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java.lang.Objectjavax.el.ELResolver
public abstract class ELResolver
Enables customization of variable and property resolution behavior for EL expression evaluation.
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, except for the last such operator in a method expression, in which
case the resolution 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 CompositeELResolver
s, to define rich semantics for evaluating an
expression. For the getValue(ELContext, Object, Object)
,
getType(ELContext, Object, Object)
, setValue(ELContext, Object, Object, Object)
and isReadOnly(ELContext, Object, 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.
The getFeatureDescriptors(ELContext, Object)
and
getCommonPropertyType(ELContext, Object)
methods are primarily designed for design-time
tool support, but must handle invocation at runtime as well. The java.beans.Beans.isDesignTime()
method can be used to determine if the resolver is being consulted at design-time or runtime.
Field Summary | |
---|---|
static java.lang.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. |
static java.lang.String |
TYPE
The attribute name of the named attribute in the FeatureDescriptor that specifies the runtime type of the variable or property. |
Constructor Summary | |
---|---|
ELResolver()
|
Method Summary | |
---|---|
abstract java.lang.Class<?> |
getCommonPropertyType(ELContext context,
java.lang.Object base)
Returns the most general type that this resolver accepts for the property argument, given a base object. |
abstract java.util.Iterator<java.beans.FeatureDescriptor> |
getFeatureDescriptors(ELContext context,
java.lang.Object base)
Returns information about the set of variables or properties that can be resolved for the given base object. |
abstract java.lang.Class<?> |
getType(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 setValue(ELContext, Object, Object, Object) method. |
abstract java.lang.Object |
getValue(ELContext context,
java.lang.Object base,
java.lang.Object property)
Attempts to resolve the given property object on the given base object. |
abstract boolean |
isReadOnly(ELContext context,
java.lang.Object base,
java.lang.Object property)
For a given base and property, attempts to determine whether a call to setValue(ELContext, Object, Object, Object) will always fail. |
abstract void |
setValue(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. |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Field Detail |
---|
public static final java.lang.String RESOLVABLE_AT_DESIGN_TIME
public static final java.lang.String TYPE
Constructor Detail |
---|
public ELResolver()
Method Detail |
---|
public abstract java.lang.Class<?> getCommonPropertyType(ELContext context, java.lang.Object base)
ArrayELResolver
will accept any int as a property, so the return value would be
Integer.class.
context
- The context of this evaluation.base
- The base object to return the most general property type for, or null to enumerate
the set of top-level variables that this resolver can evaluate.
public abstract java.util.Iterator<java.beans.FeatureDescriptor> getFeatureDescriptors(ELContext context, java.lang.Object base)
getCommonPropertyType(ELContext, Object)
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):
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.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 designtime. 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.
context
- The context of this evaluation.base
- The base object whose set of valid properties is to be enumerated, or null to
enumerate the set of top-level variables that this resolver can evaluate.
public abstract java.lang.Class<?> getType(ELContext context, java.lang.Object base, java.lang.Object property)
setValue(ELContext, Object, Object, Object)
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 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.
context
- The context of this evaluation.base
- The base object whose property value is to be analyzed, or null to analyze a
top-level variable.property
- The property or variable to return the acceptable type for.
java.lang.NullPointerException
- if context is null
PropertyNotFoundException
- if the given (base, property) pair is handled by this ELResolver 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.public abstract java.lang.Object getValue(ELContext context, java.lang.Object base, java.lang.Object property)
context
- The context of this evaluation.base
- The base object whose property value is to be returned, or null to resolve a
top-level variable.property
- The property or variable to be resolved.
java.lang.NullPointerException
- if context is null
PropertyNotFoundException
- if the given (base, property) pair is handled by this ELResolver 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.public abstract boolean isReadOnly(ELContext context, java.lang.Object base, java.lang.Object property)
setValue(ELContext, Object, Object, Object)
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.
context
- The context of this evaluation.base
- The base object whose property value is to be analyzed, or null to analyze a
top-level variable.property
- The property or variable to return the read-only status for.
java.lang.NullPointerException
- if context is null
PropertyNotFoundException
- if the given (base, property) pair is handled by this ELResolver 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.public abstract void setValue(ELContext context, java.lang.Object base, java.lang.Object property, java.lang.Object value)
context
- The context of this evaluation.base
- The base object whose property value is to be set, or null to set a top-level
variable.property
- The property or variable to be set.value
- The value to set the property or variable to.
java.lang.NullPointerException
- if context is null
PropertyNotFoundException
- if the given (base, property) pair is handled by this ELResolver but the
specified variable or property does not exist.
PropertyNotWritableException
- if the given (base, property) pair is handled by this ELResolver 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.
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Copyright © 2006-2009 Odysseus Software GmbH. | ||||||||
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