api/src/main/java/javax/el/package.html - third_party/jakartaee/expression-language - Git at Google

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 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 <html>
 <head>
 </head>

 <body bgcolor="white">
 Provides the API for the <strong>Unified Expression Language 3.0</strong>

 <p>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.</p>

 <p>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:

 <ul>
   <li><a href="#Context">EL 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">EL Functions</a></li>
   <li><a href="#Variables">EL Variables</a></li>
   <li><a href="#Standalone">EL in Stand-alone environment</li>
 </ul>

 <h3><a name="Context">EL Context</a></h3>

 <p>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.</p>

 <p>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.
 </p>

 <p>When EL used in a web container, the creation of <code>ELContext
 </code> objects is controlled through the underlying technology.
 For example, in JSP, the
  <code>JspContext.getELContext()</code> factory method is used.  In an
 stand-alone environment, a default {@link javax.el.StandardELContext} is
 provided.</p>

  <p>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 <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 the EL: <i>value expressions</i>
 and <i>method expressions</i>. A {@link javax.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 javax.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 EL expression can represent a method
 invocation, such as <code>${trader.buy("JAVA")}</code>, where the arugments to
 the method invocation are specified in the expression.</p>

 <p>All expression classes extend the base class {@link javax.el.Expression}, making them
 serializable and forcing them to implement <code>equals()</code> and
 <code>hashCode()</code>. 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.</p>

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

 <p>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.</p>

 <p>To create an expression, one must provide an {@link javax.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 an EL function
 (for example <code>${fn:toUpperCase(customer.name)}</code>) or an
 EL variable, then
 {@link javax.el.FunctionMapper} and {@link javax.el.VariableMapper}
 objects must be available within the <code>ELContext</code> so that EL functions and
 EL 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 javax.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 JSF request processing life cycle, EL 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 javax.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 javax.el.EvaluationListener}s in ELContext, a user can
 receive notifications during the EL 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 javax.el.ELResolver} base class, the EL
 features a pluggable mechanism
 to resolve model object references as well as properties and method
 invocations of these objects.</p>

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

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

 <h3><a name="Functions">EL Functions</a></h3>

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

 <h3><a name="Variables">EL Variables</a></h3>

 <p>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
 <strong>EL variables</strong>.
 </p>

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

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

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

 <code>item</code> 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
 <code>#{model.list}</code>.
 </p>

 <p>
 Assuming that there are three elements in <code>${model.list}</code>, this means
 that for
 each invocation of <code>&lt;h:inputText></code>, the following information
 about <code>item</code> must be preserved in the {@link javax.el.VariableMapper}:
 <blockquote>
     first invocation: <code>item</code> maps to first element in <code>${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 an EL variable to the EL expression from which it gets its value.
 </p>

 <h3><a name="Standalone">EL in Stand-alone environment</a></h3>
 <p>EL 3.0 includes APIs for using EL in a stand-alone environment.</p>
 <p>{@link javax.el.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.</p>

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

 </body>
 </html>
	<!--

	Copyright (c) 1997-2018 Oracle and/or its affiliates. All rights reserved.
	Copyright 2004 The Apache Software Foundation

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.

	-->

	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
	<html>
	<head>
	</head>

	<body bgcolor="white">
	Provides the API for the <strong>Unified Expression Language 3.0</strong>

	<p>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.</p>

	<p>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:

	<ul>
	<li><a href="#Context">EL 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">EL Functions</a></li>
	<li><a href="#Variables">EL Variables</a></li>
	<li><a href="#Standalone">EL in Stand-alone environment</li>
	</ul>

	<h3><a name="Context">EL Context</a></h3>

	<p>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.</p>

	<p>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.
	</p>

	<p>When EL used in a web container, the creation of <code>ELContext
	</code> objects is controlled through the underlying technology.
	For example, in JSP, the
	<code>JspContext.getELContext()</code> factory method is used. In an
	stand-alone environment, a default {@link javax.el.StandardELContext} is
	provided.</p>

	<p>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 <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 the EL: <i>value expressions</i>
	and <i>method expressions</i>. A {@link javax.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 javax.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 EL expression can represent a method
	invocation, such as <code>${trader.buy("JAVA")}</code>, where the arugments to
	the method invocation are specified in the expression.</p>

	<p>All expression classes extend the base class {@link javax.el.Expression}, making them
	serializable and forcing them to implement <code>equals()</code> and
	<code>hashCode()</code>. 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.</p>

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

	<p>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.</p>

	<p>To create an expression, one must provide an {@link javax.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 an EL function
	(for example <code>${fn:toUpperCase(customer.name)}</code>) or an
	EL variable, then
	{@link javax.el.FunctionMapper} and {@link javax.el.VariableMapper}
	objects must be available within the <code>ELContext</code> so that EL functions and
	EL 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 javax.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 JSF request processing life cycle, EL 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 javax.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 javax.el.EvaluationListener}s in ELContext, a user can
	receive notifications during the EL 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 javax.el.ELResolver} base class, the EL
	features a pluggable mechanism
	to resolve model object references as well as properties and method
	invocations of these objects.</p>

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

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

	<h3><a name="Functions">EL Functions</a></h3>

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

	<h3><a name="Variables">EL Variables</a></h3>

	<p>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
	<strong>EL variables</strong>.
	</p>

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

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

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

	<code>item</code> 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
	<code>#{model.list}</code>.
	</p>

	<p>
	Assuming that there are three elements in <code>${model.list}</code>, this means
	that for
	each invocation of <code><h:inputText></code>, the following information
	about <code>item</code> must be preserved in the {@link javax.el.VariableMapper}:
	<blockquote>
	first invocation: <code>item</code> maps to first element in <code>${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 an EL variable to the EL expression from which it gets its value.
	</p>

	<h3><a name="Standalone">EL in Stand-alone environment</a></h3>
	<p>EL 3.0 includes APIs for using EL in a stand-alone environment.</p>
	<p>{@link javax.el.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.</p>

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

	</body>
	</html>