Class ELResolver
- Direct Known Subclasses:
ArrayELResolver
,BeanELResolver
,BeanNameELResolver
,CompositeELResolver
,ImplicitObjectELResolver
,ListELResolver
,MapELResolver
,ResourceBundleELResolver
,ScopedAttributeELResolver
,StaticFieldELResolver
,TypeConverter
While evaluating an expression, the ELResolver
associated
with the 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.
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
.
In the case of method call resolution, the base
parameter
identifies the base object and the method
parameter identifies
a method on that base. In the case of overloaded methods, the
paramTypes
parameter can be optionally used to identify a method.
The params
parameter are the parameters for the method call,
and can also be used for resolving overloaded methods when the
paramTypes
parameter is not specified.
In the case of type conversion resolution, the obj
parameter
identifies the source object and the targetType
parameter
identifies the target type the source to covert to.
Though only a single ELResolver
is associated with an
ELContext
, there are usually multiple resolvers considered
for any given variable or property resolution. ELResolver
s
are combined together using CompositeELResolver
s, to define
rich semantics for evaluating an expression.
For the getValue(javax.el.ELContext, java.lang.Object, java.lang.Object)
, getType(javax.el.ELContext, java.lang.Object, java.lang.Object)
, setValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)
, and
isReadOnly(javax.el.ELContext, java.lang.Object, java.lang.Object)
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
.
Similarly, for the convertToType(javax.el.ELContext, java.lang.Object, java.lang.Class<?>)
method an
ELResolver
must set the propertyResolved
to true
to indicate
that it handles the conversion of the object to the target type.
The getFeatureDescriptors(javax.el.ELContext, java.lang.Object)
and getCommonPropertyType(javax.el.ELContext, java.lang.Object)
methods are primarily designed for design-time tool support, but must
handle invocation at runtime as well. The
Beans.isDesignTime()
method can be used to determine
if the resolver is being consulted at design-time or runtime.
- Since:
- JSP 2.1
- See Also:
-
Field Summary
Modifier and TypeFieldDescriptionstatic final String
The attribute name of the named attribute in theFeatureDescriptor
that specifies whether the variable or property can be resolved at runtime.static final String
The attribute name of the named attribute in theFeatureDescriptor
that specifies the runtime type of the variable or property. -
Constructor Summary
-
Method Summary
Modifier and TypeMethodDescriptionconvertToType
(ELContext context, Object obj, Class<?> targetType) Converts an object to a specific type.abstract Class<?>
getCommonPropertyType
(ELContext context, Object base) Returns the most general type that this resolver accepts for theproperty
argument, given abase
object.abstract Iterator<FeatureDescriptor>
getFeatureDescriptors
(ELContext context, Object base) Returns information about the set of variables or properties that can be resolved for the givenbase
object.abstract Class<?>
For a givenbase
andproperty
, attempts to identify the most general type that is acceptable for an object to be passed as thevalue
parameter in a future call to thesetValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)
method.abstract Object
Attempts to resolve the givenproperty
object on the givenbase
object.Attempts to resolve and invoke the givenmethod
on the givenbase
object.abstract boolean
isReadOnly
(ELContext context, Object base, Object property) For a givenbase
andproperty
, attempts to determine whether a call tosetValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)
will always fail.abstract void
Attempts to set the value of the givenproperty
object on the givenbase
object.
-
Field Details
-
TYPE
The attribute name of the named attribute in the
FeatureDescriptor
that specifies the runtime type of the variable or property.- See Also:
-
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.- See Also:
-
-
Constructor Details
-
ELResolver
public ELResolver()
-
-
Method Details
-
getValue
Attempts to resolve the givenproperty
object on the givenbase
object.If this resolver handles the given (base, property) pair, the
propertyResolved
property of theELContext
object must be set totrue
by the resolver, before returning. If this property is nottrue
after this method is called, the caller should ignore the return value.- Parameters:
context
- The context of this evaluation.base
- The base object whose property value is to be returned, ornull
to resolve a top-level variable.property
- The property or variable to be resolved.- Returns:
- If the
propertyResolved
property ofELContext
was set totrue
, then the result of the variable or property resolution; otherwise undefined. - Throws:
NullPointerException
- if context isnull
PropertyNotFoundException
- if the given (base, property) pair is handled by thisELResolver
but the specified variable or property does not exist or is not readable.ELException
- if an exception was thrown while performing the property or variable resolution. The thrown exception must be included as the cause property of this exception, if available.
-
invoke
public Object invoke(ELContext context, Object base, Object method, Class<?>[] paramTypes, Object[] params) Attempts to resolve and invoke the givenmethod
on the givenbase
object.If this resolver handles the given (base, method) pair, the
propertyResolved
property of theELContext
object must be set totrue
by the resolver, before returning. If this property is nottrue
after this method is called, the caller should ignore the return value.A default implementation is provided that returns null so that existing classes that extend ELResolver can continue to function.
- Parameters:
context
- The context of this evaluation.base
- The bean on which to invoke the methodmethod
- The simple name of the method to invoke. Will be coerced to aString
.paramTypes
- An array of Class objects identifying the method's formal parameter types, in declared order. Use an empty array if the method has no parameters. Can benull
, in which case the method's formal parameter types are assumed to be unknown.params
- The parameters to pass to the method, ornull
if no parameters.- Returns:
- The result of the method invocation (
null
if the method has avoid
return type). - Throws:
MethodNotFoundException
- if no suitable method can be found.ELException
- if an exception was thrown while performing (base, method) resolution. The thrown exception must be included as the cause property of this exception, if available. If the exception thrown is anInvocationTargetException
, extract itscause
and pass it to theELException
constructor.- Since:
- EL 2.2
-
getType
For a givenbase
andproperty
, attempts to identify the most general type that is acceptable for an object to be passed as thevalue
parameter in a future call to thesetValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)
method.If this resolver handles the given (base, property) pair, the
propertyResolved
property of theELContext
object must be set totrue
by the resolver, before returning. If this property is nottrue
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 anArrayELResolver
, thegetType
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.- Parameters:
context
- The context of this evaluation.base
- The base object whose property value is to be analyzed, ornull
to analyze a top-level variable.property
- The property or variable to return the acceptable type for.- Returns:
- If the
propertyResolved
property ofELContext
was set totrue
, then the most general acceptable type; otherwise undefined. - Throws:
PropertyNotFoundException
- if the given (base, property) pair is handled by thisELResolver
but the specified variable or property does not exist or is not readable.ELException
- if an exception was thrown while performing the property or variable resolution. The thrown exception must be included as the cause property of this exception, if available.
-
setValue
Attempts to set the value of the givenproperty
object on the givenbase
object.If this resolver handles the given (base, property) pair, the
propertyResolved
property of theELContext
object must be set totrue
by the resolver, before returning. If this property is nottrue
after this method is called, the caller can safely assume no value has been set.- Parameters:
context
- The context of this evaluation.base
- The base object whose property value is to be set, ornull
to set a top-level variable.property
- The property or variable to be set.value
- The value to set the property or variable to.- Throws:
NullPointerException
- if context isnull
PropertyNotFoundException
- if the given (base, property) pair is handled by thisELResolver
but the specified variable or property does not exist.PropertyNotWritableException
- if the given (base, property) pair is handled by thisELResolver
but 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.
-
isReadOnly
For a givenbase
andproperty
, attempts to determine whether a call tosetValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)
will always fail.If this resolver handles the given (base, property) pair, the
propertyResolved
property of theELContext
object must be set totrue
by the resolver, before returning. If this property is nottrue
after this method is called, the caller should ignore the return value.- Parameters:
context
- The context of this evaluation.base
- The base object whose property value is to be analyzed, ornull
to analyze a top-level variable.property
- The property or variable to return the read-only status for.- Returns:
- If the
propertyResolved
property ofELContext
was set totrue
, thentrue
if the property is read-only orfalse
if not; otherwise undefined. - Throws:
NullPointerException
- if context isnull
PropertyNotFoundException
- if the given (base, property) pair is handled by thisELResolver
but the specified variable or property does not exist.ELException
- if an exception was thrown while performing the property or variable resolution. The thrown exception must be included as the cause property of this exception, if available.
-
getFeatureDescriptors
Returns information about the set of variables or properties that can be resolved for the givenbase
object. One use for this method is to assist tools in auto-completion.If the
base
parameter isnull
, the resolver must enumerate the list of top-level variables it can resolve.The
Iterator
returned must contain zero or more instances ofFeatureDescriptor
, in no guaranteed order. In the case of primitive types such asint
, the valuenull
must be returned. This is to prevent the useless iteration through all possible primitive values. A return value ofnull
indicates that this resolver does not handle the givenbase
object or that the results are too complex to represent with this method and thegetCommonPropertyType(javax.el.ELContext, java.lang.Object)
method should be used instead.Each
FeatureDescriptor
will contain information about a single variable or property. In addition to the standard properties, theFeatureDescriptor
must have two named attributes (as set by thesetValue
method):TYPE
- The value of this named attribute must be an instance ofjava.lang.Class
and specify the runtime type of the variable or property.RESOLVABLE_AT_DESIGN_TIME
- The value of this named attribute must be an instance ofjava.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 theELResolver
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
ELResolver
s will return completely accurate results, but all must be callable at both design-time and runtime (i.e. whether or notBeans.isDesignTime()
returnstrue
), without causing errors.The
propertyResolved
property of theELContext
is not relevant to this method. The results of allELResolver
s are concatenated in the case of composite resolvers.- Parameters:
context
- The context of this evaluation.base
- The base object whose set of valid properties is to be enumerated, ornull
to enumerate the set of top-level variables that this resolver can evaluate.- Returns:
- An
Iterator
containing zero or more (possibly infinitely more)FeatureDescriptor
objects, ornull
if this resolver does not handle the givenbase
object or that the results are too complex to represent with this method - See Also:
-
getCommonPropertyType
Returns the most general type that this resolver accepts for theproperty
argument, given abase
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
ArrayELResolver
will accept anyint
as aproperty
, so the return value would beInteger.class
.- Parameters:
context
- The context of this evaluation.base
- The base object to return the most general property type for, ornull
to enumerate the set of top-level variables that this resolver can evaluate.- Returns:
null
if thisELResolver
does not know how to handle the givenbase
object; otherwiseObject.class
if any type ofproperty
is accepted; otherwise the most generalproperty
type accepted for the givenbase
.
-
convertToType
Converts an object to a specific type.An
ELException
is thrown if an error occurs during the conversion.- Parameters:
context
- The context of this evaluation.obj
- The object to convert.targetType
- The target type for the convertion.- Throws:
ELException
- thrown if errors occur.
-