java.lang.Object
   javax.el.Expression
      javax.el.ValueExpression

All Implemented Interfaces:
    Serializable

Direct Known Subclasses:
    IteratedValueExpression, IndexedValueExpression

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 ExpressionFactory#createValueExpression 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.

The #getValue , #setValue , #isReadOnly and #getType 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 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 , ELResolver#setValue , ELResolver#isReadOnly or ELResolver#getType method, depending on which was called on the ValueExpression.

See the notes about comparison, serialization and immutability in the Expression javadocs.

Also see:

ELResolver
Expression
ExpressionFactory
since: JSP - 2.1

 abstract public Class getExpectedType()
Returns the type the result of the expression will be coerced to
after evaluation.

 abstract public Class getType(ELContext context)
Evaluates the expression relative to the provided context, and
returns the most general type that is acceptable for an object to be
passed as the value parameter in a future call
to the #setValue  method.

This is not always the same as getValue().getClass().
For example, in the case of an expression that references an
array element, the getType method will return the
element type of the array, which might be a superclass of the type
of the actual element that is currently in the specified
array element.

 abstract public Object getValue(ELContext context)
Evaluates the expression relative to the provided context, and
returns the resulting value.

The resulting value is automatically coerced to the type
returned by getExpectedType(), which was
provided to the ExpressionFactory when this
expression was created.

 abstract public boolean isReadOnly(ELContext context)
Evaluates the expression relative to the provided context, and
returns true if a call to #setValue  will
always fail.

 abstract public  void setValue(ELContext context,
    Object value)
Evaluates the expression relative to the provided context, and
sets the result to the provided value.