用FactoryBean讓Spring配置動起來
編輯推薦：Spring開發資料合集
看到不少朋友討論 spring 配置時認為 spring 配置中只能靜態的設置一些參數(典型情況如數據庫配置, 定時器配置等)導致不方便, 其實 spring 已經提供了非常便利的方式來實現動態配置, 我們要做的只是實現一個自己的 FactoryBean , 來看一下 FactoryBean 接口的定義
代碼
| /** * 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(); } |
看了以后發現, FactoryBean 用于在 spring 容器中創建其他的 Bean, 我們平時用得最多的 JndiObjectFactoryBean, hibernate 的 LocalSessionFactoryBean 都是 FactoryBean 的具體實現, 既然如此, 讀取動態配置就變得易如反掌了, 假如我們要實現動態讀取數據庫配置的功能, 拿使用率最高的 BasicDatasource 為例, 簡單的實現一個 BasicDatasource FactoryBean 如下即可
代碼
| public class BasicDataSourceFactoryBean implements FactoryBean { public Object getObject() throws Exception { BasicDataSource dataSource = new BasicDataSource(); // 讀取外部配置, 設置到 dataSource 中 ... return dataSource; } public Class getObjectType() { return BasicDataSource.class; } public boolean isSingleton() { return true; } } |
然后在 spring 中如此聲明
代碼
| ＜bean id="dataSource" class="BasicDataSourceFactoryBean "＞ ... 你的配置來源 ＜/bean＞ |
就這么簡單。