ValueExpression (Java EE 5 SDK)

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javax.el Class ValueExpression

java.lang.Object
  javax.el.Expression
      javax.el.ValueExpression

所有已实现的接口:: Serializable

public abstract class ValueExpression
extends Expression

An Expression that can get or set a value.

In previous incarnations of this API, expressions could only be read. ValueExpression objects can now be used both to retrieve a value and to set a value. Expressions that can have a value set on them are referred to as l-value expressions. Those that cannot are referred to as r-value expressions. Not all r-value expressions can be used as l-value expressions (e.g. "${1+1}" or "${firstName} ${lastName}"). See the EL Specification for details. Expressions that cannot be used as l-values must always return true from isReadOnly().

The ExpressionFactory.createValueExpression(javax.el.ELContext, java.lang.String, java.lang.Class) method can be used to parse an expression string and return a concrete instance of ValueExpression that encapsulates the parsed expression. The FunctionMapper is used at parse time, not evaluation time, so one is not needed to evaluate an expression using this class. However, the ELContext is needed at evaluation time.


The getValue(javax.el.ELContext), setValue(javax.el.ELContext, java.lang.Object), isReadOnly(javax.el.ELContext) and
 getType(javax.el.ELContext) methods will evaluate the expression each time they are
 called. The ELResolver in the ELContext is used to 
 resolve the top-level variables and to determine the behavior of the
 . and [] operators. For any of the four methods,
 the ELResolver.getValue(javax.el.ELContext, java.lang.Object, java.lang.Object) method is used to resolve all properties 
 up to but excluding the last one. This provides the base 
 object. At the last resolution, the ValueExpression will 
 call the corresponding ELResolver.getValue(javax.el.ELContext, java.lang.Object, java.lang.Object), 
 ELResolver.setValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object), ELResolver.isReadOnly(javax.el.ELContext, java.lang.Object, java.lang.Object) or 
 ELResolver.getType(javax.el.ELContext, java.lang.Object, java.lang.Object) method, depending on which was called on 
 the ValueExpression.
 
See the notes about comparison, serialization and immutability in 
 the Expression javadocs.



从以下版本开始:
JSP 2.1
另请参见:
ELResolver, 
Expression, 
ExpressionFactory, 
Serialized Form







构造器摘要


ValueExpression()


           


 





方法摘要



abstract  Class<?>
getExpectedType()


          Returns the type the result of the expression will be coerced to 
 after evaluation.



abstract  Class<?>
getType(ELContext context)


          Evaluates the expression relative to the provided context, and 
 returns 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) method.



abstract  Object
getValue(ELContext context)


          Evaluates the expression relative to the provided context, and 
 returns the resulting value.



abstract  boolean
isReadOnly(ELContext context)


          Evaluates the expression relative to the provided context, and 
 returns true if a call to setValue(javax.el.ELContext, java.lang.Object) will 
 always fail.



abstract  void
setValue(ELContext context,
         Object value)


          Evaluates the expression relative to the provided context, and 
 sets the result to the provided value.


 


类方法继承 javax.el.Expression


equals, getExpressionString, hashCode, isLiteralText


 


类方法继承 java.lang.Object


clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait


 






构造器详细信息



ValueExpression
public ValueExpression()







方法详细信息



getValue
public abstract Object getValue(ELContext context)

Evaluates the expression relative to the provided context, and 
 returns the resulting value.

 The resulting value is automatically coerced to the type
 returned by getExpectedType(), which was
 provided to the ExpressionFactory when this
 expression was created.


参数:
context - The context of this evaluation.
返回:
The result of the expression evaluation.
抛出异常:
NullPointerException - if context is null.
PropertyNotFoundException - if one of the property
     resolutions failed because a specified variable or property 
     does not exist or is not readable.
ELException - if an exception was thrown while performing
     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 value)

Evaluates the expression relative to the provided context, and 
 sets the result to the provided value.


参数:
context - The context of this evaluation.
value - The new value to be set.
抛出异常:
NullPointerException - if context is null.
PropertyNotFoundException - if one of the property
     resolutions failed because a specified variable or property 
     does not exist or is not readable.
PropertyNotWritableException - if the final variable or
     property resolution failed because 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)

Evaluates the expression relative to the provided context, and 
 returns true if a call to setValue(javax.el.ELContext, java.lang.Object) will 
 always fail.


参数:
context - The context of this evaluation.
返回:
true if the expression is read-only or
     false if not.
抛出异常:
NullPointerException - if context is null.
PropertyNotFoundException - if one of the property
     resolutions failed because a specified variable or property 
     does not exist or is not readable.
ELException - if an exception was thrown while performing
     property or variable resolution. The thrown exception
     must be included as the cause property of this exception, if
     available.
     * @throws NullPointerException if context is null




getType
public abstract Class<?> getType(ELContext context)

Evaluates the expression relative to the provided context, and 
 returns 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) method.

 This is not always the same as getValue().getClass().
 For example, in the case of an expression that references an 
 array element, 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.
返回:
the most general acceptable type; otherwise undefined.
抛出异常:
NullPointerException - if context is null.
PropertyNotFoundException - if one of the property
     resolutions failed because a specified variable or property 
     does not exist or is not readable.
ELException - if an exception was thrown while performing
     property or variable resolution. The thrown exception
     must be included as the cause property of this exception, if
     available.




getExpectedType
public abstract Class<?> getExpectedType()

Returns the type the result of the expression will be coerced to 
 after evaluation.


返回:
the expectedType passed to the
     ExpressionFactory.createValueExpression method
     that created this ValueExpression.













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构造器摘要
`ValueExpression()`

方法摘要
`abstract Class<?>`	`getExpectedType()` Returns the type the result of the expression will be coerced to after evaluation.
`abstract Class<?>`	`getType(ELContext context)` Evaluates the expression relative to the provided context, and returns 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)` method.
`abstract Object`	`getValue(ELContext context)` Evaluates the expression relative to the provided context, and returns the resulting value.
`abstract boolean`	`isReadOnly(ELContext context)` Evaluates the expression relative to the provided context, and returns `true` if a call to `setValue(javax.el.ELContext, java.lang.Object)` will always fail.
`abstract void`	`setValue(ELContext context, Object value)` Evaluates the expression relative to the provided context, and sets the result to the provided value.

类方法继承 javax.el.Expression
`equals, getExpressionString, hashCode, isLiteralText`

类方法继承 java.lang.Object
`clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait`



  
  
    
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