ValueExpressionImpl (Apache Tomcat 7.0.103 API Documentation)

org.apache.el

## Class ValueExpressionImpl

- java.lang.Object
- - javax.el.Expression
  - - javax.el.ValueExpression
    - - org.apache.el.ValueExpressionImpl

- All Implemented Interfaces:
    Externalizable, Serializable

      public final class ValueExpressionImpl
      extends ValueExpression
      implements Externalizable

  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.

  Author:
    Jacob Hookom [jacob@hookom.net]
  See Also:
    `ELResolver`, `Expression`, `ExpressionFactory`, `ValueExpression`, Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `ValueExpressionImpl()`
    `ValueExpressionImpl(String expr, Node node, FunctionMapper fnMapper, VariableMapper varMapper, Class<?> expectedType)`

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `equals(Object obj)`
    `Class<?>` | `getExpectedType()`
    `String` | `getExpressionString()` Returns the type the result of the expression will be coerced to after evaluation.
    `Class<?>` | `getType(ELContext context)`
    `Object` | `getValue(ELContext context)`
    `ValueReference` | `getValueReference(ELContext context)`
    `int` | `hashCode()`
    `boolean` | `isLiteralText()`
    `boolean` | `isReadOnly(ELContext context)`
    `void` | `readExternal(ObjectInput in)`
    `void` | `setValue(ELContext context, Object value)`
    `String` | `toString()`
    `void` | `writeExternal(ObjectOutput out)`

    - ### Methods inherited from class java.lang.Object

      `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- - ### Constructor Detail

    - #### ValueExpressionImpl

          public ValueExpressionImpl()

    - #### ValueExpressionImpl

          public ValueExpressionImpl(String expr,
                             Node node,
                             FunctionMapper fnMapper,
                             VariableMapper varMapper,
                             Class<?> expectedType)

  - ### Method Detail

    - #### equals

          public boolean equals(Object obj)

      Specified by:
        `equals` in class `Expression`

    - #### getExpectedType

          public Class<?> getExpectedType()

      Specified by:
        `getExpectedType` in class `ValueExpression`

    - #### getExpressionString

          public String getExpressionString()

      Returns the type the result of the expression will be coerced to after evaluation.
      Specified by:
        `getExpressionString` in class `Expression`
      Returns:
        the `expectedType` passed to the `ExpressionFactory.createValueExpression` method that created this `ValueExpression`.
      See Also:
        `Expression.getExpressionString()`

    - #### getType

          public Class<?> getType(ELContext context)
                           throws PropertyNotFoundException,
                                  ELException

      Specified by:
        `getType` in class `ValueExpression`
      Parameters:
        `context` - The EL context for this evaluation
      Returns:
        The type of the result of this value expression
      Throws:
        `PropertyNotFoundException` - If a property/variable resolution failed because no match was found or a match was found but was not readable
        `ELException` - Wraps any exception throw whilst resolving a property or variable

    - #### getValue

          public Object getValue(ELContext context)
                          throws PropertyNotFoundException,
                                 ELException

      Specified by:
        `getValue` in class `ValueExpression`
      Parameters:
        `context` - The EL context for this evaluation
      Returns:
        The result of evaluating this value expression
      Throws:
        `PropertyNotFoundException` - If a property/variable resolution failed because no match was found or a match was found but was not readable
        `ELException` - Wraps any exception throw whilst resolving a property or variable

    - #### hashCode

          public int hashCode()

      Specified by:
        `hashCode` in class `Expression`

    - #### isLiteralText

          public boolean isLiteralText()

      Specified by:
        `isLiteralText` in class `Expression`

    - #### isReadOnly

          public boolean isReadOnly(ELContext context)
                             throws PropertyNotFoundException,
                                    ELException

      Specified by:
        `isReadOnly` in class `ValueExpression`
      Parameters:
        `context` - The EL context for this evaluation
      Returns:
        `true` if this expression is read only otherwise `false`
      Throws:
        `PropertyNotFoundException` - If a property/variable resolution failed because no match was found or a match was found but was not readable
        `ELException` - Wraps any exception throw whilst resolving a property or variable

    - #### readExternal

          public void readExternal(ObjectInput in)
                            throws IOException,
                                   ClassNotFoundException

      Specified by:
        `readExternal` in interface `Externalizable`
      Throws:
        `IOException`
        `ClassNotFoundException`

    - #### setValue

          public void setValue(ELContext context,
                      Object value)
                        throws PropertyNotFoundException,
                               PropertyNotWritableException,
                               ELException

      Specified by:
        `setValue` in class `ValueExpression`
      Parameters:
        `context` - The EL context for this evaluation
        `value` - The value to set the property to which this value expression refers
      Throws:
        `PropertyNotFoundException` - If a property/variable resolution failed because no match was found
        `PropertyNotWritableException` - If a property/variable resolution failed because a match was found but was not writable
        `ELException` - Wraps any exception throw whilst resolving a property or variable

    - #### writeExternal

          public void writeExternal(ObjectOutput out)
                             throws IOException

      Specified by:
        `writeExternal` in interface `Externalizable`
      Throws:
        `IOException`

    - #### toString

          public String toString()

      Overrides:
        `toString` in class `Object`

    - #### getValueReference

          public ValueReference getValueReference(ELContext context)

      Overrides:
        `getValueReference` in class `ValueExpression`
      Parameters:
        `context` - The EL context for this evaluation
      Returns:
        This default implementation always returns `null`
      Since:
        EL 2.2

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