使用 FactoryBean 让你的 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Java代码 收藏代码 /** * 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. * *
NB: A bean that implements this interface cannot be used * as a normal bean. A FactoryBean is defined in a bean style, * but the object exposed for bean references is always the object * that it creates. * *

FactoryBeans can support singletons and prototypes, and can * either create objects lazily on demand or eagerly on startup. * *

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. *

If this method returns

null

, the factory will consider * the FactoryBean as not fully initialized and throw a corresponding * FactoryBeanNotInitializedException. * @return an instance of the bean (should not be

null

; * a

null

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

null

* if not known in advance. This allows to check for specific types * of beans without instantiating objects, for example on autowiring. *

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. *

This method can be called before 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. *

NOTE: Autowiring will simply ignore FactoryBeans that return *

null

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

null

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

getObject()

always return the same object * (a reference that can be cached)? *

NOTE: If a FactoryBean indicates to hold a singleton object, * the object returned from

getObject()

might get cached * by the owning BeanFactory. Hence, do not return

true

* unless the FactoryBean always exposes the same reference. *

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Java代码 收藏代码 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Java代码 收藏代码 ... 你的配置来源 就这么简单