Expression factory implementation.
This class is also used as an EL "service provider".
The JUEL jar files specify this class as their expression factory
implementation in META-INF/services/javax.el.ExpressionFactory.
Calling
ExpressionFactory.getExpressionFactory will then return
an instance of this class, configured as described below.
The default constructor loads properties from resources
JAVA_HOME/lib/el.properties -
If this file exists and if it contains property javax.el.ExpressionFactory
whose value is the name of this class, these properties are taken as default properties.