javax.el: abstract public class: ELResolver

javax.el
abstract public class: ELResolver [javadoc | source]

java.lang.Object
   javax.el.ELResolver

Direct Known Subclasses:
ImplicitObjectELResolver, BeanELResolver, MapELResolver, ListELResolver, ResourceBundleELResolver, CompositeELResolver, ScopedAttributeELResolver, ArrayELResolver

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 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 CompositeELResolver s, to define rich semantics for evaluating an expression.

For the #getValue , #getType , #setValue and #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 #getFeatureDescriptors and #getCommonPropertyType 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.

Also see:

CompositeELResolver

ELContext#getELResolver

since: JSP - 2.1

Field 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.

Method from javax.el.ELResolver Summary:
getCommonPropertyType, getFeatureDescriptors, getType, getValue, isReadOnly, setValue

Methods from java.lang.Object:
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method from javax.el.ELResolver Detail:
abstract public Class getCommonPropertyType(ELContext context, 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 ArrayELResolver will accept any `int` as a `property`, so the return value would be `Integer.class`.
abstract public Iterator getFeatureDescriptors(ELContext context, 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 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 #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): #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 `ELResolver`s 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 `ELResolver`s are concatenated in the case of composite resolvers.
abstract public Class getType(ELContext context, Object base, 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 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.
abstract public Object getValue(ELContext context, Object base, 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.
abstract public boolean isReadOnly(ELContext context, Object base, Object property) For a given `base` and `property`, attempts to determine whether a call to #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.
abstract public void setValue(ELContext context, Object base, Object property, 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.