JavaEE API 8.0.1 · javax.el.ValueExpression

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, setValue, isReadOnly, getType and getValueReference 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 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.

Constructor Summary

Access	Constructor and Description
public	ValueExpression()

Method Summary

Modifier and Type	Method and Description
public abstract Class<?>	Returns: the `expectedType` passed to the `ExpressionFactory.createValueExpression` method that created this `ValueExpression`. getExpectedType() Returns the type the result of the expression will be coerced to after evaluation.
public abstract Class<?>	Returns: the most general acceptable type; otherwise undefined. getType(ELContext The context of this evaluation. 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` method.
public abstract Object	Returns: The result of the expression evaluation. getValue(ELContext The context of this evaluation. context) Evaluates the expression relative to the provided context, and returns the resulting value.
public ValueReference	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. getValueReference(ELContext the context of this evaluation context) Returns a `ValueReference` for this expression instance.
public abstract boolean	Returns: `true` if the expression is read-only or `false` if not. isReadOnly(ELContext The context of this evaluation. context) Evaluates the expression relative to the provided context, and returns `true` if a call to `setValue` will always fail.
public abstract void	setValue(ELContext The context of this evaluation. context, Object The new value to be set. value) Evaluates the expression relative to the provided context, and sets the result to the provided value.

Inherited from javax.el.Expression:: equals getExpressionString hashCode isLiteralText

Constructor Detail

ValueExpression	back to summary
public ValueExpression()

Method Detail

getExpectedType back to summary

getExpectedType	back to summary
public abstract Class<?> getExpectedType() Returns the type the result of the expression will be coerced to after evaluation. Returns:Class<?> the `expectedType` passed to the `ExpressionFactory.createValueExpression` method that created this `ValueExpression`.

public abstract Class<?> getExpectedType()

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

Returns:Class<?>: the expectedType passed to the ExpressionFactory.createValueExpression method that created this ValueExpression.

getType back to summary

getType	back to summary
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` 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:ELContext The context of this evaluation. Returns:Class<?> the most general acceptable type; otherwise undefined. Exceptions 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.

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 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:ELContext: The context of this evaluation.

Returns:Class<?>

the most general acceptable type; otherwise undefined.

Exceptions

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.

getValue back to summary

getValue	back to summary
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. Parameters context:ELContext The context of this evaluation. Returns:Object The result of the expression evaluation. Exceptions 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.

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.

Parameters

context:ELContext: The context of this evaluation.

Returns:Object

The result of the expression evaluation.

Exceptions

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.

getValueReference back to summary

getValueReference	back to summary
public ValueReference getValueReference(ELContext context) Returns a `ValueReference` for this expression instance. Parameters context:ELContext the context of this evaluation Returns:ValueReference 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

public ValueReference getValueReference(ELContext context)

Returns a ValueReference for this expression instance.

Parameters

context:ELContext: the context of this evaluation

Returns:ValueReference

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

isReadOnly back to summary

isReadOnly	back to summary
public abstract boolean isReadOnly(ELContext context) Evaluates the expression relative to the provided context, and returns `true` if a call to `setValue` will always fail. Parameters context:ELContext The context of this evaluation. Returns:boolean `true` if the expression is read-only or `false` if not. Exceptions 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`

public abstract boolean isReadOnly(ELContext context)

Evaluates the expression relative to the provided context, and returns true if a call to setValue will always fail.

Parameters

context:ELContext: The context of this evaluation.

Returns:boolean

true if the expression is read-only or false if not.

Exceptions

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

setValue back to summary

setValue	back to summary
public abstract void setValue(ELContext context, Object value) Evaluates the expression relative to the provided context, and sets the result to the provided value. Parameters context:ELContext The context of this evaluation. value:Object The new value to be set. Exceptions 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.

public abstract void setValue(ELContext context, Object value)

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

Parameters

context:ELContext: The context of this evaluation.
value:Object: The new value to be set.

Exceptions

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.

public abstract Class ValueExpression

Constructor Summary

Method Summary

Constructor Detail

Method Detail