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。

===============================================================================

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）