link jakarta.el.Expression}, making them serializable and forcing them to implement `equals()` and `hashCode()`. 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.

### Creation of Expressions

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.

To create an expression, one must provide an {@link jakarta.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 a Jakarta Expression Language function (for example `${fn:toUpperCase(customer.name)}`) or a Jakarta Expression Language variable, then {@link jakarta.el.FunctionMapper} and {@link jakarta.el.VariableMapper} objects must be available within the `ELContext` so that Jakarta Expression Language functions and Jakarta Expression Language 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 {@link jakarta.el.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 Jakarta Faces request processing life cycle, Jakarta Expression Language 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 {@link jakarta.el.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 {@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:

- Before evaluation
- After evaluation
- When (base, property) is resolved

### Resolution of Model Objects and their Properties

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.

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

### Jakarta Expression Language Functions

If a Jakarta Expression Language expression uses a function (for example `${fn:toUpperCase(customer.name)}`), then a {@link jakarta.el.FunctionMapper} object must also be specified within the `ELContext`. The `FunctionMapper` is responsible to map `${prefix:name()}` style functions to static methods that can execute the specified functions.

### Jakarta Expression Language Variables

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 Jakarta Expression Language variables.

A Jakarta Expression Language variable does not directly refer to a model object that can then be resolved by an `ELResolver`. 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.

For example, in the following code snippet

> `<h:inputText value="#{handler.customer.name}"/>`

`handler` refers to a model object that can be resolved by a Jakarta Expression Language Resolver.

However, in this other example:

>     <c:forEach var="item" items="#{model.list}">
>        <h:inputText value="#{item.name}"/>
>     </c:forEach>

`item` 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 `#{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 jakarta.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 a Jakarta Expression Language variable to the Jakarta Expression Language expression from which it gets its value.

### Jakarta Expression Language in Stand-alone environment

Jakarta Expression Language 4.0 includes APIs for using Jakarta Expression Language in a stand-alone environment.

{@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.

{@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}.