1 /**
2 *
3 * Copyright 2003-2004 The Apache Software Foundation
4 *
5 * Licensed under the Apache License, Version 2.0 (the "License");
6 * you may not use this file except in compliance with the License.
7 * You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17
18
19
20
21
22
23
24 package javax.servlet.jsp.el;
25
26
27 /**
28 * <p>The abstract base class for an expression-language evaluator.
29 * Classes that implement an expression language expose their functionality
30 * via this abstract class.</p>
31 *
32 * <p>An instance of the ExpressionEvaluator can be obtained via the
33 * JspContext / PageContext</p>
34 *
35 * <p>The parseExpression() and evaluate() methods must be thread-safe.
36 * That is, multiple threads may call these methods on the same
37 * ExpressionEvaluator object simultaneously. Implementations should
38 * synchronize access if they depend on transient state. Implementations
39 * should not, however, assume that only one object of each
40 * ExpressionEvaluator type will be instantiated; global caching should
41 * therefore be static.</p>
42 *
43 * <p>Only a single EL expression, starting with '${' and ending with
44 * '}', can be parsed or evaluated at a time. EL expressions
45 * cannot be mixed with static text. For example, attempting to
46 * parse or evaluate "<code>abc${1+1}def${1+1}ghi</code>" or even
47 * "<code>${1+1}${1+1}</code>" will cause an <code>ELException</code> to
48 * be thrown.</p>
49 *
50 * <p>The following are examples of syntactically legal EL expressions:
51 *
52 * <ul>
53 * <li><code>${person.lastName}</code></li>
54 * <li><code>${8 * 8}</code></li>
55 * <li><code>${my:reverse('hello')}</code></li>
56 * </ul>
57 * </p>
58 *
59 * @since 2.0
60 */
61 public abstract class ExpressionEvaluator {
62
63 /**
64 * Prepare an expression for later evaluation. This method should perform
65 * syntactic validation of the expression; if in doing so it detects
66 * errors, it should raise an ELParseException.
67 *
68 * @param expression The expression to be evaluated.
69 * @param expectedType The expected type of the result of the evaluation
70 * @param fMapper A FunctionMapper to resolve functions found in
71 * the expression. It can be null, in which case no functions
72 * are supported for this invocation. The ExpressionEvaluator
73 * must not hold on to the FunctionMapper reference after
74 * returning from <code>parseExpression()</code>. The
75 * <code>Expression</code> object returned must invoke the same
76 * functions regardless of whether the mappings in the
77 * provided <code>FunctionMapper</code> instance change between
78 * calling <code>ExpressionEvaluator.parseExpression()</code>
79 * and <code>Expression.evaluate()</code>.
80 * @return The Expression object encapsulating the arguments.
81 *
82 * @exception ELException Thrown if parsing errors were found.
83 */
84 public abstract Expression parseExpression( String expression,
85 Class expectedType,
86 FunctionMapper fMapper )
87 throws ELException;
88
89
90 /**
91 * Evaluates an expression. This method may perform some syntactic
92 * validation and, if so, it should raise an ELParseException error if
93 * it encounters syntactic errors. EL evaluation errors should cause
94 * an ELException to be raised.
95 *
96 * @param expression The expression to be evaluated.
97 * @param expectedType The expected type of the result of the evaluation
98 * @param vResolver A VariableResolver instance that can be used at
99 * runtime to resolve the name of implicit objects into Objects.
100 * @param fMapper A FunctionMapper to resolve functions found in
101 * the expression. It can be null, in which case no functions
102 * are supported for this invocation.
103 * @return The result of the expression evaluation.
104 *
105 * @exception ELException Thrown if the expression evaluation failed.
106 */
107 public abstract Object evaluate( String expression,
108 Class expectedType,
109 VariableResolver vResolver,
110 FunctionMapper fMapper )
111 throws ELException;
112 }
113