Spring中的FactoryBean
1.概述
Spring中有两种类型的Bean,一种是普通Bean,另一种是工厂Bean,即FactoryBean,这两种Bean都被容器管理,但工厂Bean跟普通Bean不同,其返回的对象不是指定类的一个实例,其返回的是该FactoryBean的getObject方法所返回的对象。在Spring框架内部,有很多地方有FactoryBean的实现类,它们在很多应用如(Spring的AOP、ORM、事务管理)及与其它第三框架(ehCache)集成时都有体现,下面简单分析FactoryBean的用法。
2.实例
以下SimpleFactoryBean类实现了FactoryBean接口中的三个方法。并将该类配置在XML中。
public class SimpleFactoryBean implements FactoryBean { private boolean flag; public Object getObject() throws Exception { if (flag) { return new Date(); } return new String("false"); } @SuppressWarnings("unchecked") public Class getObjectType() { return flag ? Date.class : String.class; } public boolean isSingleton() { return false; } public void setFlag(boolean flag) { this.flag = flag; } } <bean id="factoryBeanOne" class="com.study.demo.factorybean.SimpleFactoryBean" > <property name="flag"> <value>true</value> </property> </bean> <bean id="factoryBeanTwo" class="com.study.demo.factorybean.SimpleFactoryBean" > <property name="flag"> <value>false</value> </property> </bean> public class MainTest { public static void main(String[] args) { Resource res = new ClassPathResource("bean.xml"); BeanFactory factory = new XmlBeanFactory(res); System.out.println(factory.getBean("factoryBeanOne").getClass()); System.out.println(factory.getBean("factoryBeanTwo").getClass()); } }
通过简单的测试可知,该类输出如下:
classjava.util.Date
classjava.lang.String
也就是说,容器通过getBean方法返回的不是FactoryBean本身,而是FactoryBean实现类中getObject()方法所返回的对象。
3.FactoryBean的扩展应用
以Spring集成ehcache为例,看下org.springframework.cache.ehcache.EhCacheManagerFactoryBean类与org.springframework.cache.ehcache.EhCacheFactoryBean类。EhCacheManagerFactoryBean类中的getObject()和getObjectType()方法返回的如下:
public Object getObject() { return this.cacheManager; } public Class getObjectType() { return (this.cacheManager != null ? this.cacheManager.getClass() : CacheManager.class); }
EhCacheFactoryBean类中的getObject()和getObjectType()方法返回的如下:
public Object getObject() { return this.cache; } public Class getObjectType() { return (this.cache != null ? this.cache.getClass() : Ehcache.class); }
有了这两个FactoryBean,在Spring容器中使用ehcache就变得很简单了。配置如下:
<bean id="cacheManager" class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean"> <property name="configLocation"> <value>classpath:ehcache.xml</value> </property> </bean> <bean id="levelOneCache" class="org.springframework.cache.ehcache.EhCacheFactoryBean"> <property name="cacheManager"> <ref local="cacheManager" /> </property> <property name="cacheName"> <value>levelOneCache</value> </property> </bean>
应用代码如下:
public class MainTest { public static void main(String[] args) { Resource res = new ClassPathResource("bean.xml"); BeanFactory factory = new XmlBeanFactory(res); //取到CacheManager类的实例 CacheManager cacheManager = (CacheManager) factory .getBean("cacheManager"); //取到Cache类的实例 Cache levelOneCache = cacheManager.getCache("levelOneCache"); } }