juneau-core/juneau-svl/src/main/java/org/apache/juneau/svl/package.html - juneau - Git at Google

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 <p>Simple Variable Language</p>

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 <a id='TOC'></a><h5 class='toc'>Table of Contents</h5>
 <ol class='toc'>
 	<li><p><a class='doclink' href='#SimpleVarLanguage'>Simple Variable Language</a></p>
 		<ol>
 			<li><p><a class='doclink' href='#Vars'>Vars</a></p>
 			<li><p><a class='doclink' href='#VarResolvers'>VarResolvers and VarResolverSessions</a></p>
 			<li><p><a class='doclink' href='#OtherNotes'>Other Notes</a></p>
 		</ol>
 	</li>
 </ol>

 <!-- ======================================================================================================== -->
 <a id="SimpleVarLanguage"></a>
 <h2 class='topic' onclick='toggle(this)'>1 - Simple Variable Language</h2>
 <div class='topic'>
 	<p>
 		The <code>org.apache.juneau.svl</code> package defines an API for a language called "Simple Variable Language".
 		In a nutshell, Simple Variable Language (or SVL) is text that contains variables of the form <js>"$varName{varKey}"</js>.
 	</p>
 	<p>
 		Variables can be recursively nested within the varKey (e.g. <js>"$FOO{$BAR{xxx},$BAZ{xxx}}"</js>).
 		Variables can also return values that themselves contain more variables.
 	</p>
 	<p>
 		The {@link org.apache.juneau.svl.VarResolver} class is used to resolve variables.
 		The {@link org.apache.juneau.svl.VarResolver#DEFAULT} resolver will resolve <js>"$S{systemProperty}"</js>
 		and <js>"$E{envVariable}"</js> variables.
 	</p>
 	<p class='bcode'>
 	<jc>// Use the default variable resolver to resolve a string that contains $S (system property) variables</jc>
 	String myProperty = VarResolver.<jsf>DEFAULT</jsf>.resolve(<js>"The Java home directory is $S{java.home}"</js>);
 	</p>
 	<p>
 		The following shows how variables can be arbitrarily nested...
 	</p>
 	<p class='bcode'>
 	<jc>// Look up a property in the following order:
 	// 1) MYPROPERTY environment variable.
 	// 2) 'my.property' system property if environment variable not found.
 	// 3) 'not found' string if system property not found.</jc>
 	String myproperty = VarResolver.<jsf>DEFAULT</jsf>.resolve(<js>"$E{MYPROPERTY,$S{my.property,not found}}"</js>);
  	</p>

 	<!-- ======================================================================================================== -->
 	<a id="Vars"></a>
 	<h3 class='topic' onclick='toggle(this)'>1.1 - Vars</h3>
 	<div class='topic'>
 		<p>
 			Variables are defined through the {@link org.apache.juneau.svl.Var} API.
 		</p>
 		<p class='bcode'>
 	<jc>// Create a var resolver that extends the default resolver and appends our own "$URLEncode{...}" variable</jc>

 	<jc>// First create our var.</jc>
 	<jk>public class</jk> UrlEncodeVar <jk>extends</jk> SimpleVar {

 		<jc>// Must have a no-arg constructor!</jc>
 		<jk>public</jk> UrlEncodeVar() {
 			<jk>super</jk>(<js>"URLEncode"</js>);
 		}

 		<jc>// The method we must implement</jc>
 		<ja>@Override</ja>
 		<jk>public</jk> String resolve(VarResolverSession session, String varVal) {
 			<jk>return</jk> URLEncoder.<jsm>encode</jsm>(varVal, <js>"UTF-8"</js>);
 		}
 	}

 	<jc>// Next create a var resolver that extends the existing DEFAULT resolver
 	// that supports resolving system properties.</jc>
 	VarResolver r = VarResolver.<jsf>DEFAULT</jsf>.builder().vars(UrlEncodeVar.<jk>class</jk>).build();

 	<jc>// Retrieve a system property and URL-encode it if necessary.</jc>
 	String myProperty = r.resolve(<js>"$URLEncode{$S{my.property}}"</js>);
 		</p>
 		<p>
 			The following shows the class hierarchy of the {@link org.apache.juneau.svl.Var} class and all current
 			predefined implementations.
 		</p>
 		<ul class='doctree'>
 			<li class='jac'>
 				{@link org.apache.juneau.svl.Var}
 				- Superclass of all vars.
 				<ul>
 					<li class='jac'>
 						{@link org.apache.juneau.svl.SimpleVar}
 						- Superclass of all vars that return strings.
 						<ul>
 							<li class='jac'>
 								{@link org.apache.juneau.svl.DefaultingVar}
 								- Variables that define a default value if the resolve method returns null.
 								<ul>
 									<li class='jac'>
 										{@link org.apache.juneau.svl.MapVar}
 										- Variables that pull values from maps.
 										<ul>
 											<li class='jc'>
 												{@link org.apache.juneau.svl.vars.SystemPropertiesVar}
 												- Resolves system properties.
 										</ul>
 									</li>
 									<li class='jc'>
 										{@link org.apache.juneau.svl.vars.ArgsVar}
 										- Resolves variables from an {@link org.apache.juneau.utils.Args} object.
 									<li class='jc'>
 										<code>ConfigFileVar</code>
 										- Resolves variables from a {@link org.apache.juneau.ini.ConfigFile} object.
 									<li class='jc'>
 										{@link org.apache.juneau.svl.vars.EnvVariablesVar}
 										- Resolves environment variables.
 									<li class='jc'>
 										{@link org.apache.juneau.svl.vars.ManifestFileVar}
 										- Resolves variables from a {@link org.apache.juneau.utils.ManifestFile} object.
 									<li class='jc'>
 										{@link org.apache.juneau.rest.vars.ServletInitParamVar}
 										- Resolves servlet initialization parameters.
 								</ul>
 							</li>
 							<li class='jac'>
 								{@link org.apache.juneau.svl.MultipartVar}
 								- Variables that consist of 2 or more comma-delimited arguments.
 								<ul>
 									<li class='jc'>
 										{@link org.apache.juneau.rest.vars.LocalizationVar}
 										- Resolves localized strings for an HTTP request.
 								</ul>
 							</li>
 							<li class='jc'>
 								{@link org.apache.juneau.rest.vars.RequestVar}
 								- Resolves specialized HTTP request values.
 							<li class='jc'>
 								{@link org.apache.juneau.rest.vars.UrlEncodeVar}
 								- URL-encodes the value inside the variable.
 						</ul>
 					</li>
 					<li class='jac'>
 						{@link org.apache.juneau.svl.StreamedVar}
 						- Superclass of all vars that stream their value to writers.
 						<ul>
 							<li class='jc'>
 								{@link org.apache.juneau.rest.vars.SerializedRequestAttrVar}
 								- Resolves HTTP request attribute values passed through a
 								{@link org.apache.juneau.serializer.Serializer}.
 						</ul>
 					</li>
 				</ul>
 			</li>
 		</ul>
 	</div>

 	<!-- ======================================================================================================== -->
 	<a id="VarResolvers"></a>
 	<h3 class='topic' onclick='toggle(this)'>1.2 - VarResolvers and VarResolverSessions</h3>
 	<div class='topic'>
 		<p>
 			The main class for performing variable resolution is {@link org.apache.juneau.svl.VarResolver}.
 			Two methods are provided for resolving variables:
 		</p>
 		<ul class='doctree'>
 			<li class='jm'>
 				{@link org.apache.juneau.svl.VarResolver#resolve(String)}
 				- Resolves variables and returns the results as a simple string.
 			<li class='jm'>
 				{@link org.apache.juneau.svl.VarResolver#resolveTo(String,Writer)}
 				- Resolves variables and sends results to a writer.
 		</ul>
 		<p>
 			Var resolvers can have zero or more context objects associated with them.
 			Some {@link org.apache.juneau.svl.Var Vars} rely on the existence of some other object, such as an
 			{@link org.apache.juneau.utils.Args} object
 			for {@link org.apache.juneau.svl.vars.ArgsVar} or a {@link org.apache.juneau.ini.ConfigFile} for a
 			<code>ConfigFileVar</code>.
 			These object dependencies are made by setting context objects on the var resolver.
 		</p>
 		<p>
 			Context objects are set through the {@link org.apache.juneau.svl.VarResolverBuilder
 			#contextObject(String,Object)} method.
 			They can be any class type.
 		</p>
 		<p>
 			Context objects are used by {@link org.apache.juneau.svl.Var Vars} by calling the
 			{@link org.apache.juneau.svl.VarResolverSession#getSessionObject(Class, String)} method.
 		</p>
 		<p>
 			In addition to context objects, there are also session objects.
 			Session objects are considered more ephemeral than context objects.
 			While a context object is unlikely to ever change, a session object may change on every use of the var
 			resolver.
 			For example, the server API defines various <code>Var</code> objects that use the <code>RestRequest</code>
 				object as a session object for the duration of a single HTTP request.
 		</p>
 		<p>
 			Session objects are used by calling the {@link org.apache.juneau.svl.VarResolver#createSession()} or
 			{@link org.apache.juneau.svl.VarResolver#createSession(Map)} methods to create an instance of a
 			{@link org.apache.juneau.svl.VarResolverSession} object that contains
 			{@link org.apache.juneau.svl.VarResolverSession#resolve(String)} and
 			{@link org.apache.juneau.svl.VarResolverSession#resolveTo(String,Writer)} methods that are identical to
 			{@link org.apache.juneau.svl.VarResolver#resolve(String)} and
 			{@link org.apache.juneau.svl.VarResolver#resolveTo(String, Writer)} except that the <code>Var</code> objects
 			have access to the session objects through the
 			{@link org.apache.juneau.svl.VarResolverSession#getSessionObject(Class, String)} method.
 			Session objects are specified through either the {@link org.apache.juneau.svl.VarResolver#createSession(Map)}
 			method or the {@link org.apache.juneau.svl.VarResolverSession#sessionObject(String, Object)} methods.
 		</p>
 		<p>
 			Like Context object, Session objects are used by {@link org.apache.juneau.svl.Var Vars} by calling the
 			{@link org.apache.juneau.svl.VarResolverSession#getSessionObject(Class, String)} method.
 		</p>
 		<p>
 			Var resolvers can be cloned and extended by using the {@link org.apache.juneau.svl.VarResolver#builder()}
 			method.
 			Cloning a resolver will copy it's {@link org.apache.juneau.svl.Var} class names and context objects.
 		</p>

 		<h6 class='topic'>Example:</h6>
 		<p class='bcode'>
 			<jc>// Create a resolver that copies the default resolver and adds $C and $ARG vars.</jc>
 			VarResolver myVarResolver = VarResolver.<jsf>DEFAULT</jsf>.builder().vars(ConfigFileVar.<jk>class</jk>,
 				ArgsVar.<jk>class</jk>).build();
 		</p>
 	</div>

 	<!-- ======================================================================================================== -->
 	<a id="OtherNotes"></a>
 	<h3 class='topic' onclick='toggle(this)'>1.3 - Other Notes</h3>
 	<div class='topic'>
 		<ul class='spaced-list'>
 			<li>
 				The escape character <js>'\'</js> can be used when necessary to escape the following characters:
 				<code>$ , { }</code>
 			<li>
 				<b>WARNING:</b>  It is possible to cause {@link java.lang.StackOverflowError StackOverflowErrors} if
 				your nested variables result in a recursive loop (e.g. the environment variable
 				<code>'MYPROPERTY'</code> has the value <code>'$E{MYPROPERTY}'</code>).
 				So don't do that!
 			<li>
 				As a general rule, this class tries to be as efficient as possible by not creating new strings when not
 				needed.
 				<br>For example, calling the resolve method on a string that doesn't contain variables (e.g.
 				<code>resolver.resolve(<js>"foobar"</js>)</code>) will simply be a no-op and return the same string.
 		</ul>
 	</div>
 </div>
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	* with the License. You may obtain a copy of the License at
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	* http://www.apache.org/licenses/LICENSE-2.0
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	* Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an
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	</script>
	</head>
	<body>
	<p>Simple Variable Language</p>

	<script>
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	<a id='TOC'></a><h5 class='toc'>Table of Contents</h5>
	<ol class='toc'>
	<li><p><a class='doclink' href='#SimpleVarLanguage'>Simple Variable Language</a></p>
	<ol>
	<li><p><a class='doclink' href='#Vars'>Vars</a></p>
	<li><p><a class='doclink' href='#VarResolvers'>VarResolvers and VarResolverSessions</a></p>
	<li><p><a class='doclink' href='#OtherNotes'>Other Notes</a></p>
	</ol>
	</li>
	</ol>

	<!-- ======================================================================================================== -->
	<a id="SimpleVarLanguage"></a>
	<h2 class='topic' onclick='toggle(this)'>1 - Simple Variable Language</h2>
	<div class='topic'>
	<p>
	The <code>org.apache.juneau.svl</code> package defines an API for a language called "Simple Variable Language".
	In a nutshell, Simple Variable Language (or SVL) is text that contains variables of the form <js>"$varName{varKey}"</js>.
	</p>
	<p>
	Variables can be recursively nested within the varKey (e.g. <js>"$FOO{$BAR{xxx},$BAZ{xxx}}"</js>).
	Variables can also return values that themselves contain more variables.
	</p>
	<p>
	The {@link org.apache.juneau.svl.VarResolver} class is used to resolve variables.
	The {@link org.apache.juneau.svl.VarResolver#DEFAULT} resolver will resolve <js>"$S{systemProperty}"</js>
	and <js>"$E{envVariable}"</js> variables.
	</p>
	<p class='bcode'>
	<jc>// Use the default variable resolver to resolve a string that contains $S (system property) variables</jc>
	String myProperty = VarResolver.<jsf>DEFAULT</jsf>.resolve(<js>"The Java home directory is $S{java.home}"</js>);
	</p>
	<p>
	The following shows how variables can be arbitrarily nested...
	</p>
	<p class='bcode'>
	<jc>// Look up a property in the following order:
	// 1) MYPROPERTY environment variable.
	// 2) 'my.property' system property if environment variable not found.
	// 3) 'not found' string if system property not found.</jc>
	String myproperty = VarResolver.<jsf>DEFAULT</jsf>.resolve(<js>"$E{MYPROPERTY,$S{my.property,not found}}"</js>);
	</p>

	<!-- ======================================================================================================== -->
	<a id="Vars"></a>
	<h3 class='topic' onclick='toggle(this)'>1.1 - Vars</h3>
	<div class='topic'>
	<p>
	Variables are defined through the {@link org.apache.juneau.svl.Var} API.
	</p>
	<p class='bcode'>
	<jc>// Create a var resolver that extends the default resolver and appends our own "$URLEncode{...}" variable</jc>

	<jc>// First create our var.</jc>
	<jk>public class</jk> UrlEncodeVar <jk>extends</jk> SimpleVar {

	<jc>// Must have a no-arg constructor!</jc>
	<jk>public</jk> UrlEncodeVar() {
	<jk>super</jk>(<js>"URLEncode"</js>);
	}

	<jc>// The method we must implement</jc>
	<ja>@Override</ja>
	<jk>public</jk> String resolve(VarResolverSession session, String varVal) {
	<jk>return</jk> URLEncoder.<jsm>encode</jsm>(varVal, <js>"UTF-8"</js>);
	}
	}

	<jc>// Next create a var resolver that extends the existing DEFAULT resolver
	// that supports resolving system properties.</jc>
	VarResolver r = VarResolver.<jsf>DEFAULT</jsf>.builder().vars(UrlEncodeVar.<jk>class</jk>).build();

	<jc>// Retrieve a system property and URL-encode it if necessary.</jc>
	String myProperty = r.resolve(<js>"$URLEncode{$S{my.property}}"</js>);
	</p>
	<p>
	The following shows the class hierarchy of the {@link org.apache.juneau.svl.Var} class and all current
	predefined implementations.
	</p>
	<ul class='doctree'>
	<li class='jac'>
	{@link org.apache.juneau.svl.Var}
	- Superclass of all vars.
	<ul>
	<li class='jac'>
	{@link org.apache.juneau.svl.SimpleVar}
	- Superclass of all vars that return strings.
	<ul>
	<li class='jac'>
	{@link org.apache.juneau.svl.DefaultingVar}
	- Variables that define a default value if the resolve method returns null.
	<ul>
	<li class='jac'>
	{@link org.apache.juneau.svl.MapVar}
	- Variables that pull values from maps.
	<ul>
	<li class='jc'>
	{@link org.apache.juneau.svl.vars.SystemPropertiesVar}
	- Resolves system properties.
	</ul>
	</li>
	<li class='jc'>
	{@link org.apache.juneau.svl.vars.ArgsVar}
	- Resolves variables from an {@link org.apache.juneau.utils.Args} object.
	<li class='jc'>
	<code>ConfigFileVar</code>
	- Resolves variables from a {@link org.apache.juneau.ini.ConfigFile} object.
	<li class='jc'>
	{@link org.apache.juneau.svl.vars.EnvVariablesVar}
	- Resolves environment variables.
	<li class='jc'>
	{@link org.apache.juneau.svl.vars.ManifestFileVar}
	- Resolves variables from a {@link org.apache.juneau.utils.ManifestFile} object.
	<li class='jc'>
	{@link org.apache.juneau.rest.vars.ServletInitParamVar}
	- Resolves servlet initialization parameters.
	</ul>
	</li>
	<li class='jac'>
	{@link org.apache.juneau.svl.MultipartVar}
	- Variables that consist of 2 or more comma-delimited arguments.
	<ul>
	<li class='jc'>
	{@link org.apache.juneau.rest.vars.LocalizationVar}
	- Resolves localized strings for an HTTP request.
	</ul>
	</li>
	<li class='jc'>
	{@link org.apache.juneau.rest.vars.RequestVar}
	- Resolves specialized HTTP request values.
	<li class='jc'>
	{@link org.apache.juneau.rest.vars.UrlEncodeVar}
	- URL-encodes the value inside the variable.
	</ul>
	</li>
	<li class='jac'>
	{@link org.apache.juneau.svl.StreamedVar}
	- Superclass of all vars that stream their value to writers.
	<ul>
	<li class='jc'>
	{@link org.apache.juneau.rest.vars.SerializedRequestAttrVar}
	- Resolves HTTP request attribute values passed through a
	{@link org.apache.juneau.serializer.Serializer}.
	</ul>
	</li>
	</ul>
	</li>
	</ul>
	</div>

	<!-- ======================================================================================================== -->
	<a id="VarResolvers"></a>
	<h3 class='topic' onclick='toggle(this)'>1.2 - VarResolvers and VarResolverSessions</h3>
	<div class='topic'>
	<p>
	The main class for performing variable resolution is {@link org.apache.juneau.svl.VarResolver}.
	Two methods are provided for resolving variables:
	</p>
	<ul class='doctree'>
	<li class='jm'>
	{@link org.apache.juneau.svl.VarResolver#resolve(String)}
	- Resolves variables and returns the results as a simple string.
	<li class='jm'>
	{@link org.apache.juneau.svl.VarResolver#resolveTo(String,Writer)}
	- Resolves variables and sends results to a writer.
	</ul>
	<p>
	Var resolvers can have zero or more context objects associated with them.
	Some {@link org.apache.juneau.svl.Var Vars} rely on the existence of some other object, such as an
	{@link org.apache.juneau.utils.Args} object
	for {@link org.apache.juneau.svl.vars.ArgsVar} or a {@link org.apache.juneau.ini.ConfigFile} for a
	<code>ConfigFileVar</code>.
	These object dependencies are made by setting context objects on the var resolver.
	</p>
	<p>
	Context objects are set through the {@link org.apache.juneau.svl.VarResolverBuilder
	#contextObject(String,Object)} method.
	They can be any class type.
	</p>
	<p>
	Context objects are used by {@link org.apache.juneau.svl.Var Vars} by calling the
	{@link org.apache.juneau.svl.VarResolverSession#getSessionObject(Class, String)} method.
	</p>
	<p>
	In addition to context objects, there are also session objects.
	Session objects are considered more ephemeral than context objects.
	While a context object is unlikely to ever change, a session object may change on every use of the var
	resolver.
	For example, the server API defines various <code>Var</code> objects that use the <code>RestRequest</code>
	object as a session object for the duration of a single HTTP request.
	</p>
	<p>
	Session objects are used by calling the {@link org.apache.juneau.svl.VarResolver#createSession()} or
	{@link org.apache.juneau.svl.VarResolver#createSession(Map)} methods to create an instance of a
	{@link org.apache.juneau.svl.VarResolverSession} object that contains
	{@link org.apache.juneau.svl.VarResolverSession#resolve(String)} and
	{@link org.apache.juneau.svl.VarResolverSession#resolveTo(String,Writer)} methods that are identical to
	{@link org.apache.juneau.svl.VarResolver#resolve(String)} and
	{@link org.apache.juneau.svl.VarResolver#resolveTo(String, Writer)} except that the <code>Var</code> objects
	have access to the session objects through the
	{@link org.apache.juneau.svl.VarResolverSession#getSessionObject(Class, String)} method.
	Session objects are specified through either the {@link org.apache.juneau.svl.VarResolver#createSession(Map)}
	method or the {@link org.apache.juneau.svl.VarResolverSession#sessionObject(String, Object)} methods.
	</p>
	<p>
	Like Context object, Session objects are used by {@link org.apache.juneau.svl.Var Vars} by calling the
	{@link org.apache.juneau.svl.VarResolverSession#getSessionObject(Class, String)} method.
	</p>
	<p>
	Var resolvers can be cloned and extended by using the {@link org.apache.juneau.svl.VarResolver#builder()}
	method.
	Cloning a resolver will copy it's {@link org.apache.juneau.svl.Var} class names and context objects.
	</p>

	<h6 class='topic'>Example:</h6>
	<p class='bcode'>
	<jc>// Create a resolver that copies the default resolver and adds $C and $ARG vars.</jc>
	VarResolver myVarResolver = VarResolver.<jsf>DEFAULT</jsf>.builder().vars(ConfigFileVar.<jk>class</jk>,
	ArgsVar.<jk>class</jk>).build();
	</p>
	</div>

	<!-- ======================================================================================================== -->
	<a id="OtherNotes"></a>
	<h3 class='topic' onclick='toggle(this)'>1.3 - Other Notes</h3>
	<div class='topic'>
	<ul class='spaced-list'>
	<li>
	The escape character <js>'\'</js> can be used when necessary to escape the following characters:
	<code>$ , { }</code>
	<li>
	<b>WARNING:</b> It is possible to cause {@link java.lang.StackOverflowError StackOverflowErrors} if
	your nested variables result in a recursive loop (e.g. the environment variable
	<code>'MYPROPERTY'</code> has the value <code>'$E{MYPROPERTY}'</code>).
	So don't do that!
	<li>
	As a general rule, this class tries to be as efficient as possible by not creating new strings when not
	needed.
	<br>For example, calling the resolve method on a string that doesn't contain variables (e.g.
	<code>resolver.resolve(<js>"foobar"</js>)</code>) will simply be a no-op and return the same string.
	</ul>
	</div>
	</div>
	</body>
	</html>