Spring BeanPostProcessor与动态加载数据源配置

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：

db.driverClass=****
db.jdbcUrl=****
db.user=****
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);