ValueExpression (Java EE 5 SDK)
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javax.el Class ValueExpression
java.lang.Objectjavax.el.Expression
- 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.
Java EE 5 SDK深圳电信培训中心.徐海蛟老师.
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详细信息: 字段 | 构造器 | 方法
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