Spring中BeanPostProcessor

Spring中BeanPostProcessor

前言：

本文旨在介绍Spring动态配置数据源的方式，即对一个DataSource的配置诸如jdbcUrl,user,password,driverClass都通过运行时指定，而非由xml静态配置定死。

Spring构造Context的参数一般只包含配置文件路径和类加载器，如果需要达到动态传入配置参数的目的，需要Spring在初始化数据源相关bean的时候能够对原有配置执行修改或替换，为方便处理，本文将定义一个名为DynamicDataSourceConfigHolder的公共类提供配置数据存储。

本文替换数据源为c3p0配置。

BeanPostProcessor简介：

Spring BeanPostProcesssor通常被称为Spring Bean回调处理器，它一般用于在实例化一个bean的前后增加一些附加操作，它会对全局的Spring bean配置生效。

Spring Bean的生命周期处理：

Spring Bean生命周期通常对应两种处理方式，一种是init-method &destroy-method, 另一种是InitializingBean的afterPropertiesSet()方法和DisposeBean的destroy()方法，BeanPostProcessor的出现使得批处理Spring bean定义有了可能。

BeanPostProcessor定义：

 /**
  * Factory hook that allows for custom modification of new bean instances,
  * e.g. checking for marker interfaces or wrapping them with proxies.
  *
  * <p>ApplicationContexts can autodetect BeanPostProcessor beans in their
  * bean definitions and apply them to any beans subsequently created.
  * Plain bean factories allow for programmatic registration of post-processors,
  * applying to all beans created through this factory.
  *
  * <p>Typically, post-processors that populate beans via marker interfaces
  * or the like will implement {@link #postProcessBeforeInitialization},
  * while post-processors that wrap beans with proxies will normally
  * implement {@link #postProcessAfterInitialization}.
  *
  * @author Juergen Hoeller
  * @since 10.10.2003
  * @see InstantiationAwareBeanPostProcessor
  * @see DestructionAwareBeanPostProcessor
  * @see ConfigurableBeanFactory#addBeanPostProcessor
  * @see BeanFactoryPostProcessor
  */
 public interface BeanPostProcessor {

     /**
      * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
      * initialization callbacks (like InitializingBean's <code>afterPropertiesSet</code>
      * or a custom init-method). The bean will already be populated with property values.
      * The returned bean instance may be a wrapper around the original.
      * @param bean the new bean instance
      * @param beanName the name of the bean
      * @return the bean instance to use, either the original or a wrapped one; if
      * <code>null</code>, no subsequent BeanPostProcessors will be invoked
      * @throws org.springframework.beans.BeansException in case of errors
      * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
      */
     Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;

     /**
      * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
      * initialization callbacks (like InitializingBean's <code>afterPropertiesSet</code>
      * or a custom init-method). The bean will already be populated with property values.
      * The returned bean instance may be a wrapper around the original.
      * <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean
      * instance and the objects created by the FactoryBean (as of Spring 2.0). The
      * post-processor can decide whether to apply to either the FactoryBean or created
      * objects or both through corresponding <code>bean instanceof FactoryBean</code> checks.
      * <p>This callback will also be invoked after a short-circuiting triggered by a
      * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
      * in contrast to all other BeanPostProcessor callbacks.
      * @param bean the new bean instance
      * @param beanName the name of the bean
      * @return the bean instance to use, either the original or a wrapped one; if
      * <code>null</code>, no subsequent BeanPostProcessors will be invoked
      * @throws org.springframework.beans.BeansException in case of errors
      * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
      * @see org.springframework.beans.factory.FactoryBean
      */
     Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

 }

以上为Spring源代码，我们重点关注它和Spring bean初始化的关系，即postProcessBeforeInitialization将会在Spring 执行bean初始化钩子(init-method或者afterPropertiesSet)之前被调用。

DynamicDataSourceConfigHolder：

 package org.wit.ff;

 import java.util.Map;

 /**
  * 动态数据源配置存储.
  * @author ff
  *
  */
 public class DynamicDataSourceConfigHolder {

     /**
      * 定义本地变量，加入存在多个Spring Context连接多个不同的数据源时,可以共用此类。
      */
     private static final ThreadLocal<Map<String,String>> dynamicDataSourceConfigHolder = new ThreadLocal<Map<String,String>>();

     public static void setDynamicConfig(Map<String,String> dynamicDataSourceConfig) {
         dynamicDataSourceConfigHolder.set(dynamicDataSourceConfig);
     }

     public static Map<String,String> getDynamicDataSourceConfig() {
         return (dynamicDataSourceConfigHolder.get());
     }

     public static void clear() {
         dynamicDataSourceConfigHolder.remove();
     }

 }

数据源配置文件：

1 db.driverClass=****
2 db.jdbcUrl=****
3 db.user=****
4 db.password=****

自定义bean回调处理器：

 package org.wit.ff;

 import java.lang.reflect.Method;
 import java.util.HashMap;
 import java.util.Map;

 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.config.BeanPostProcessor;
 import org.springframework.util.ReflectionUtils;

 import com.mchange.v2.c3p0.ComboPooledDataSource;

 /**
  * Bean回调处理器.
  * @author ff
  *
  */
 public class ComboPooledDataSourceBeanPostProcessor implements BeanPostProcessor {

     private String dataSourceName;

     @Override
     public Object postProcessAfterInitialization(Object bean, String paramString) throws BeansException {
         return bean;
     }

     @Override
     public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
         // 限制数据源名称和类型.
         if (bean instanceof ComboPooledDataSource && dataSourceName.equals(beanName)) {
             final Map<String,String> methodMatchField = new HashMap<String,String>();
             methodMatchField.put("setDriverClass", "db.driverClass");
             methodMatchField.put("setJdbcUrl", "db.jdbcUrl");
             methodMatchField.put("setUser", "db.user");
             methodMatchField.put("setPassword", "db.password");
             // 从公共存储区中加载.
             final Map<String, String> config = DynamicDataSourceConfigHolder.getDynamicDataSourceConfig();
             ReflectionUtils.doWithMethods(bean.getClass(), new ReflectionUtils.MethodCallback() {
                 @Override
                 public void doWith(Method paramMethod) throws IllegalArgumentException, IllegalAccessException {
                     if(methodMatchField.containsKey(paramMethod.getName())){
                         ReflectionUtils.invokeMethod(paramMethod, bean, config.get(methodMatchField.get(paramMethod.getName())));
                     }
                 }
             });
         }
         return bean;
     }

     public void setDataSourceName(String dataSourceName) {
         this.dataSourceName = dataSourceName;
     }

 }

Spring 配置文件dynamicDatasource/applicationContext.xml：

 <!-- 加载properties配置文件 -->
     <bean id="propertyConfigurer"
         class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
         <property name="locations">
             <list>
                 <!-- 这里支持多种寻址方式：classpath和file -->
                 <value>classpath:dynamicDatasource/dbconfig.properties</value>
             </list>
         </property>
     </bean>
     <!-- 回调处理器.-->
     <bean id="dynamicDataSourceBp" class="org.wit.ff.ComboPooledDataSourceBeanPostProcessor" >
         <property name="dataSourceName" value="dataSource" />
     </bean>

     <!-- 数据库连接池 -->
     <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource"
         destroy-method="close">
         <property name="driverClass" value="${db.driverClass}" />
         <property name="jdbcUrl" value="${db.jdbcUrl}" />
         <property name="user" value="${db.user}" />
         <property name="password" value="${db.password}" />
     </bean>

测试示例：

 Map<String,String> dynamicDataSourceConfig = new HashMap<String,String>();
 dynamicDataSourceConfig.put("db.driverClass", "com.mysql.jdbc.Driver");
 dynamicDataSourceConfig.put("db.jdbcUrl", "jdbc:mysql://127.0.0.1:3306/menlo3?autoReconnect=true&amp;characterEncoding=utf-8");
 dynamicDataSourceConfig.put("db.user", "root");
 dynamicDataSourceConfig.put("db.password", "root");
 DynamicDataSourceConfigHolder.setDynamicConfig(dynamicDataSourceConfig);
 ApplicationContext applicationContext = new ClassPathXmlApplicationContext(new String[]{"classpath:dynamicDatasource/applicationContext.xml"});

 //执行一段操作数据库的逻辑验证即可.

 assertNotNull(applicationContext);

Spring提供了很多扩展接口，BeanPostProcessor接口和InstantiationAwareBeanPostProcessor接口就是其中两个。

BeanPostProcessor

BeanPostProcessor接口作用是：如果我们需要在Spring容器完成Bean的实例化、配置和其他的初始化前后添加一些自己的逻辑处理，我们就可以定义一个或者多个BeanPostProcessor接口的实现，然后注册到容器中。

Spring中Bean的实例化过程图示：

由上图可以看到，Spring中的BeanPostProcessor在实例化过程处于的位置，BeanPostProcessor接口有两个方法需要实现：postProcessBeforeInitialization和postProcessAfterInitialization，

 import org.springframework.beans.factory.config.BeanPostProcessor;  

 public class MyBeanPostProcessor implements BeanPostProcessor {  

      public MyBeanPostProcessor() {
         super();
         System.out.println("这是BeanPostProcessor实现类构造器！！");
      }  

      @Override
      public Object postProcessAfterInitialization(Object bean, String arg1)
              throws BeansException {
          System.out.println("bean处理器：bean创建之后..");
          return bean;
      }  

      @Override
      public Object postProcessBeforeInitialization(Object bean, String arg1)
              throws BeansException {
          System.out.println("bean处理器：bean创建之前..");  

          return bean;
      }
  }

由方法名字也可以看出，前者在实例化及依赖注入完成后、在任何初始化代码（比如配置文件中的init-method）调用之前调用；后者在初始化代码调用之后调用。

注意：

1、接口中的两个方法都要将传入的bean返回，而不能返回null，如果返回的是null那么我们通过getBean方法将得不到目标。

2、BeanFactory和ApplicationContext对待bean后置处理器稍有不同。ApplicationContext会自动检测在配置文件中实现了BeanPostProcessor接口的所有bean，并把它们注册为后置处理器，然后在容器创建bean的适当时候调用它，因此部署一个后置处理器同部署其他的bean并没有什么区别。而使用BeanFactory实现的时候，bean 后置处理器必须通过代码显式地去注册，在IoC容器继承体系中的ConfigurableBeanFactory接口中定义了注册方法

 /**
  * Add a new BeanPostProcessor that will get applied to beans created
  * by this factory. To be invoked during factory configuration.
  * <p>Note: Post-processors submitted here will be applied in the order of
  * registration; any ordering semantics expressed through implementing the
  * {@link org.springframework.core.Ordered} interface will be ignored. Note
  * that autodetected post-processors (e.g. as beans in an ApplicationContext)
  * will always be applied after programmatically registered ones.
  * @param beanPostProcessor the post-processor to register
  */
 void addBeanPostProcessor(BeanPostProcessor beanPostProcessor); 

另外，不要将BeanPostProcessor标记为延迟初始化。因为如果这样做，Spring容器将不会注册它们，自定义逻辑也就无法得到应用。假如你在<beans />元素的定义中使用了'default-lazy-init'属性，请确信你的各个BeanPostProcessor标记为'lazy-init="false"'。