ELResolver (Java EE 5 SDK)

javax.el Class ELResolver

java.lang.Object
  javax.el.ELResolver

直接已知子类:

ArrayELResolver, BeanELResolver, CompositeELResolver, ImplicitObjectELResolver, ListELResolver, MapELResolver, ResourceBundleELResolver, ScopedAttributeELResolver

public abstract class ELResolver

extends Object

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 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.

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.

从以下版本开始:

JSP 2.1

另请参见:

CompositeELResolver, ELContext.getELResolver()

TYPE

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.

另请参见:

常量字段

RESOLVABLE_AT_DESIGN_TIME

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.

另请参见:

常量字段

ELResolver

public ELResolver()

getValue

public abstract 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.

参数:

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.

返回:

If the propertyResolved property of ELContext was set to true, then the result of the variable or property resolution; otherwise undefined.

抛出异常:

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.

getType

public abstract 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(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.

参数:

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.

返回:

If the propertyResolved property of ELContext was set to true, then the most general acceptable type; otherwise undefined.

抛出异常:

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.

setValue

public abstract 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.

参数:

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.

抛出异常:

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.

isReadOnly

public abstract boolean isReadOnly(ELContext context,
                                   Object base,
                                   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.

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.

参数:

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.

返回:

If the propertyResolved property of ELContext was set to true, then true if the property is read-only or false if not; otherwise undefined.

抛出异常:

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.

getFeatureDescriptors

public abstract Iterator<FeatureDescriptor> 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 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.

参数:

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.

返回:

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

另请参见:

FeatureDescriptor

getCommonPropertyType

public abstract 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.

参数:

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.

返回:

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.

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