objectfactorycreatingfactorybean.java

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/*
 * Copyright 2002-2006 the original author or authors.
 * 
 * Licensed 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.
 */

package org.springframework.beans.factory.config;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.beans.factory.ObjectFactory;
import org.springframework.util.Assert;

/**
 * A {@link org.springframework.beans.factory.FactoryBean} implementation that
 * returns a value which is an {@link org.springframework.beans.factory.ObjectFactory}
 * that in turn returns a bean sourced from a {@link org.springframework.beans.factory.BeanFactory}.
 * 
 * <p>As such, this may be used to avoid having a client object directly calling
 * {@link org.springframework.beans.factory.BeanFactory#getBean(String)} to get
 * a (typically prototype) bean from a
 * {@link org.springframework.beans.factory.BeanFactory}, which would be a
 * violation of the inversion of control principle. Instead, with the use
 * of this class, the client object can be fed an
 * {@link org.springframework.beans.factory.ObjectFactory} instance as a
 * property which directly returns only the one target bean (again, which is
 * typically a prototype bean).
 * 
 * <p>A sample config in an XML-based
 * {@link org.springframework.beans.factory.BeanFactory} might look as follows:
 * 
 * <pre class="code">&lt;beans>
 *
 *   &lt;!-- Prototype bean since we have state -->
 *   &lt;bean id="myService" class="a.b.c.MyService" singleton="false"/>
 * 
 *   &lt;bean id="myServiceFactory"
 *            class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean">
 *     &lt;property name="targetBeanName">&lt;idref local="myService"/>&lt;/property>
 *   &lt;/bean> 
 *  
 *   &lt;bean id="clientBean" class="a.b.c.MyClientBean">
 *     &lt;property name="myServiceFactory" ref="myServiceFactory"/>
 *   &lt;/bean>
 *
 *&lt;/beans></pre>
 * 
 * <p>The attendant <code>MyClientBean</code> class implementation might look
 * something like this:
 * 
 * <pre class="code">package a.b.c;
 * 
 *import org.springframework.beans.factory.ObjectFactory;
 * 
 *public class MyClientBean {
 * 
 *    private ObjectFactory myServiceFactory;
 * 
 *    public void setMyServiceFactory(ObjectFactory myServiceFactory) {
 *        this.myServiceFactory = myServiceFactory;
 *    }
 * 
 *    public void someBusinessMethod() {
 *        // get a 'fresh', brand new MyService instance
 *        MyService service = this.myServiceFactory.getObject();
 *        // use the service object to effect the business logic...
 *    }
 *}</pre>
 * 
 * <p>An alternate approach to this application of an object creational pattern
 * would be to use the {@link ServiceLocatorFactoryBean}
 * to source (prototype) beans. The {@link ServiceLocatorFactoryBean} approach
 * has the advantage of the fact that one doesn't have to depend on any
 * Spring-specific interface such as {@link org.springframework.beans.factory.ObjectFactory},
 * but has the disadvantage of requiring runtime class generation. Please do
 * consult the
 * {@link ServiceLocatorFactoryBean ServiceLocatorFactoryBean JavaDoc} for a
 * fuller discussion of this issue.
 *
 * @author Colin Sampaleanu
 * @since 2004-05-11
 * @see org.springframework.beans.factory.ObjectFactory
 * @see ServiceLocatorFactoryBean
 */
public class ObjectFactoryCreatingFactoryBean extends AbstractFactoryBean
		implements BeanFactoryAware {

	private String targetBeanName;

	private BeanFactory beanFactory;


	/**
	 * Set the name of the target bean.
	 * <p>
	 * The target does not <i>have</> to be a prototype bean, but realisticially
	 * always will be (because if the target bean were a singleton, then said
	 * singleton bean could simply be injected straight into the dependent object,
	 * thus obviating the need for the extra level of indirection afforded by
	 * the approach encapsulated by this class). Please note that no exception
	 * will be thrown if the supplied <code>targetBeanName</code> does not
	 * reference a prototype bean.
	 */
	public void setTargetBeanName(String targetBeanName) {
		this.targetBeanName = targetBeanName;
	}

	public void setBeanFactory(BeanFactory beanFactory) {
		this.beanFactory = beanFactory;
	}

	protected Object createInstance() {
		return new ObjectFactory() {
			public Object getObject() throws BeansException {
				return beanFactory.getBean(targetBeanName);
			}
		};
	}

	public void afterPropertiesSet() throws Exception {
		Assert.hasText(targetBeanName, "targetBeanName is required");
		super.afterPropertiesSet();
	}

	public Class getObjectType() {
		return ObjectFactory.class;
	}

}