Source code: com/presumo/jms/selector/SelectorFalseException.java
1 /**
2 * This file is part of Presumo.
3 *
4 * Presumo is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
8 *
9 * Presumo is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
13 *
14 * You should have received a copy of the GNU General Public License
15 * along with Presumo; if not, write to the Free Software
16 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
17 *
18 *
19 * Copyright 2001 Dan Greff
20 */
21 package com.presumo.jms.selector;
22
23 /**
24 * JMS defines some conditions that will always render a selector
25 * false. One example is trying to add two strings. While the
26 * parser can detect errors in literals at parse time, it can not
27 * tell what type an identifier is until evaluation time. If an
28 * identifier type is incorrectly used JMS says that the selector
29 * is invalid. Another example would be referencing an object
30 * property from anywhere in the sql expression.
31 *
32 * The evaluate methods will throw this exception if they encounter
33 * a situation like the ones above. This will effectively short-circuit
34 * the evaluation. If you call evaluate on the node in the expression
35 * tree you must catch this exception. If it occurs you can assume
36 * the selector is false. If the selector does not throw an
37 * exception you still have to check the return type to see if
38 * the evaluation was true or false or unknown. (unknown == false).
39 *
40 * @author Dan Greff
41 */
42 public final class SelectorFalseException extends Exception
43 {
44 //
45 // No reason to have multiple instance of this class so it
46 // is a singleton.
47 //
48
49 private static SelectorFalseException onlyInstance;
50
51 static SelectorFalseException getInstance()
52 {
53 if (onlyInstance == null)
54 onlyInstance = new SelectorFalseException();
55 return onlyInstance;
56 }
57
58 private SelectorFalseException() {}
59
60 }