public abstract class ValueExpression extends Expression
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)
,
getType(javax.el.ELContext)
and getValueReference(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 five 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. For all methods other than the getValueReference(javax.el.ELContext)
method,
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)
or
ELResolver.getType(javax.el.ELContext, java.lang.Object, java.lang.Object)
method, depending on which was called on
the ValueExpression
.
For the getValueReference
method, the (base, property) is
not resolved by the ELResolver, but an instance of ValueReference
is created to encapsulate this (base ,property), and returned.
See the notes about comparison, serialization and immutability in
the Expression
javadocs.
ELResolver
,
Expression
,
ExpressionFactory
,
Serialized FormConstructor and Description |
---|
ValueExpression() |
Modifier and Type | Method and Description |
---|---|
abstract Class<?> |
getExpectedType()
Returns the type the result of the expression will be coerced to
after evaluation.
|
abstract 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(javax.el.ELContext, java.lang.Object) method. |
abstract Object |
getValue(ELContext context)
Evaluates the expression relative to the provided context, and
returns the resulting value.
|
ValueReference |
getValueReference(ELContext context)
Returns a
ValueReference for this expression instance. |
abstract boolean |
isReadOnly(ELContext context)
Evaluates the expression relative to the provided context, and
returns
true if a call to setValue(javax.el.ELContext, java.lang.Object) will
always fail. |
abstract void |
setValue(ELContext context,
Object value)
Evaluates the expression relative to the provided context, and
sets the result to the provided value.
|
equals, getExpressionString, hashCode, isLiteralText
public abstract Object getValue(ELContext context)
The resulting value is automatically coerced to the type
returned by getExpectedType()
, which was
provided to the ExpressionFactory
when this
expression was created.
context
- The context of this evaluation.NullPointerException
- if context is null
.PropertyNotFoundException
- if one of the property
resolutions failed because a specified variable or property
does not exist or is not readable.ELException
- if an exception was thrown while performing
property or variable resolution. The thrown exception
must be included as the cause property of this exception, if
available.public abstract void setValue(ELContext context, Object value)
context
- The context of this evaluation.value
- The new value to be set.NullPointerException
- if context is null
.PropertyNotFoundException
- if one of the property
resolutions failed because a specified variable or property
does not exist or is not readable.PropertyNotWritableException
- if the final variable or
property resolution failed because the specified
variable or property is not writable.ELException
- if an exception was thrown while attempting to
set the property or variable. The thrown exception
must be included as the cause property of this exception, if
available.public abstract boolean isReadOnly(ELContext context)
true
if a call to setValue(javax.el.ELContext, java.lang.Object)
will
always fail.context
- The context of this evaluation.true
if the expression is read-only or
false
if not.NullPointerException
- if context is null
.PropertyNotFoundException
- if one of the property
resolutions failed because a specified variable or property
does not exist or is not readable.ELException
- if an exception was thrown while performing
property or variable resolution. The thrown exception
must be included as the cause property of this exception, if
available.
* @throws NullPointerException if context is null
public abstract Class<?> getType(ELContext context)
value
parameter in a future call
to the setValue(javax.el.ELContext, java.lang.Object)
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.
context
- The context of this evaluation.NullPointerException
- if context is null
.PropertyNotFoundException
- if one of the property
resolutions failed because a specified variable or property
does not exist or is not readable.ELException
- if an exception was thrown while performing
property or variable resolution. The thrown exception
must be included as the cause property of this exception, if
available.public abstract Class<?> getExpectedType()
expectedType
passed to the
ExpressionFactory.createValueExpression
method
that created this ValueExpression
.public ValueReference getValueReference(ELContext context)
ValueReference
for this expression instance.context
- the context of this evaluationValueReference
for this
ValueExpression
, or null
if this
ValueExpression
is not a reference to
a base (null or non-null) and a property.
If the base is null, and the property is a EL variable, return
the ValueReference
for the
ValueExpression
associated with this EL variable.Copyright © 2012 JBoss by Red Hat. All Rights Reserved.