# Interface JxltEngine.Expression

Enclosing class:
  `JxltEngine`

public static interface JxltEngine.Expression
A unified expression that can mix immediate, deferred and nested sub-expressions as well as string constants;

- The "immediate" syntax is of the form `"...${jexl-expr}..."`
- The "deferred" syntax is of the form `"...#{jexl-expr}..."`
- The "nested" syntax is of the form `"...#{...${jexl-expr0}...}..."`
- The "composite" syntax is of the form `"...${jexl-expr0}... #{jexl-expr1}..."`

Deferred and immediate expression carry different intentions:

- An immediate expression indicate that evaluation is intended to be performed close to the definition/parsing point.
- A deferred expression indicate that evaluation is intended to occur at a later stage.

For instance: `"Hello ${name}, now is #{time}"` is a composite "deferred" expression since one of its subexpressions is deferred. Furthermore, this (composite) expression intent is to perform two evaluations; one close to its definition and another one in a later phase.

The API reflects this feature in 2 methods, prepare and evaluate. The prepare method will evaluate the immediate subexpression and return an expression that contains only the deferred subexpressions (and constants), a prepared expression. Such a prepared expression is suitable for a later phase evaluation that may occur with a different JexlContext. Note that it is valid to call evaluate without prepare in which case the same JexlContext is used for the 2 evaluation phases.

In the most common use-case where deferred expressions are to be kept around as properties of objects, one should createExpression and prepare an expression before storing it and evaluate it each time the property storing it is accessed.

Note that nested expression use the JEXL syntax as in:

> `"#{${bar}+'.charAt(2)'}"`

The most common mistake leading to an invalid expression being the following:

> `"#{${bar}charAt(2)}"`

Also note that methods that createExpression evaluate expressions may throw unchecked exceptions; The `JxltEngine.Exception` are thrown when the engine instance is in "non-silent" mode but since these are RuntimeException, user-code should catch them where appropriate.

Since:
  2.0

- ## Method Summary

  Modifier and Type
  Method
  Description
  `String`
  `asString()`
  Generates this expression's string representation.
  `StringBuilder`
  `asString(StringBuilder strb)`
  Adds this expression's string representation to a StringBuilder.
  `Object`
  `evaluate(JexlContext context)`
  Evaluates this expression.
  `JxltEngine.Expression`
  `getSource()`
  Gets this expression's source expression.
  `Set<List<String>>`
  `getVariables()`
  Gets the list of variables accessed by this expression.
  `boolean`
  `isDeferred()`
  Checks whether this expression is deferred.
  `boolean`
  `isImmediate()`
  Checks whether this expression is immediate.
  `JxltEngine.Expression`
  `prepare(JexlContext context)`
  Evaluates the immediate sub-expressions.
  `String`
  `toString()`
  Formats this expression, adding its source string representation in comments if available: 'expression /*= source *\/'' .

- ## Method Details

  - ### asString

    String asString()
    Generates this expression's string representation.
    Returns:
      the string representation

  - ### asString

    StringBuilder asString(StringBuilder strb)
    Adds this expression's string representation to a StringBuilder.
    Parameters:
      `strb` - the builder to fill
    Returns:
      the builder argument

  - ### evaluate

    Object evaluate(JexlContext context)
    Evaluates this expression.

    If the underlying JEXL engine is silent, errors will be logged through its logger as warning.

    Parameters:
      `context` - the variable context
    Returns:
      the result of this expression evaluation or null if an error occurs and the `JexlEngine` is running in silent mode
    Throws:
      `JxltEngine.Exception` - if an error occurs and the `JexlEngine` is not silent

  - ### getSource

    JxltEngine.Expression getSource()
    Gets this expression's source expression.

    If this expression was prepared, this allows to retrieve the original expression that lead to it.

    Other expressions return themselves.

    Returns:
      the source expression

  - ### getVariables

    Set<List<String>> getVariables()
    Gets the list of variables accessed by this expression.

    This method will visit all nodes of the sub-expressions and extract all variables whether they are written in 'dot' or 'bracketed' notation. (a.b is equivalent to a['b']).

    Returns:
      the set of variables, each as a list of strings (ant-ish variables use more than 1 string) or the empty set if no variables are used

  - ### isDeferred

    boolean isDeferred()
    Checks whether this expression is deferred.
    Returns:
      true if deferred, false otherwise

  - ### isImmediate

    boolean isImmediate()
    Checks whether this expression is immediate.
    Returns:
      true if immediate, false otherwise

  - ### prepare

    JxltEngine.Expression prepare(JexlContext context)
    Evaluates the immediate sub-expressions.

    When the expression is dependant upon immediate and deferred sub-expressions, evaluates the immediate sub-expressions with the context passed as parameter and returns this expression deferred form.

    In effect, this binds the result of the immediate sub-expressions evaluation in the context, allowing to differ evaluation of the remaining (deferred) expression within another context. This only has an effect to nested and composite expressions that contain differed and immediate sub-expressions.

    If the underlying JEXL engine is silent, errors will be logged through its logger as warning.*

    Parameters:
      `context` - the context to use for immediate expression evaluations
    Returns:
      an `JxltEngine.Expression` or null if an error occurs and the `JexlEngine` is running in silent mode
    Throws:
      `JxltEngine.Exception` - if an error occurs and the `JexlEngine` is not in silent mode

  - ### toString

    String toString()
    Formats this expression, adding its source string representation in comments if available: 'expression /*= source *\/'' .
    Overrides:
      `toString` in class `Object`
    Returns:
      the formatted expression string