Spring中Bean多种实现切换方案
一个公共工程中的Spring配置文件，可能会被多个工程引用。因为每个工程可能只需要公共工程中的一部分Bean，所以这些工程的Spring容器启动时，需要区分开哪些Bean要创建出来。另一种场景是：想通过Properties文件中的配置开关，就将Spring配置文件中Bean的实现切换成另一套。
方法一：Qulifier区分Bean
1.1应用实例
以Apache开源框架Jetspeed中的一段配置为例：page-manager.xml
===============================================================================
<bean name="xmlPageManager"class="org.apache.jetspeed.page.psml.CastorXmlPageManager"init-method="init" destroy-method="destroy">
<meta key="j2:cat" value="xmlPageManager orpageSerializer" />
<constructor-arg index="0">
<ref bean="IdGenerator"/>
</constructor-arg>
<constructor-arg index="1">
<refbean="xmlDocumentHandlerFactory" />
</constructor-arg>
……
</bean>
<bean id="dbPageManager"class="org.apache.jetspeed.page.impl.DatabasePageManager"init-method="init" destroy-method="destroy">
<meta key="j2:cat" value="dbPageManager orpageSerializer" />
<!-- OJB configuration file resourcepath -->
<constructor-arg index="0">
<value>JETSPEED-INF/ojb/page-manager-repository.xml</value>
</constructor-arg>
<!-- fragment id generator -->
<constructor-arg index="1">
<ref bean="IdGenerator"/>
</constructor-arg>
……
</bean>
1.2 Bean过滤器
JetspeedBeanDefinitionFilter在Spring容器解析每个Bean定义时，会取出上面Bean配置中j2:cat对应的值，例如dbPageManageror pageSerializer。然后将这部分作为正则表达式与当前的Key(从配置文件中读出)进行匹配。只有匹配上的Bean，才会被Spring容器创建出来。
JetspeedBeanDefinitionFilter
===============================================================================
public boolean match(BeanDefinition bd)
{
String beanCategoriesExpression = (String)bd.getAttribute(CATEGORY_META_KEY);
boolean matched = true;
if (beanCategoriesExpression != null)
{
matched = ((matcher != null)&& matcher.match(beanCategoriesExpression));
}
return matched;
}
public void registerDynamicAlias(BeanDefinitionRegistry registry, String beanName,BeanDefinition bd)
{
String aliases =(String)bd.getAttribute(ALIAS_META_KEY);
if (aliases != null)
{
StringTokenizer st = newStringTokenizer(aliases, " ,");
while (st.hasMoreTokens())
{
String alias = st.nextToken();
if (!alias.equals(beanName))
{
registry.registerAlias(beanName, alias);
}
}
}
}
===============================================================================
match()方法中的CATEGORY_META_KEY的值就是j2:cat，matcher类中保存的就是当前的Key，并负责将当前Key与每个Bean的进行正则表达式匹配。
registerDynamicAlias的作用是：在Bean匹配成功后，定制的Spring容器会调用此方法为Bean注册别名。详见下面1.3中的源码。
1.3定制Spring容器
定制一个Spring容器，重写registerBeanDefinition()方法，在Spring注册Bean时进行拦截。
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public class FilteringXmlWebApplicationContextextends XmlWebApplicationContext
{
private JetspeedBeanDefinitionFilterfilter;
publicFilteringXmlWebApplicationContext(JetspeedBeanDefinitionFilter filter, String[]configLocations, Properties initProperties, ServletContext servletContext)
{
this(filter, configLocations,initProperties, servletContext, null);
}
publicFilteringXmlWebApplicationContext(JetspeedBeanDefinitionFilter filter, String[]configLocations, Properties initProperties, ServletContext servletContext,ApplicationContext parent)
{
super();
if (parent != null)
{
this.setParent(parent);
}
if (initProperties != null)
{
PropertyPlaceholderConfigurer ppc =new PropertyPlaceholderConfigurer();
ppc.setIgnoreUnresolvablePlaceholders(true);
ppc.setSystemPropertiesMode(PropertyPlaceholderConfigurer.SYSTEM_PROPERTIES_MODE_FALLBACK);
ppc.setProperties(initProperties);
addBeanFactoryPostProcessor(ppc);
}
setConfigLocations(configLocations);
setServletContext(servletContext);
this.filter = filter;
}
protected DefaultListableBeanFactorycreateBeanFactory()
{
return new FilteringListableBeanFactory(filter,getInternalParentBeanFactory());
}
}
public classFilteringListableBeanFactory extends DefaultListableBeanFactory
{
private JetspeedBeanDefinitionFilterfilter;
public FilteringListableBeanFactory(JetspeedBeanDefinitionFilterfilter, BeanFactory parentBeanFactory)
{
super(parentBeanFactory);
this.filter = filter;
if (this.filter == null)
{
this.filter = newJetspeedBeanDefinitionFilter();
}
this.filter.init();
}
/**
* Override of the registerBeanDefinitionmethod to optionally filter out a BeanDefinition and
* if requested dynamically register anbean alias
*/
public void registerBeanDefinition(StringbeanName, BeanDefinition bd)
throws BeanDefinitionStoreException
{
if (filter.match(bd))
{
super.registerBeanDefinition(beanName, bd);
if (filter != null)
{
filter.registerDynamicAlias(this, beanName, bd);
}
}
}
}
1.4为Bean起别名
使用BeanReferenceFactoryBean工厂Bean，将上面配置的两个Bean(xmlPageManager和dbPageManager)包装起来。将Key配成各自的，实现通过配置当前Key来切换两种实现。别名都配成一个，这样引用他们的Bean就直接引用这个别名就行了。例如下面的PageLayoutComponent。
page-manager.xml
===============================================================================
<bean class="org.springframework.beans.factory.config.BeanReferenceFactoryBean">
<meta key="j2:cat"value="xmlPageManager" />
<meta key="j2:alias"value="org.apache.jetspeed.page.PageManager" />
<propertyname="targetBeanName" value="xmlPageManager" />
</bean>
<bean class="org.springframework.beans.factory.config.BeanReferenceFactoryBean">
<meta key="j2:cat"value="dbPageManager" />
<meta key="j2:alias"value="org.apache.jetspeed.page.PageManager" />
<propertyname="targetBeanName" value="dbPageManager" />
</bean>
<bean id="org.apache.jetspeed.layout.PageLayoutComponent"
class="org.apache.jetspeed.layout.impl.PageLayoutComponentImpl">
<meta key="j2:cat"value="default" />
<constructor-arg index="0">
<refbean="org.apache.jetspeed.page.PageManager" />
</constructor-arg>
<constructor-arg index="1">
<value>jetspeed-layouts::VelocityOneColumn</value>
</constructor-arg>
</bean>
方法二：使用注解区分Bean
（未完 待续）
