https://tomcat.apache.org/: JSP package 
/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package jakarta.servlet.jsp.el;



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.




Only a single EL expression, starting with '${' and ending with '}', can be
parsed or evaluated at a time. EL expressions cannot be mixed with static
text. For example, attempting to parse or evaluate "
abc${1+1}def${1+1}ghi" or even "${1+1}${1+1}" will
cause an ELException to be thrown.




The following are examples of syntactically legal EL expressions:



    

  • ${person.lastName}
    • 

  • ${8 * 8}
    • 

  • ${my:reverse('hello')}
    • 



Since:2.0
Deprecated:As of JSP 2.1, replaced by jakarta.el.ExpressionFactory
/**
 * <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}${1+1}</code>" will
 * cause an <code>ELException</code> to be thrown.
 * </p>
 * <p>
 * The following are examples of syntactically legal EL expressions:
 * </p>
 * <ul>
 * <li><code>${person.lastName}</code></li>
 * <li><code>${8 * 8}</code></li>
 * <li><code>${my:reverse('hello')}</code></li>
 * </ul>
 *
 * @since 2.0
 * @deprecated As of JSP 2.1, replaced by jakarta.el.ExpressionFactory
 */
@Deprecated
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.

Params:
  • 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 ExpressionEvaluator must not hold on to
           the FunctionMapper reference after returning from
           parseExpression(). The Expression
           object returned must invoke the same functions regardless of
           whether the mappings in the provided
           FunctionMapper instance change between calling
           ExpressionEvaluator.parseExpression() and
           Expression.evaluate().
Throws:
Returns:The Expression object encapsulating the arguments.
/**
     * 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.

Params:
  • 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.
Throws:
  • ELException – 
               Thrown if the expression evaluation failed.
Returns:The result of the expression evaluation.
/**
     * 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;
}