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
- /**//**
- * 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>
【编辑推荐】