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(jakarta.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(jakarta.el.ELContext)
, setValue(jakarta.el.ELContext, java.lang.Object)
, isReadOnly(jakarta.el.ELContext)
, getType(jakarta.el.ELContext)
and getValueReference(jakarta.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(jakarta.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(jakarta.el.ELContext)
method, at the last resolution, the ValueExpression
will call the
corresponding ELResolver.getValue(jakarta.el.ELContext, java.lang.Object, java.lang.Object)
, ELResolver.setValue(jakarta.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)
, ELResolver.isReadOnly(jakarta.el.ELContext, java.lang.Object, java.lang.Object)
or
ELResolver.getType(jakarta.el.ELContext, java.lang.Object, java.lang.Object)
method, depending on which was called on the ValueExpression
. For the
getValueReference(jakarta.el.ELContext)
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(jakarta.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(jakarta.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(jakarta.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(jakarta.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 Jakarta Expression Language variable, return the ValueReference
for the
ValueExpression
associated with this Jakarta Expression Language variable.Copyright © 2018,2020 Eclipse Foundation.
Use is subject to license terms.