用FactoryBean让Spring配置动起来

　　看到不少朋友讨论 spring 配置时认为 spring 配置中只能静态的设置一些参数(典型情况如数据库配置, 定时器配置等)导致不方便, 其实 spring 已经提供了非常便利的方式来实现动态配置, 我们要做的只是实现一个自己的 FactoryBean , 来看一下 FactoryBean 接口的定义

　　代码

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/** * Interface to be implemented by objects used within a BeanFactory * that are themselves factories. If a bean implements this interface, * it is used as a factory, not directly as a bean. * * ＜p＞＜b＞NB: A bean that implements this interface cannot be used * as a normal bean.＜/b＞ A FactoryBean is defined in a bean style, * but the object exposed for bean references is always the object * that it creates. * * ＜p＞FactoryBeans can support singletons and prototypes, and can * either create objects lazily on demand or eagerly on startup. * * ＜p＞This interface is heavily used within the framework, for * example for the AOP ProxyFactoryBean or JndiObjectFactoryBean. * It can be used for application components, but this is not common * outside of infrastructure code. * * @author Rod Johnson * @author Juergen Hoeller * @since 08.03.2003 * @see org.springframework.beans.factory.BeanFactory * @see org.springframework.aop.framework.ProxyFactoryBean * @see org.springframework.jndi.JndiObjectFactoryBean */ public interface FactoryBean { 　/** 　* Return an instance (possibly shared or independent) of the object 　* managed by this factory. As with a BeanFactory, this allows 　* support for both the Singleton and Prototype design pattern. 　* ＜p＞If this method returns ＜code＞null＜/code＞, the factory will consider 　* the FactoryBean as not fully initialized and throw a corresponding 　* FactoryBeanNotInitializedException. 　* @return an instance of the bean (should not be ＜code＞null＜/code＞; 　* a ＜code＞null＜/code＞ value will be considered as an indication of 　* incomplete initialization) 　* @throws Exception in case of creation errors 　* @see FactoryBeanNotInitializedException 　*/ 　Object getObject() throws Exception; 　/** 　* Return the type of object that this FactoryBean creates, or ＜code＞null＜/code＞ 　* if not known in advance. This allows to check for specific types 　* of beans without instantiating objects, for example on autowiring. 　* ＜p＞For a singleton, this should try to avoid singleton creation 　* as far as possible; it should rather estimate the type in advance. 　* For prototypes, returning a meaningful type here is advisable too. 　* ＜p＞This method can be called ＜i＞before＜/i＞ this FactoryBean has 　* been fully initialized. It must not rely on state created during 　* initialization; of course, it can still use such state if available. 　* ＜p＞＜b＞NOTE:＜/b＞ Autowiring will simply ignore FactoryBeans that return 　* ＜code＞null＜/code＞ here. Therefore it is highly recommended to implement 　* this method properly, using the current state of the FactoryBean. 　* @return the type of object that this FactoryBean creates, 　* or ＜code＞null＜/code＞ if not known at the time of the call 　* @see ListableBeanFactory#getBeansOfType 　*/ 　Class getObjectType(); 　/** 　* Is the bean managed by this factory a singleton or a prototype? 　* That is, will ＜code＞getObject()＜/code＞ always return the same object 　* (a reference that can be cached)? 　* ＜p＞＜b＞NOTE:＜/b＞ If a FactoryBean indicates to hold a singleton object, 　* the object returned from ＜code＞getObject()＜/code＞ might get cached 　* by the owning BeanFactory. Hence, do not return ＜code＞true＜/code＞ 　* unless the FactoryBean always exposes the same reference. 　* ＜p＞The singleton status of the FactoryBean itself will generally 　* be provided by the owning BeanFactory; usually, it has to be 　* defined as singleton there. 　* @return if this bean is a singleton 　* @see #getObject() 　*/ 　boolean isSingleton(); }

　　看了以后发现, FactoryBean 用于在 spring 容器中创建其他的 Bean, 我们平时用得最多的 JndiObjectFactoryBean, hibernate 的 LocalSessionFactoryBean 都是 FactoryBean 的具体实现, 既然如此, 读取动态配置就变得易如反掌了, 假如我们要实现动态读取数据库配置的功能, 拿使用率最高的 BasicDatasource 为例, 简单的实现一个 BasicDatasource FactoryBean 如下即可

　　代码

public class BasicDataSourceFactoryBean implements FactoryBean { 　public Object getObject() throws Exception { 　　BasicDataSource dataSource = new BasicDataSource(); 　　// 读取外部配置, 设置到 dataSource 中 ... 　　return dataSource; 　} 　public Class getObjectType() { 　　return BasicDataSource.class; 　} 　public boolean isSingleton() { 　　return true; 　} }

　　然后在 spring 中如此声明

　　代码

＜bean id="dataSource" class="BasicDataSourceFactoryBean "＞ ... 你的配置来源 ＜/bean＞

　　就这么简单。

原文转自：http://www.ltesting.net