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Java > Open Source Codes > BookFactory


1 /*
2  * Copyright 2003-2004 The Apache Software Foundation.
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  * http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16
17 import org.apache.commons.digester.AbstractObjectCreationFactory;
18
19 /**
20  * The Book class doesn't have a no-argument constructor, so the
21  * standard digester ObjectCreateRule can't be used to create instances
22  * of it.
23  * <p>
24  * To resolve this issue, the FactoryCreateRule can be used in
25  * conjunction with an appropriate factory class, like this one.
26  * The "createObject" method of the factory is invoked to generate
27  * object instances when required.
28  * <p>
29  * The factory object can access any xml attributes, plus of course
30  * any values set up within it before digester parsing starts (like
31  * JNDI references, database connections, etc) that it may in the
32  * process of generating an appropriate object.
33  * <p>
34  * Note that it is <i>not</i> possible for any data to be extracted
35  * from the body or subelements of the xml element that caused the
36  * createObject method on this factory to be invoked. For example:
37  * <pre>
38  * [book isdn="12345"]
39  * </pre>
40  * is fine; the isdn value can be accessed during the createObject method.
41  * However, given the xml:
42  * <pre>
43  * [book]
44  * [isdn]12345[/isdn]
45  * ...
46  * </pre>
47  * it is not possible to access the isdn number until after the
48  * Book instance has been created.
49  * <p>
50  * Note that even if the class to be created does have a default constructor,
51  * you may wish to use a factory class, in order to initialise the created
52  * object in specific ways, or insert created objects into a central
53  * register, etc.
54  * <p>
55  * And don't forget, either, that factories may be implemented as
56  * inner classes or anonymous classes if appropriate, reducing the
57  * overhead of using this functionality in many cases.
58  */
59 public class BookFactory extends AbstractObjectCreationFactory {
60
61     public Object JavaDoc createObject(org.xml.sax.Attributes JavaDoc attributes)
62     throws Exception JavaDoc {
63         String JavaDoc isbn = attributes.getValue("isbn");
64         
65         if (isbn == null) {
66             throw new Exception JavaDoc(
67                 "Mandatory isbn attribute not present on book tag.");
68         }
69         
70         return new Book(isbn);
71     }
72 }
73
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