javax.el.ELResolver Maven / Gradle / Ivy
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package javax.el;
import java.util.Iterator;
import java.beans.FeatureDescriptor;
/**
* 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.
*
* @see CompositeELResolver
* @see ELContext#getELResolver
* @since JSP 2.1
*/
public abstract class ELResolver {
// --------------------------------------------------------- Constants
/**
* The attribute name of the named attribute in the
* FeatureDescriptor that specifies the runtime type of
* the variable or property.
*/
public static final String TYPE = "type";
/**
* The attribute name of the named attribute in the
* FeatureDescriptor that specifies whether the
* variable or property can be resolved at runtime.
*/
public static final String RESOLVABLE_AT_DESIGN_TIME = "resolvableAtDesignTime";
/**
* 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.
*
* @param context The context of this evaluation.
* @param base The base object whose property value is to be returned,
* or null to resolve a top-level variable.
* @param property The property or variable to be resolved.
* @return If the propertyResolved property of
* ELContext was set to true, then
* the result of the variable or property resolution; otherwise
* undefined.
* @throws NullPointerException if context is null
* @throws 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.
* @throws 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 Object getValue(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 {@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.
*
* @param context The context of this evaluation.
* @param base The base object whose property value is to be analyzed,
* or null to analyze a top-level variable.
* @param property The property or variable to return the acceptable
* type for.
* @return If the propertyResolved property of
* ELContext was set to true, then
* the most general acceptable type; otherwise undefined.
* @throws NullPointerException if context is null
* @throws 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.
* @throws 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 Class getType(ELContext context,
Object base,
Object property);
/**
* 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.
*
* @param context The context of this evaluation.
* @param base The base object whose property value is to be set,
* or null to set a top-level variable.
* @param property The property or variable to be set.
* @param value The value to set the property or variable to.
* @throws NullPointerException if context is null
* @throws PropertyNotFoundException if the given (base, property) pair
* is handled by this ELResolver but the specified
* variable or property does not exist.
* @throws PropertyNotWritableException if the given (base, property)
* pair is handled by this ELResolver but the specified
* variable or property is not writable.
* @throws 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 void setValue(ELContext context,
Object base,
Object property,
Object value);
/**
* 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.
*
* @param context The context of this evaluation.
* @param base The base object whose property value is to be analyzed,
* or null to analyze a top-level variable.
* @param property The property or variable to return the read-only status
* for.
* @return If the propertyResolved property of
* ELContext was set to true, then
* true if the property is read-only or
* false if not; otherwise undefined.
* @throws NullPointerException if context is null
* @throws PropertyNotFoundException if the given (base, property) pair
* is handled by this ELResolver but the specified
* variable or property does not exist.
* @throws 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,
Object base,
Object property);
/**
* 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.
*
* @param context The context of this evaluation.
* @param 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.
* @return An 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 method
* @see java.beans.FeatureDescriptor
*/
public abstract Iterator getFeatureDescriptors(
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
* {@link ArrayELResolver} will accept any int as a
* property, so the return value would be
* Integer.class.
*
* @param context The context of this evaluation.
* @param 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.
* @return 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 abstract Class getCommonPropertyType(ELContext context,
Object base);
}