javax.el
Class ValueExpression

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

All Implemented Interfaces:

java.io.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.

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.
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.