CompositeELResolver (Java EE 5 SDK)

javax.el Class CompositeELResolver

java.lang.Object
  javax.el.ELResolver
      javax.el.CompositeELResolver

public class CompositeELResolver

extends ELResolver

Maintains an ordered composite list of child ELResolvers.

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 a CompositeELResolver, 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 CompositeELResolver initializes the ELContext.propertyResolved flag to false, and uses it as a stop condition for iterating through its component resolvers.

The ELContext.propertyResolved flag is not used for the design-time methods getFeatureDescriptors(javax.el.ELContext, java.lang.Object) and getCommonPropertyType(javax.el.ELContext, java.lang.Object). Instead, results are collected and combined from all child ELResolvers for these methods.

从以下版本开始:

JSP 2.1

另请参见:

ELContext, ELResolver

CompositeELResolver

public CompositeELResolver()

add

public void add(ELResolver elResolver)

Adds the given resolver to the list of component resolvers.

Resolvers are consulted in the order in which they are added.

参数:

elResolver - The component resolver to add.

抛出异常:

NullPointerException - If the provided resolver is null.

getValue

public Object getValue(ELContext context,
                       Object base,
                       Object property)

Attempts to resolve the given property object on the given base object by querying all component resolvers.

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.

First, propertyResolved is set to false on the provided ELContext.

Next, for each component resolver in this composite:

1. The getValue() method is called, passing in the provided context, base and property.
2. If the ELContext's propertyResolved flag is false then iteration continues.
3. Otherwise, iteration stops and no more component resolvers are considered. The value returned by getValue() is returned by this method.

If none of the component resolvers were able to perform this operation, the value null is returned and the propertyResolved flag remains set to false

.

Any exception thrown by component resolvers during the iteration is propagated to the caller of this method.

规范说明:

getValue in class ELResolver

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 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. The result is obtained by querying all component resolvers.

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.

First, propertyResolved is set to false on the provided ELContext.

Next, for each component resolver in this composite:

1. The getType() method is called, passing in the provided context, base and property.
2. If the ELContext's propertyResolved flag is false then iteration continues.
3. Otherwise, iteration stops and no more component resolvers are considered. The value returned by getType() is returned by this method.

If none of the component resolvers were able to perform this operation, the value null is returned and the propertyResolved flag remains set to false

.

Any exception thrown by component resolvers during the iteration is propagated to the caller of this method.

规范说明:

getType in class ELResolver

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 void setValue(ELContext context,
                     Object base,
                     Object property,
                     Object val)

Attempts to set the value of the given property object on the given base object. All component resolvers are asked to attempt to set the value.

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.

First, propertyResolved is set to false on the provided ELContext.

Next, for each component resolver in this composite:

1. The setValue() method is called, passing in the provided context, base, property and value.
2. If the ELContext's propertyResolved flag is false then iteration continues.
3. Otherwise, iteration stops and no more component resolvers are considered.

If none of the component resolvers were able to perform this operation, the propertyResolved flag remains set to false

.

Any exception thrown by component resolvers during the iteration is propagated to the caller of this method.

规范说明:

setValue in class ELResolver

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.

val - 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 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. The result is obtained by querying all component resolvers.

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.

First, propertyResolved is set to false on the provided ELContext.

Next, for each component resolver in this composite:

1. The isReadOnly() method is called, passing in the provided context, base and property.
2. If the ELContext's propertyResolved flag is false then iteration continues.
3. Otherwise, iteration stops and no more component resolvers are considered. The value returned by isReadOnly() is returned by this method.

If none of the component resolvers were able to perform this operation, the value false is returned and the propertyResolved flag remains set to false

.

Any exception thrown by component resolvers during the iteration is propagated to the caller of this method.

规范说明:

isReadOnly in class ELResolver

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 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. The results are collected from all component resolvers.

The propertyResolved property of the ELContext is not relevant to this method. The results of all ELResolvers are concatenated.

The Iterator returned is an iterator over the collection of FeatureDescriptor objects returned by the iterators returned by each component resolver's getFeatureDescriptors method. If null is returned by a resolver, it is skipped.

规范说明:

getFeatureDescriptors in class ELResolver

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 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. The result is obtained by querying all component resolvers.

The Class returned is the most specific class that is a common superclass of all the classes returned by each component resolver's getCommonPropertyType method. If null is returned by a resolver, it is skipped.

规范说明:

getCommonPropertyType in class ELResolver

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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