public abstract class ELResolver
extends java.lang.Object
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 Jakarta Expression Language 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 paramsparameter 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. ELResolvers are combined together
using CompositeELResolvers, 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.
CompositeELResolver,
ELContext.getELResolver()| Modifier and Type | Field and Description |
|---|---|
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 and Description |
|---|
ELResolver() |
| Modifier and Type | Method and Description |
|---|---|
java.lang.Object |
convertToType(ELContext context,
java.lang.Object obj,
java.lang.Class<?> targetType)
Converts an object to a specific type.
|
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(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.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. |
java.lang.Object |
invoke(ELContext context,
java.lang.Object base,
java.lang.Object method,
java.lang.Class<?>[] paramTypes,
java.lang.Object[] params)
Attempts to resolve and invoke the given
method 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(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.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. |
public static final java.lang.String TYPE
FeatureDescriptor that specifies the runtime type of
the variable or property.public static final java.lang.String RESOLVABLE_AT_DESIGN_TIME
FeatureDescriptor that specifies whether the variable
or property can be resolved at runtime.public abstract java.lang.Object getValue(ELContext context, java.lang.Object base, java.lang.Object property)
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.
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.propertyResolved property of ELContext was set to true, then
the result of the variable or property resolution; otherwise undefined.java.lang.NullPointerException - if context is nullPropertyNotFoundException - 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 java.lang.Object invoke(ELContext context, java.lang.Object base, java.lang.Object method, java.lang.Class<?>[] paramTypes, java.lang.Object[] params)
method on the given base object.
If this resolver handles the given (base, method) 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.
A default implementation is provided that returns null so that existing classes that extend ELResolver can continue to function.
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 a String.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 be null, in which case the method's formal parameter
types are assumed to be unknown.params - The parameters to pass to the method, or null if no parameters.null if the method has a void return type).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 an
InvocationTargetException, extract its cause and pass it to the ELException
constructor.public abstract java.lang.Class<?> getType(ELContext context, java.lang.Object base, java.lang.Object property)
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(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 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.propertyResolved property of ELContext was set to true, then
the most general acceptable type; otherwise undefined.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 void setValue(ELContext context, java.lang.Object base, java.lang.Object property, java.lang.Object value)
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.
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 nullPropertyNotFoundException - 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.public abstract boolean isReadOnly(ELContext context, java.lang.Object base, java.lang.Object property)
base and property, attempts to determine whether a call to setValue(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 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.propertyResolved property of ELContext was set to true, then
true if the property is read-only or false if not; otherwise undefined.java.lang.NullPointerException - if context is nullPropertyNotFoundException - 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 java.util.Iterator<java.beans.FeatureDescriptor> getFeatureDescriptors(ELContext context, java.lang.Object base)
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 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 getCommonPropertyType(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, 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 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.
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.Iterator containing zero or more (possibly infinitely more) FeatureDescriptor
objects, or null if this resolver does not handle the given base object or that the results
are too complex to represent with this methodFeatureDescriptorpublic abstract java.lang.Class<?> getCommonPropertyType(ELContext context, java.lang.Object base)
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 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.null if this ELResolver does not know how to handle the given base
object; otherwise Object.class if any type of property is accepted; otherwise the most
general property type accepted for the given base.public java.lang.Object convertToType(ELContext context, java.lang.Object obj, java.lang.Class<?> targetType)
An ELException is thrown if an error occurs during the conversion.
context - The context of this evaluation.obj - The object to convert.targetType - The target type for the conversion.targetTypeELException - thrown if errors occur.