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：

 1 /**
 2  * Factory hook that allows for custom modification of new bean instances,
 3  * e.g. checking for marker interfaces or wrapping them with proxies.
 4  *
 5  * <p>ApplicationContexts can autodetect BeanPostProcessor beans in their
 6  * bean definitions and apply them to any beans subsequently created.
 7  * Plain bean factories allow for programmatic registration of post-processors,
 8  * applying to all beans created through this factory.
 9  *
10  * <p>Typically, post-processors that populate beans via marker interfaces
11  * or the like will implement {@link #postProcessBeforeInitialization},
12  * while post-processors that wrap beans with proxies will normally
13  * implement {@link #postProcessAfterInitialization}.
14  *
15  * @author Juergen Hoeller
16  * @since 10.10.2003
17  * @see InstantiationAwareBeanPostProcessor
18  * @see DestructionAwareBeanPostProcessor
19  * @see ConfigurableBeanFactory#addBeanPostProcessor
20  * @see BeanFactoryPostProcessor
21  */
22 public interface BeanPostProcessor {
23
24     /**
25      * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
26      * initialization callbacks (like InitializingBean‘s <code>afterPropertiesSet</code>
27      * or a custom init-method). The bean will already be populated with property values.
28      * The returned bean instance may be a wrapper around the original.
29      * @param bean the new bean instance
30      * @param beanName the name of the bean
31      * @return the bean instance to use, either the original or a wrapped one; if
32      * <code>null</code>, no subsequent BeanPostProcessors will be invoked
33      * @throws org.springframework.beans.BeansException in case of errors
34      * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
35      */
36     Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
37
38     /**
39      * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
40      * initialization callbacks (like InitializingBean‘s <code>afterPropertiesSet</code>
41      * or a custom init-method). The bean will already be populated with property values.
42      * The returned bean instance may be a wrapper around the original.
43      * <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean
44      * instance and the objects created by the FactoryBean (as of Spring 2.0). The
45      * post-processor can decide whether to apply to either the FactoryBean or created
46      * objects or both through corresponding <code>bean instanceof FactoryBean</code> checks.
47      * <p>This callback will also be invoked after a short-circuiting triggered by a
48      * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
49      * in contrast to all other BeanPostProcessor callbacks.
50      * @param bean the new bean instance
51      * @param beanName the name of the bean
52      * @return the bean instance to use, either the original or a wrapped one; if
53      * <code>null</code>, no subsequent BeanPostProcessors will be invoked
54      * @throws org.springframework.beans.BeansException in case of errors
55      * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
56      * @see org.springframework.beans.factory.FactoryBean
57      */
58     Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
59
60 }

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：

 1 package org.wit.ff;
 2
 3 import java.util.Map;
 4
 5 /**
 6  * 动态数据源配置存储.
 7  * @author ff
 8  *
 9  */
10 public class DynamicDataSourceConfigHolder {
11
12     /**
13      * 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。
14      */
15     private static final ThreadLocal<Map<String,String>> dynamicDataSourceConfigHolder = new ThreadLocal<Map<String,String>>();
16
17     public static void setDynamicConfig(Map<String,String> dynamicDataSourceConfig) {
18         dynamicDataSourceConfigHolder.set(dynamicDataSourceConfig);
19     }
20
21     public static Map<String,String> getDynamicDataSourceConfig() {
22         return (dynamicDataSourceConfigHolder.get());
23     }
24
25     public static void clear() {
26         dynamicDataSourceConfigHolder.remove();
27     }
28
29 }

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：

 1 package org.wit.ff;
 2
 3 import java.lang.reflect.Method;
 4 import java.util.HashMap;
 5 import java.util.Map;
 6
 7 import org.springframework.beans.BeansException;
 8 import org.springframework.beans.factory.config.BeanPostProcessor;
 9 import org.springframework.util.ReflectionUtils;
10
11 import com.mchange.v2.c3p0.ComboPooledDataSource;
12
13 /**
14  * Bean回调处理器.
15  * @author ff
16  *
17  */
18 public class ComboPooledDataSourceBeanPostProcessor implements BeanPostProcessor {
19
20     private String dataSourceName;
21
22     @Override
23     public Object postProcessAfterInitialization(Object bean, String paramString) throws BeansException {
24         return bean;
25     }
26
27     @Override
28     public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
29         // 限制数据源名称和类型.
30         if (bean instanceof ComboPooledDataSource && dataSourceName.equals(beanName)) {
31             final Map<String,String> methodMatchField = new HashMap<String,String>();
32             methodMatchField.put("setDriverClass", "db.driverClass");
33             methodMatchField.put("setJdbcUrl", "db.jdbcUrl");
34             methodMatchField.put("setUser", "db.user");
35             methodMatchField.put("setPassword", "db.password");
36             // 从公共存储区中加载.
37             final Map<String, String> config = DynamicDataSourceConfigHolder.getDynamicDataSourceConfig();
38             ReflectionUtils.doWithMethods(bean.getClass(), new ReflectionUtils.MethodCallback() {
39                 @Override
40                 public void doWith(Method paramMethod) throws IllegalArgumentException, IllegalAccessException {
41                     if(methodMatchField.containsKey(paramMethod.getName())){
42                         ReflectionUtils.invokeMethod(paramMethod, bean, config.get(methodMatchField.get(paramMethod.getName())));
43                     }
44                 }
45             });
46         }
47         return bean;
48     }
49
50     public void setDataSourceName(String dataSourceName) {
51         this.dataSourceName = dataSourceName;
52     }
53
54 }

Spring 配置文件dynamicDatasource/applicationContext.xml：

 1 <!-- 加载properties配置文件 -->
 2     <bean id="propertyConfigurer"
 3         class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
 4         <property name="locations">
 5             <list>
 6                 <!-- 这里支持多种寻址方式：classpath和file -->
 7                 <value>classpath:dynamicDatasource/dbconfig.properties</value>
 8             </list>
 9         </property>
10     </bean>
11     <!-- 回调处理器.-->
12     <bean id="dynamicDataSourceBp" class="org.wit.ff.ComboPooledDataSourceBeanPostProcessor" >
13         <property name="dataSourceName" value="dataSource" />
14     </bean>
15
16     <!-- 数据库连接池 -->
17     <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource"
18         destroy-method="close">
19         <property name="driverClass" value="${db.driverClass}" />
20         <property name="jdbcUrl" value="${db.jdbcUrl}" />
21         <property name="user" value="${db.user}" />
22         <property name="password" value="${db.password}" />
23     </bean>

测试示例：

 1 Map<String,String> dynamicDataSourceConfig = new HashMap<String,String>();
 2 dynamicDataSourceConfig.put("db.driverClass", "com.mysql.jdbc.Driver");
 3 dynamicDataSourceConfig.put("db.jdbcUrl", "jdbc:mysql://127.0.0.1:3306/menlo3?autoReconnect=true&amp;characterEncoding=utf-8");
 4 dynamicDataSourceConfig.put("db.user", "root");
 5 dynamicDataSourceConfig.put("db.password", "root");
 6 DynamicDataSourceConfigHolder.setDynamicConfig(dynamicDataSourceConfig);
 7 ApplicationContext applicationContext = new ClassPathXmlApplicationContext(new String[]{"classpath:dynamicDatasource/applicationContext.xml"});
 8
 9 //执行一段操作数据库的逻辑验证即可.
10
11 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，

 1 import org.springframework.beans.factory.config.BeanPostProcessor;
 2
 3 public class MyBeanPostProcessor implements BeanPostProcessor {
 4
 5      public MyBeanPostProcessor() {
 6         super();
 7         System.out.println("这是BeanPostProcessor实现类构造器！！");
 8      }
 9
10      @Override
11      public Object postProcessAfterInitialization(Object bean, String arg1)
12              throws BeansException {
13          System.out.println("bean处理器：bean创建之后..");
14          return bean;
15      }
16
17      @Override
18      public Object postProcessBeforeInitialization(Object bean, String arg1)
19              throws BeansException {
20          System.out.println("bean处理器：bean创建之前..");
21
22          return bean;
23      }
24  }

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

 1 /**
 2  * Add a new BeanPostProcessor that will get applied to beans created
 3  * by this factory. To be invoked during factory configuration.
 4  * <p>Note: Post-processors submitted here will be applied in the order of
 5  * registration; any ordering semantics expressed through implementing the
 6  * {@link org.springframework.core.Ordered} interface will be ignored. Note
 7  * that autodetected post-processors (e.g. as beans in an ApplicationContext)
 8  * will always be applied after programmatically registered ones.
 9  * @param beanPostProcessor the post-processor to register
10  */
11 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"‘。