Simple Variable Language

The juneau-svl module 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 "$varName{varKey}".
It is used extensively in the Config, REST and Microservice APIs.

Most variables can be recursively nested within the varKey (e.g. "$FOO{$BAR{xxx},$BAZ{xxx}}") and can return values that themselves contain more variables.

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:

- {@link oaj.svl.vars.SystemPropertiesVar} - `$S{key[,default]}`
- {@link oaj.svl.vars.EnvVariablesVar} - `$E{key[,default]}`

The following logic variables are also provided:

- {@link oaj.svl.vars.IfVar} - `$IF{arg,then[,else]}`
- {@link oaj.svl.vars.SwitchVar} - `$SW{arg,pattern1:then1[,pattern2:then2...]}`
- {@link oaj.svl.vars.CoalesceVar} - `$CO{arg1[,arg2...]}`
- {@link oaj.svl.vars.PatternMatchVar} - `$PM{arg,pattern}`
- {@link oaj.svl.vars.NotEmptyVar} - `$NE{arg}`
- {@link oaj.svl.vars.UpperCaseVar} - `$UC{arg}`
- {@link oaj.svl.vars.LowerCaseVar} - `$LC{arg}`

##### Example:

// Use the default variable resolver to resolve a string that contains $S (system property) variables String myProperty = VarResolver.DEFAULT.resolve("The Java home directory is $S{java.home}");

The following shows how variables can be arbitrarily nested...

// 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. String myproperty = VarResolver.DEFAULT.resolve("$E{MYPROPERTY,$S{my.property,not found}}");