## 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 `ELResolver`s.

Though only a single `ELResolver` is associated with an `ELContext`, there are usually multiple resolvers considered for any given variable or property resolution. `ELResolver`s 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 `ELResolver`s for these methods.

Since:
  JSP 2.1
See Also:
  `ELContext`, `ELResolver`

Field Summary

Fields inherited from class javax.el.ELResolver
`RESOLVABLE_AT_DESIGN_TIME, TYPE`

Constructor Summary
`CompositeELResolver()`

Method Summary
`void` `add(ELResolver elResolver)` Adds the given resolver to the list of component resolvers.
`Class<?>` `getCommonPropertyType(ELContext context, Object base)` Returns the most general type that this resolver accepts for the `property` argument, given a `base` object.
`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.
`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.
`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.
`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.
`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.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### CompositeELResolver

    public CompositeELResolver()

Method Detail

### 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.

  Parameters:
    `elResolver` - The component resolver to add.
  Throws:
    `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.

  Specified by:
    `getValue` in class `ELResolver`
  Parameters:
    `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.
  Returns:
    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`
    `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.

  Specified by:
    `getType` in class `ELResolver`
  Parameters:
    `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.
  Returns:
    If the `propertyResolved` property of `ELContext` was set to `true`, then the most general acceptable type; otherwise undefined.
  Throws:
    `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.

  Specified by:
    `setValue` in class `ELResolver`
  Parameters:
    `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.
  Throws:
    `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.

  Specified by:
    `isReadOnly` in class `ELResolver`
  Parameters:
    `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.
  Returns:
    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`
    `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 `ELResolver`s 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.

  Specified by:
    `getFeatureDescriptors` in class `ELResolver`
  Parameters:
    `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.
  Returns:
    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 Also:
    `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.

  Specified by:
    `getCommonPropertyType` in class `ELResolver`
  Parameters:
    `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.
  Returns:
    `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`.

Copyright © 2008 Mort Bay Consulting. All Rights Reserved.