javax.el
Class ValueExpression

java.lang.Object
  extended by javax.el.Expression
      extended by javax.el.ValueExpression
All Implemented Interfaces:
java.io.Serializable
Direct Known Subclasses:
IndexedValueExpression, IteratedValueExpression

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 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), getType(javax.el.ELContext) and getValueReference(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 five methods, the ELResolver#getValue method is used to resolve all properties up to but excluding the last one. This provides the base object. For all methods other than the getValueReference(javax.el.ELContext) method, at the last resolution, the ValueExpression will call the corresponding ELResolver#getValue, ELResolver#setValue, ELResolver#isReadOnly or ELResolver#getType method, depending on which was called on the ValueExpression. For the getValueReference method, the (base, property) is not resolved by the ELResolver, but an instance of ValueReference is created to encapsulate this (base ,property), and returned.

See the notes about comparison, serialization and immutability in the Expression javadocs.

Since:
JSP 2.1
See Also:
ELResolver, Expression, ExpressionFactory, Serialized Form

Constructor Summary
ValueExpression()
           
 
Method Summary
abstract  java.lang.Class<?> getExpectedType()
          Returns the type the result of the expression will be coerced to after evaluation.
abstract  java.lang.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  java.lang.Object getValue(ELContext context)
          Evaluates the expression relative to the provided context, and returns the resulting value.
 ValueReference getValueReference(ELContext context)
          Returns a ValueReference for this expression instance.
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, java.lang.Object value)
          Evaluates the expression relative to the provided context, and sets the result to the provided value.
 
Methods inherited from class javax.el.Expression
equals, getExpressionString, hashCode, isLiteralText
 
Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

ValueExpression

public ValueExpression()
Method Detail

getValue

public abstract java.lang.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.

Parameters:
context - The context of this evaluation.
Returns:
The result of the expression evaluation.
Throws:
java.lang.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,
                              java.lang.Object value)
Evaluates the expression relative to the provided context, and sets the result to the provided value.

Parameters:
context - The context of this evaluation.
value - The new value to be set.
Throws:
java.lang.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.

Parameters:
context - The context of this evaluation.
Returns:
true if the expression is read-only or false if not.
Throws:
java.lang.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 java.lang.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.

Parameters:
context - The context of this evaluation.
Returns:
the most general acceptable type; otherwise undefined.
Throws:
java.lang.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 java.lang.Class<?> getExpectedType()
Returns the type the result of the expression will be coerced to after evaluation.

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

getValueReference

public ValueReference getValueReference(ELContext context)
Returns a ValueReference for this expression instance.

Parameters:
context - the context of this evaluation
Returns:
the ValueReference for this ValueExpression, or null if this ValueExpression is not a reference to a base (null or non-null) and a property. If the base is null, and the property is a EL variable, return the ValueReference for the ValueExpression associated with this EL variable.
Since:
EL 2.2


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Generated on 10-February-2011 12:41


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