In previous incarnations of this API, expressions could only be
read. ValueExpression objects can now be used both to
retrieve a value and to set a value. Expressions that can have a value
set on them are referred to as l-value expressions. Those that
cannot are referred to as r-value expressions. Not all r-value expressions
can be used as l-value expressions (e.g. "${1+1}" or
"${firstName} ${lastName}"). See the EL Specification for
details. Expressions that cannot be used as l-values must always
return true from isReadOnly().
The createValueExpression(ELContext, String, Class) method
can be used to parse an expression string and return a concrete instance
of ValueExpression that encapsulates the parsed expression.
The FunctionMapper is used at parse time, not evaluation time,
so one is not needed to evaluate an expression using this class.
However, the ELContext is needed at evaluation time.
Expressionthat can get or set a value.In previous incarnations of this API, expressions could only be read.
ValueExpressionobjects can now be used both to retrieve a value and to set a value. Expressions that can have a value set on them are referred to as l-value expressions. Those that cannot are referred to as r-value expressions. Not all r-value expressions can be used as l-value expressions (e.g."${1+1}"or"${firstName} ${lastName}"). See the EL Specification for details. Expressions that cannot be used as l-values must always returntruefromisReadOnly().The
createValueExpression(ELContext, String, Class)method can be used to parse an expression string and return a concrete instance ofValueExpressionthat encapsulates the parsed expression. The FunctionMapper is used at parse time, not evaluation time, so one is not needed to evaluate an expression using this class. However, the ELContext is needed at evaluation time.The getValue(ELContext) , setValue(ELContext, Object) , isReadOnly(ELContext) and getType(ELContext) methods will evaluate the expression each time they are called. The ELResolver in the
ELContextis used to resolve the top-level variables and to determine the behavior of the.and[]operators. For any of the four methods, the getValue(ELContext, Object, Object) method is used to resolve all properties up to but excluding the last one. This provides thebaseobject. At the last resolution, theValueExpressionwill call the corresponding getValue(ELContext, Object, Object) , setValue(ELContext, Object, Object, Object) , isReadOnly(ELContext, Object, Object) or getType(ELContext, Object, Object) method, depending on which was called on theValueExpression.See the notes about comparison, serialization and immutability in the Expression javadocs.