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038:
039: package javax.servlet.jsp.el;
040:
041: /**
042: * <p>The abstract base class for an expression-language evaluator.
043: * Classes that implement an expression language expose their functionality
044: * via this abstract class.</p>
045: *
046: * <p>An instance of the ExpressionEvaluator can be obtained via the
047: * JspContext / PageContext</p>
048: *
049: * <p>The parseExpression() and evaluate() methods must be thread-safe.
050: * That is, multiple threads may call these methods on the same
051: * ExpressionEvaluator object simultaneously. Implementations should
052: * synchronize access if they depend on transient state. Implementations
053: * should not, however, assume that only one object of each
054: * ExpressionEvaluator type will be instantiated; global caching should
055: * therefore be static.</p>
056: *
057: * <p>Only a single EL expression, starting with '${' and ending with
058: * '}', can be parsed or evaluated at a time. EL expressions
059: * cannot be mixed with static text. For example, attempting to
060: * parse or evaluate "<code>abc${1+1}def${1+1}ghi</code>" or even
061: * "<code>${1+1}${1+1}</code>" will cause an <code>ELException</code> to
062: * be thrown.</p>
063: *
064: * <p>The following are examples of syntactically legal EL expressions:
065: *
066: * <ul>
067: * <li><code>${person.lastName}</code></li>
068: * <li><code>${8 * 8}</code></li>
069: * <li><code>${my:reverse('hello')}</code></li>
070: * </ul>
071: * </p>
072: *
073: * @deprecated As of JSP 2.1, replaced by {@link javax.el.ExpressionFactory}
074: * @since JSP 2.0
075: */
076: public abstract class ExpressionEvaluator {
077:
078: /**
079: * Prepare an expression for later evaluation. This method should perform
080: * syntactic validation of the expression; if in doing so it detects
081: * errors, it should raise an ELParseException.
082: *
083: * @param expression The expression to be evaluated.
084: * @param expectedType The expected type of the result of the evaluation
085: * @param fMapper A FunctionMapper to resolve functions found in
086: * the expression. It can be null, in which case no functions
087: * are supported for this invocation. The ExpressionEvaluator
088: * must not hold on to the FunctionMapper reference after
089: * returning from <code>parseExpression()</code>. The
090: * <code>Expression</code> object returned must invoke the same
091: * functions regardless of whether the mappings in the
092: * provided <code>FunctionMapper</code> instance change between
093: * calling <code>ExpressionEvaluator.parseExpression()</code>
094: * and <code>Expression.evaluate()</code>.
095: * @return The Expression object encapsulating the arguments.
096: *
097: * @exception ELException Thrown if parsing errors were found.
098: */
099: public abstract Expression parseExpression(String expression,
100: Class expectedType, FunctionMapper fMapper)
101: throws ELException;
102:
103: /**
104: * Evaluates an expression. This method may perform some syntactic
105: * validation and, if so, it should raise an ELParseException error if
106: * it encounters syntactic errors. EL evaluation errors should cause
107: * an ELException to be raised.
108: *
109: * @param expression The expression to be evaluated.
110: * @param expectedType The expected type of the result of the evaluation
111: * @param vResolver A VariableResolver instance that can be used at
112: * runtime to resolve the name of implicit objects into Objects.
113: * @param fMapper A FunctionMapper to resolve functions found in
114: * the expression. It can be null, in which case no functions
115: * are supported for this invocation.
116: * @return The result of the expression evaluation.
117: *
118: * @exception ELException Thrown if the expression evaluation failed.
119: */
120: public abstract Object evaluate(String expression,
121: Class expectedType, VariableResolver vResolver,
122: FunctionMapper fMapper) throws ELException;
123: }
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