ELResolverpublic abstract class ELResolver extends Object | Enables customization of variable and property resolution behavior for EL
expression evaluation.
While evaluating an expression, the ELResolver associated
with the {@link ELContext} is consulted to do the initial resolution of
the first variable of an expression. It is also consulted when a
. or [] operator is encountered, except for the
last such operator in a method expression, in which case the resultion
rules are hard coded.
For example, in the EL expression ${employee.lastName},
the ELResolver determines what object employee
refers to, and what it means to get the lastName property on
that object.
Most methods in this class accept a base
and property parameter. In the case of variable resolution
(e.g. determining what employee refers to in
${employee.lastName}), the base parameter will
be null and the property parameter will always
be of type String. In this case, if the property
is not a String, the behavior of the ELResolver
is undefined.
In the case of property resolution, the base parameter
identifies the base object and the property object identifies
the property on that base. For example, in the expression
${employee.lastName}, base is the result of the
variable resolution for employee and property
is the string "lastName". In the expression
${y[x]}, base is the result of the variable
resolution for y and property is the result of
the variable resolution for x.
Though only a single ELResolver is associated with an
ELContext, there are usually multiple resolvers considered
for any given variable or property resolution. ELResolvers
are combined together using {@link CompositeELResolver}s, to define
rich semantics for evaluating an expression.
For the {@link #getValue}, {@link #getType}, {@link #setValue} and
{@link #isReadOnly} methods, an ELResolver is not
responsible for resolving all possible (base, property) pairs. In fact,
most resolvers will only handle a base of a single type.
To indicate that a resolver has successfully resolved a particular
(base, property) pair, it must set the propertyResolved
property of the ELContext to true. If it could
not handle the given pair, it must leave this property alone. The caller
must ignore the return value of the method if propertyResolved
is false.
The {@link #getFeatureDescriptors} and {@link #getCommonPropertyType}
methods are primarily designed for design-time tool support, but must
handle invocation at runtime as well. The
{@link java.beans.Beans#isDesignTime} method can be used to determine
if the resolver is being consulted at design-time or runtime. |
| Fields Summary |
|---|
| public static final String | TYPE The attribute name of the named attribute in the
FeatureDescriptor that specifies the runtime type of
the variable or property. | | public static final String | RESOLVABLE_AT_DESIGN_TIME The attribute name of the named attribute in the
FeatureDescriptor that specifies whether the
variable or property can be resolved at runtime. |
| Methods Summary |
|---|
| public abstract java.lang.Class | getCommonPropertyType(javax.el.ELContext context, java.lang.Object base)Returns the most general type that this resolver accepts for the
property argument, given a base object.
One use for this method is to assist tools in auto-completion.
This assists tools in auto-completion and also provides a
way to express that the resolver accepts a primitive value,
such as an integer index into an array. For example, the
{@link ArrayELResolver} will accept any int as a
property, so the return value would be
Integer.class.
| | public abstract java.util.Iterator | getFeatureDescriptors(javax.el.ELContext context, java.lang.Object base)Returns information about the set of variables or properties that
can be resolved for the given base object. One use for
this method is to assist tools in auto-completion.
If the base parameter is null, the
resolver must enumerate the list of top-level variables it can
resolve.
The Iterator returned must contain zero or more
instances of {@link java.beans.FeatureDescriptor}, in no guaranteed
order. In the case of primitive types such as int, the
value null must be returned. This is to prevent the
useless iteration through all possible primitive values. A
return value of null indicates that this resolver does
not handle the given base object or that the results
are too complex to represent with this method and the
{@link #getCommonPropertyType} method should be used instead.
Each FeatureDescriptor will contain information about
a single variable or property. In addition to the standard
properties, the FeatureDescriptor must have two
named attributes (as set by the setValue method):
- {@link #TYPE} - The value of this named attribute must be
an instance of
java.lang.Class and specify the
runtime type of the variable or property.
- {@link #RESOLVABLE_AT_DESIGN_TIME} - The value of this
named attribute must be an instance of
java.lang.Boolean and indicates whether it is safe
to attempt to resolve this property at design-time. For
instance, it may be unsafe to attempt a resolution at design
time if the ELResolver needs access to a resource
that is only available at runtime and no acceptable simulated
value can be provided.
The caller should be aware that the Iterator
returned might iterate through a very large or even infinitely large
set of properties. Care should be taken by the caller to not get
stuck in an infinite loop.
This is a "best-effort" list. Not all ELResolvers
will return completely accurate results, but all must be callable
at both design-time and runtime (i.e. whether or not
Beans.isDesignTime() returns true),
without causing errors.
The propertyResolved property of the
ELContext is not relevant to this method.
The results of all ELResolvers are concatenated
in the case of composite resolvers.
| | public abstract java.lang.Class | getType(javax.el.ELContext context, java.lang.Object base, java.lang.Object property)For a given base and property, attempts to
identify the most general type that is acceptable for an object to be
passed as the value parameter in a future call
to the {@link #setValue} method.
If this resolver handles the given (base, property) pair,
the propertyResolved property of the
ELContext object must be set to true
by the resolver, before returning. If this property is not
true after this method is called, the caller should ignore
the return value.
This is not always the same as getValue().getClass().
For example, in the case of an {@link ArrayELResolver}, 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.
| | public abstract java.lang.Object | getValue(javax.el.ELContext context, java.lang.Object base, java.lang.Object property)Attempts to resolve the given property object on the given
base object.
If this resolver handles the given (base, property) pair,
the propertyResolved property of the
ELContext object must be set to true
by the resolver, before returning. If this property is not
true after this method is called, the caller should ignore
the return value.
| | public abstract boolean | isReadOnly(javax.el.ELContext context, java.lang.Object base, java.lang.Object property)For a given base and property, attempts to
determine whether a call to {@link #setValue} will always fail.
If this resolver handles the given (base, property) pair,
the propertyResolved property of the
ELContext object must be set to true
by the resolver, before returning. If this property is not
true after this method is called, the caller should ignore
the return value.
| | public abstract void | setValue(javax.el.ELContext context, java.lang.Object base, java.lang.Object property, java.lang.Object value)Attempts to set the value of the given property
object on the given base object.
If this resolver handles the given (base, property) pair,
the propertyResolved property of the
ELContext object must be set to true
by the resolver, before returning. If this property is not
true after this method is called, the caller can
safely assume no value has been set.
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