用FactoryBean让Spring配置动起来

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spring配置, 我们要做的只是实现一个自己的 Factory Bean , 来看一下 Factory Bean 接口的定义

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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, Factory Bean 用于在spring容器中创建其他的Bean, 我们平时用得最多的 JndiObjectFactory Bean, hibernate 的 LocalSessionFactory Bean 都是 Factory Bean 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***的 BasicDatasource 为例, 简单的实现一个 BasicDatasource Factory Bean 如下即可

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

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

文章名称：用FactoryBean让Spring配置动起来
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