javax.servlet.jsp.el
Class ExpressionEvaluator

java.lang.Object
  |
  +--javax.servlet.jsp.el.ExpressionEvaluator

public abstract class ExpressionEvaluator
extends java.lang.Object

The abstract base class for an expression-language evaluator. Classes that implement an expression language expose their functionality via this abstract class.

An instance of the ExpressionEvaluator can be obtained via the JspContext / PageContext

The parseExpression() and evaluate() methods must be thread-safe. That is, multiple threads may call these methods on the same ExpressionEvaluator object simultaneously. Implementations should synchronize access if they depend on transient state. Implementations should not, however, assume that only one object of each ExpressionEvaluator type will be instantiated; global caching should therefore be static.

For JSP EL expressions, an expression string without '${' and '}' tokens is considered to be a static string. One or more occurrences of '${' and '}' can be used in the expression string to delimit dynamic expressions. Examples:

Since:
2.0

Constructor Summary
ExpressionEvaluator()
           
 
Method Summary
abstract  java.lang.Object evaluate(java.lang.String expression, java.lang.Class expectedType, VariableResolver vResolver, FunctionMapper fMapper, java.lang.String defaultPrefix)
          Evaluates an expression.
abstract  Expression parseExpression(java.lang.String expression, java.lang.Class expectedType, FunctionMapper fMapper, java.lang.String defaultPrefix)
          Prepare an expression for later evaluation.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

ExpressionEvaluator

public ExpressionEvaluator()
Method Detail

parseExpression

public abstract Expression parseExpression(java.lang.String expression,
                                           java.lang.Class expectedType,
                                           FunctionMapper fMapper,
                                           java.lang.String defaultPrefix)
                                    throws ELException
Prepare an expression for later evaluation. This method should perform syntactic validation of the expression; if in doing so it detects errors, it should raise an ELParseException.

Parameters:
expression - The expression to be evaluated.
expectedType - The expected type of the result of the evaluation
fMapper - A FunctionMapper to resolve functions found in the expression. It can be null, in which case no functions are supported for this invocation. The FunctionMapper will be invoked one or more times between parsing the expression and evaluating it, and must return a consistent value each time it is invoked.
defaultPrefix - The default prefix to use when a function is encountered with no prefix, or "" if the default is no prefix.
Returns:
The Expression object encapsulating the arguments.
Throws:
ELException - Thrown if parsing errors were found.

evaluate

public abstract java.lang.Object evaluate(java.lang.String expression,
                                          java.lang.Class expectedType,
                                          VariableResolver vResolver,
                                          FunctionMapper fMapper,
                                          java.lang.String defaultPrefix)
                                   throws ELException
Evaluates an expression. This method may perform some syntactic validation and, if so, it should raise an ELParseException error if it encounters syntactic errors. EL evaluation errors should cause an ELException to be raised.

Parameters:
expression - The expression to be evaluated.
expectedType - The expected type of the result of the evaluation
vResolver - A VariableResolver instance that can be used at runtime to resolve the name of implicit objects into Objects.
fMapper - A FunctionMapper to resolve functions found in the expression. It can be null, in which case no functions are supported for this invocation. The FunctionMapper will be invoked one or more times between parsing the expression and evaluating it, and must return a consistent value each time it is invoked.
defaultPrefix - The default prefix to use when a function is encountered with no prefix, or "" if the default is no prefix.
Returns:
The result of the expression evaluation.
Throws:
ELException - Thrown if the expression evaluation failed.


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