Commons Digester example source code file (BookFactory.java)

This example Commons Digester source code file (BookFactory.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Commons Digester tags/keywords

abstractobjectcreationfactory, abstractobjectcreationfactory, book, book, bookfactory, exception, exception, mandatory, object, object, string

The Commons Digester BookFactory.java source code

/*
 * 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.
 */ 

import org.apache.commons.digester.AbstractObjectCreationFactory;

/**
 * The Book class doesn't have a no-argument constructor, so the
 * standard digester ObjectCreateRule can't be used to create instances
 * of it.
 * <p>
 * To resolve this issue, the FactoryCreateRule can be used in 
 * conjunction with an appropriate factory class, like this one.
 * The "createObject" method of the factory is invoked to generate
 * object instances when required.
 * <p>
 * The factory object can access any xml attributes, plus of course
 * any values set up within it before digester parsing starts (like
 * JNDI references, database connections, etc) that it may in the
 * process of generating an appropriate object.
 * <p>
 * Note that it is <i>not possible for any data to be extracted
 * from the body or subelements of the xml element that caused the
 * createObject method on this factory to be invoked. For example:
 * <pre>
 *  [book isdn="12345"]
 * </pre>
 * is fine; the isdn value can be accessed during the createObject method.
 * However, given the xml:
 * <pre>
 * [book]
 *   [isdn]12345[/isdn]
 *   ...
 * </pre>
 * it is not possible to access the isdn number until after the
 * Book instance has been created.
 * <p>
 * Note that even if the class to be created does have a default constructor,
 * you may wish to use a factory class, in order to initialise the created
 * object in specific ways, or insert created objects into a central
 * register, etc.
 * <p>
 * And don't forget, either, that factories may be implemented as
 * inner classes or anonymous classes if appropriate, reducing the
 * overhead of using this functionality in many cases. 
 */
public class BookFactory extends AbstractObjectCreationFactory {

    public Object createObject(org.xml.sax.Attributes attributes) 
    throws Exception {
        String isbn = attributes.getValue("isbn");
        
        if (isbn == null) {
            throw new Exception(
                "Mandatory isbn attribute not present on book tag.");
        }
        
        return new Book(isbn);
    }
}