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Simple Variable Language

<p>
	The <l>org.apache.juneau.svl</l> packages defines an API for a language called <l>Simple Variable Language</l>.
	In a nutshell, Simple Variable Language (or SVL) is text that contains variables of the form <js>"$varName{varKey}"</js>.
	It is used extensively in the Config, REST and Microservice APIs.
</p>
<p>
	Most variables can be recursively nested within the varKey (e.g. <js>"$FOO{$BAR{xxx},$BAZ{xxx}}"</js>)
	and can return values that themselves contain more variables.
</p>
<p>
	The {@link oaj.svl.VarResolver} class is used to resolve variables.
	The {@link oaj.svl.VarResolver#DEFAULT} resolver is a reusable instance of this class 
	configured with the following basic variables:
</p>
<ul class='javatree'>
	<li class='jc'>{@link oaj.svl.vars.SystemPropertiesVar} - <c>$S{key[,default]}</c>
	<li class='jc'>{@link oaj.svl.vars.EnvVariablesVar} - <c>$E{key[,default]}</c>
</ul>
<p>
	The following logic variables are also provided:
</p>
<ul class='javatree'>
	<li class='jc'>{@link oaj.svl.vars.IfVar} - <c>$IF{arg,then[,else]}</c>
	<li class='jc'>{@link oaj.svl.vars.SwitchVar} - <c>$SW{arg,pattern1:then1[,pattern2:then2...]}</c>
	<li class='jc'>{@link oaj.svl.vars.CoalesceVar} - <c>$CO{arg1[,arg2...]}</c>
	<li class='jc'>{@link oaj.svl.vars.PatternMatchVar} - <c>$PM{arg,pattern}</c> 
	<li class='jc'>{@link oaj.svl.vars.PatternReplaceVar} - <c>$PR{arg,pattern,replace}</c> 
	<li class='jc'>{@link oaj.svl.vars.PatternExtractVar} - <c>$PE{arg,pattern,groupIndex}</c> 
	<li class='jc'>{@link oaj.svl.vars.NotEmptyVar} - <c>$NE{arg}</c>
	<li class='jc'>{@link oaj.svl.vars.UpperCaseVar} - <c>$UC{arg}</c>
	<li class='jc'>{@link oaj.svl.vars.LowerCaseVar} - <c>$LC{arg}</c>
	<li class='jc'>{@link oaj.svl.vars.LenVar} - <c>$LN{arg[,delimiter]}</c>
	<li class='jc'>{@link oaj.svl.vars.SubstringVar} - <c>$ST{arg,start[,end]}</c>
</ul>
<h5 class='figure'>Example:</h5>
<p class='bpcode w800'>
	<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 $S{java.home}"</js>);
</p>
<p>
	The following shows how variables can be arbitrarily nested...
</p>
<p class='bpcode w800'>
	<jc>// Look up a property in the following order:
	// 1) MYPROPERTY environment variable.
	// 2) 'my.property' system property if environment variable not found.
	// 3) 'not found' string if system property not found.</jc>
	String myproperty = VarResolver.<jsf>DEFAULT</jsf>.resolve(<js>"$E{MYPROPERTY,$S{my.property,not found}}"</js>);
</p>