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package javax.servlet.jsp.el;
/**
* <p>The abstract base class for an expression-language evaluator.
* Classes that implement an expression language expose their functionality
* via this abstract class.</p>
*
* <p>An instance of the ExpressionEvaluator can be obtained via the
* JspContext / PageContext</p>
*
* <p>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.</p>
*
* <p>Only a single EL expression, starting with '${' and ending with
* '}', can be parsed or evaluated at a time. EL expressions
* cannot be mixed with static text. For example, attempting to
* parse or evaluate "<code>abc${1+1}def${1+1}ghi</code>" or even
* "<code>${1+1}${1+1}</code>" will cause an <code>ELException</code> to
* be thrown.</p>
*
* <p>The following are examples of syntactically legal EL expressions:
*
* <ul>
* <li><code>${person.lastName}</code></li>
* <li><code>${8 * 8}</code></li>
* <li><code>${my:reverse('hello')}</code></li>
* </ul>
* </p>
*
* @deprecated As of JSP 2.1, replaced by {@link javax.el.ExpressionFactory}
* @since JSP 2.0
*/
public abstract class ExpressionEvaluator {
/**
* 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.
*
* @param expression The expression to be evaluated.
* @param expectedType The expected type of the result of the evaluation
* @param 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 ExpressionEvaluator
* must not hold on to the FunctionMapper reference after
* returning from <code>parseExpression()</code>. The
* <code>Expression</code> object returned must invoke the same
* functions regardless of whether the mappings in the
* provided <code>FunctionMapper</code> instance change between
* calling <code>ExpressionEvaluator.parseExpression()</code>
* and <code>Expression.evaluate()</code>.
* @return The Expression object encapsulating the arguments.
*
* @exception ELException Thrown if parsing errors were found.
*/
public abstract Expression parseExpression( String expression,
Class expectedType,
FunctionMapper fMapper )
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.
*
* @param expression The expression to be evaluated.
* @param expectedType The expected type of the result of the evaluation
* @param vResolver A VariableResolver instance that can be used at
* runtime to resolve the name of implicit objects into Objects.
* @param fMapper A FunctionMapper to resolve functions found in
* the expression. It can be null, in which case no functions
* are supported for this invocation.
* @return The result of the expression evaluation.
*
* @exception ELException Thrown if the expression evaluation failed.
*/
public abstract Object evaluate( String expression,
Class expectedType,
VariableResolver vResolver,
FunctionMapper fMapper )
throws ELException;
}
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