001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *  http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing,
013     * software distributed under the License is distributed on an
014     * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015     * KIND, either express or implied.  See the License for the
016     * specific language governing permissions and limitations
017     * under the License.
018     */
019    
020    //
021    // This source code implements specifications defined by the Java
022    // Community Process. In order to remain compliant with the specification
023    // DO NOT add / change / or delete method signatures!
024    //
025    
026    package javax.servlet.jsp.el;
027    
028    
029    /**
030     * <p>The abstract class for a prepared expression.</p>
031     *
032     * <p>An instance of an Expression can be obtained via from an 
033     * ExpressionEvaluator instance.</p>
034     *
035     * <p>An Expression may or not have done a syntactic parse of the expression.
036     * A client invoking the evaluate() method should be ready for the case 
037     * where ELParseException exceptions are raised. </p>
038     *
039     * @since 2.0
040     */
041    public abstract class Expression {
042    
043        /** 
044         * Evaluates an expression that was previously prepared.  In some 
045         * implementations preparing an expression involves full syntactic 
046         * validation, but others may not do so.  Evaluating the expression may 
047         * raise an ELParseException as well as other ELExceptions due to 
048         * run-time evaluation.
049         *
050         * @param vResolver A VariableResolver instance that can be used at 
051         *   runtime to resolve the name of implicit objects into Objects.
052         * @return The result of the expression evaluation.
053         *
054         * @exception ELException Thrown if the expression evaluation failed.
055         */ 
056        public abstract Object evaluate( VariableResolver vResolver )
057            throws ELException;
058    }
059