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();
}
看了以後發現, FactoryBean 用於在 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;
}
}
然後在 spring 中如此聲明
代碼
<bean id="dataSource" class="BasicDataSourceFactoryBean ">
... 你的配置來源
</bean>
就這麼簡單。
