javax.el Class ValueExpression
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javax.el
Class ValueExpression
java.lang.Object javax.el.Expression javax.el.ValueExpression
- All Implemented Interfaces:
- Serializable
public abstract class ValueExpression
- extends Expression
An Expression
that can get or set a value.
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
method can be used to parse an expression string and return a concrete instance of ExpressionFactory.createValueExpression(javax.el.ELContext, java.lang.String, java.lang.Class)
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.
The getValue(javax.el.ELContext)
, setValue(javax.el.ELContext, java.lang.Object)
, isReadOnly(javax.el.ELContext)
and getType(javax.el.ELContext)
methods will evaluate the expression each time they are called. The ELResolver
in the ELContext
is used to resolve the top-level variables and to determine the behavior of the .
and []
operators. For any of the four methods, the ELResolver.getValue(javax.el.ELContext, java.lang.Object, java.lang.Object)
method is used to resolve all properties up to but excluding the last one. This provides the base
object. At the last resolution, the ValueExpression
will call the corresponding ELResolver.getValue(javax.el.ELContext, java.lang.Object, java.lang.Object)
, ELResolver.setValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)
, ELResolver.isReadOnly(javax.el.ELContext, java.lang.Object, java.lang.Object)
orELResolver.getType(javax.el.ELContext, java.lang.Object, java.lang.Object)
method, depending on which was called on the ValueExpression
.
See the notes about comparison, serialization and immutability in the Expression
javadocs.
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