public abstract class ValueExpression extends Expression
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(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.
The getValue(ELContext)
, setValue(ELContext, Object)
,
isReadOnly(ELContext)
, getType(ELContext)
and
getValueReference(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(ELContext, Object, 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(ELContext)
method, at the last resolution, the ValueExpression will
call the corresponding ELResolver.getValue(ELContext, Object, Object)
,
ELResolver.setValue(ELContext, Object, Object, Object)
,
ELResolver.isReadOnly(ELContext, Object, Object)
or
ELResolver.getType(ELContext, Object, Object)
method, depending on which was called on
the ValueExpression. For the getValueReference(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(ELContext, 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(ELContext, 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 Class getExpectedType()
public abstract Class getType(ELContext context)
setValue(ELContext, 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 Object getValue(ELContext context)
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 boolean isReadOnly(ELContext context)
setValue(ELContext, Object)
will always fail.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 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.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.