Spring实现动态数据源切换

一、前言

    近期一项目A需实现数据同步到另一项目B数据库中，在不改变B项目的情况下，只好选择项目A中切换数据源，直接把数据写入项目B的数据库中。这种需求，在数据同步与定时任务中经常需要。

    那么问题来了，该如何解决多数据源问题呢？不光是要配置多个数据源，还得能灵活动态的切换数据源。以spring+hibernate框架项目为例（引用：http://blog.csdn.net/wangpeng047/article/details/8866239博客的图片）：

    

    单个数据源绑定给sessionFactory，再在Dao层操作，若多个数据源的话，那不是就成了下图：

    

    可见，sessionFactory都写死在了Dao层，若我再添加个数据源的话，则又得添加一个sessionFactory。所以比较好的做法应该是下图：

    接下来就为大家讲解下如何用spring来整合这些数据源，同样以spring+hibernate配置为例。

二、实现原理

    1、扩展Spring的AbstractRoutingDataSource抽象类（该类充当了DataSource的路由中介, 能有在运行时, 根据某种key值来动态切换到真正的DataSource上。）

    从AbstractRoutingDataSource的源码中：

public  abstract  class  AbstractRoutingDataSource  extends  AbstractDataSource  implements  InitializingBean

    我们可以看到，它继承了AbstractDataSource，而AbstractDataSource不就是javax.sql.DataSource的子类，So我们可以分析下它的getConnection方法：

public  Connection getConnection()  throws  SQLException {
    return  determineTargetDataSource().getConnection();
}

public  Connection getConnection(String username, String password)  throws  SQLException {
     return  determineTargetDataSource().getConnection(username, password);
}

    获取连接的方法中，重点是determineTargetDataSource()方法，看源码：

/**
     * Retrieve the current target DataSource. Determines the
     * {@link #determineCurrentLookupKey() current lookup key}, performs
     * a lookup in the {@link #setTargetDataSources targetDataSources} map,
     * falls back to the specified
     * {@link #setDefaultTargetDataSource default target DataSource} if necessary.
     * @see #determineCurrentLookupKey()
     */
    protected  DataSource determineTargetDataSource() {
        Assert.notNull( this .resolvedDataSources,  "DataSource router not initialized" );
        Object lookupKey = determineCurrentLookupKey();
        DataSource dataSource =  this .resolvedDataSources.get(lookupKey);
        if  (dataSource ==  null  && ( this .lenientFallback || lookupKey ==  null )) {
            dataSource =  this .resolvedDefaultDataSource;
        }
        if  (dataSource ==  null ) {
            throw  new  IllegalStateException( "Cannot determine target DataSource for lookup key ["  + lookupKey +  "]" );
        }
        return  dataSource;
    }

    上面这段源码的重点在于determineCurrentLookupKey()方法，这是AbstractRoutingDataSource类中的一个抽象方法，而它的返回值是你所要用的数据源dataSource的key值，有了这个key值，resolvedDataSource（这是个map,由配置文件中设置好后存入的）就从中取出对应的DataSource，如果找不到，就用配置默认的数据源。

    看完源码，应该有点启发了吧，没错！你要扩展AbstractRoutingDataSource类，并重写其中的determineCurrentLookupKey()方法，来实现数据源的切换：

package  com.datasource.test.util.database;

import  org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource;

/**
 * 获取数据源（依赖于spring）
 * @author linhy
 */
public  class  DynamicDataSource  extends  AbstractRoutingDataSource{
    @Override
    protected  Object determineCurrentLookupKey() {
        return  DataSourceHolder.getDataSource();
    }
}

    DataSourceHolder这个类则是我们自己封装的对数据源进行操作的类：

package  com.datasource.test.util.database;

/**
 * 数据源操作
 * @author linhy
 */
public  class  DataSourceHolder {
    //线程本地环境
    private  static  final  ThreadLocal<String> dataSources =  new  ThreadLocal<String>();
    //设置数据源
    public  static  void  setDataSource(String customerType) {
        dataSources.set(customerType);
    }
    //获取数据源
    public  static  String getDataSource() {
        return  (String) dataSources.get();
    }
    //清除数据源
    public  static  void  clearDataSource() {
        dataSources.remove();
    }

}

    2、有人就要问，那你setDataSource这方法是要在什么时候执行呢？当然是在你需要切换数据源的时候执行啦。手动在代码中调用写死吗？这是多蠢的方法，当然要让它动态咯。所以我们可以应用spring aop来设置，把配置的数据源类型都设置成为注解标签，在service层中需要切换数据源的方法上，写上注解标签，调用相应方法切换数据源咯（就跟你设置事务一样）：

@DataSource (name=DataSource.slave1)
public  List getProducts(){

    当然，注解标签的用法可能很少人用到，但它可是个好东西哦，大大的帮助了我们开发：

package  com.datasource.test.util.database;

import  java.lang.annotation.*;

@Target ({ElementType.METHOD, ElementType.TYPE})
@Retention (RetentionPolicy.RUNTIME)
@Documented
public  @interface  DataSource {
    String name()  default  DataSource.master;

    public  static  String master =  "dataSource1" ;

    public  static  String slave1 =  "dataSource2" ;

    public  static  String slave2 =  "dataSource3" ;

}

三、配置文件

    为了精简篇幅，省略了无关本内容主题的配置。

    项目中单独分离出application-database.xml，关于数据源配置的文件。

<? xml  version = "1.0"  encoding = "UTF-8" ?>
<!-- Spring 数据库相关配置 放在这里 -->
< beans  xmlns = "http://www.springframework.org/schema/beans"
       xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop = "http://www.springframework.org/schema/aop"
       xmlns:tx = "http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd
       http://www.springframework.org/schema/aop
        http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
        http://www.springframework.org/schema/tx
        http://www.springframework.org/schema/tx/spring-tx-3.0.xsd">

    < bean  id = "dataSource1" class = "com.mysql.jdbc.jdbc2.optional.MysqlDataSource">
        < property  name = "url"  value = "${db1.url}" />
        < property  name = "user" value = "${db1.user}"/>
        < property  name = "password" value = "${db1.pwd}"/>
        < property  name = "autoReconnect"  value = "true" />
        < property  name = "useUnicode"   value = "true" />
        < property  name = "characterEncoding"  value = "UTF-8" />
    </ bean >

    < bean  id = "dataSource2" class = "com.mysql.jdbc.jdbc2.optional.MysqlDataSource">
        < property  name = "url"  value = "${db2.url}" />
        < property  name = "user" value = "${db2.user}"/>
        < property  name = "password" value = "${db2.pwd}"/>
        < property  name = "autoReconnect"  value = "true" />
        < property  name = "useUnicode"   value = "true" />
        < property  name = "characterEncoding"  value = "UTF-8" />
    </ bean >

    < bean  id = "dataSource3" class = "com.mysql.jdbc.jdbc2.optional.MysqlDataSource">
        < property  name = "url"  value = "${db3.url}" />
        < property  name = "user" value = "${db3.user}"/>
        < property  name = "password" value = "${db3.pwd}"/>
        < property  name = "autoReconnect"  value = "true" />
        < property  name = "useUnicode"   value = "true" />
        < property  name = "characterEncoding"  value = "UTF-8" />
    </ bean >
    <!-- 配置多数据源映射关系 -->
    < bean  id = "dataSource"  class = "com.datasource.test.util.database.DynamicDataSource" >
        < property  name = "targetDataSources" >
            < map  key-type = "java.lang.String" >
        < entry  key = "dataSource1"  value-ref = "dataSource1" ></ entry >
                < entry  key = "dataSource2"  value-ref = "dataSource2" ></ entry >
                < entry  key = "dataSource3"  value-ref = "dataSource3" ></ entry >
            </ map >
        </ property >
    <!-- 默认目标数据源为你主库数据源 -->
        < property  name = "defaultTargetDataSource"  ref = "dataSource1" />
    </ bean >

    < bean  id = "sessionFactoryHibernate"  class = "org.springframework.orm.hibernate3.LocalSessionFactoryBean" >
        < property  name = "dataSource"  ref = "dataSource" />
        < property  name = "hibernateProperties" >
            < props >
                < prop  key = "hibernate.dialect" >com.datasource.test.util.database.ExtendedMySQLDialect</ prop >
                < prop  key = "hibernate.show_sql" >${SHOWSQL}</ prop >
                < prop  key = "hibernate.format_sql" >${SHOWSQL}</ prop >
                < prop  key = "query.factory_class" >org.hibernate.hql.classic.ClassicQueryTranslatorFactory</ prop >
                < prop  key = "hibernate.connection.provider_class" >org.hibernate.connection.C3P0ConnectionProvider</ prop >
                < prop  key = "hibernate.c3p0.max_size" >30</ prop >
                < prop  key = "hibernate.c3p0.min_size" >5</ prop >
                < prop  key = "hibernate.c3p0.timeout" >120</ prop >
                < prop  key = "hibernate.c3p0.idle_test_period" >120</ prop >
                < prop  key = "hibernate.c3p0.acquire_increment" >2</ prop >
                < prop  key = "hibernate.c3p0.validate" >true</ prop >
                < prop  key = "hibernate.c3p0.max_statements" >100</ prop >
            </ props >
        </ property >
    </ bean >

    < bean  id = "hibernateTemplate"  class = "org.springframework.orm.hibernate3.HibernateTemplate" >
        < property  name = "sessionFactory"  ref = "sessionFactoryHibernate" />
    </ bean >

    < bean  id = "dataSourceExchange"  class = "com.datasource.test.util.database.DataSourceExchange" />

    < bean  id = "transactionManager"  class = "org.springframework.orm.hibernate3.HibernateTransactionManager" >
        < property  name = "sessionFactory"  ref = "sessionFactoryHibernate" />
    </ bean >

    < tx:advice  id = "txAdvice"  transaction-manager = "transactionManager" >
        < tx:attributes >
            < tx:method  name = "insert*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "add*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "update*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "modify*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "edit*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "del*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "save*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "send*"  propagation = "NESTED"  rollback-for = "Exception" />
            < tx:method  name = "get*"  read-only = "true" />
            < tx:method  name = "find*"  read-only = "true" />
            < tx:method  name = "query*"  read-only = "true" />
            < tx:method  name = "search*"  read-only = "true" />
            < tx:method  name = "select*"  read-only = "true" />
            < tx:method  name = "count*"  read-only = "true" />
        </ tx:attributes >
    </ tx:advice >

    < aop:config >
        < aop:pointcut  id = "service"  expression = "execution(* com.datasource..*.service.*.*(..))" />
        <!-- 关键配置，切换数据源一定要比持久层代码更先执行（事务也算持久层代码） -->
        < aop:advisor  advice-ref = "txAdvice"  pointcut-ref = "service"  order = "2" />
        < aop:advisor  advice-ref = "dataSourceExchange"  pointcut-ref = "service"  order = "1" />
    </ aop:config >

</ beans >

四、疑问

    多数据源切换是成功了，但牵涉到事务呢？单数据源事务是ok的，但如果多数据源需要同时使用一个事务呢？这个问题有点头大，网络上有人提出用atomikos开源项目实现JTA分布式事务处理。你怎么看？

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