Spring动态数据源路由实现

简单的翻译, 也算是一篇笔记. 
原文:http://blog.springsource.com/2007/01/23/dynamic-datasource-routing/ 
在Spring 2.0.1中引入了AbstractRoutingDataSource, 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, 同时对于不支持事务隔离级别的JTA事务来说, Spring还提供了另外一个类IsolationLevelDataSourceRouter来处理这个问题. 下面的例子将通过context来切换不同的数据源. 
首先定义一个Catalog的Dao: 

 package blog.datasource;

import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.List;

import org.springframework.jdbc.core.simple.ParameterizedRowMapper;
import org.springframework.jdbc.core.simple.SimpleJdbcDaoSupport;

public class Catalog extends SimpleJdbcDaoSupport {

   public List<Item> getItems() {
      String query = "select name, price from item";
      return getSimpleJdbcTemplate().query(query, new ParameterizedRowMapper<Item>() {
            public Item mapRow(ResultSet rs, int row) throws SQLException {
               String name = rs.getString(1);
               double price = rs.getDouble(2);
               return new Item(name, price);
            }
      });
   }
}

然后定义一个Item的JavaBean 

package blog.datasource;

public class Item {

   private String name;
   private double price;

   public Item(String name, double price) {
      this.name = name;
      this.price = price;
   }

   public String getName() {
      return name;
   }

   public double getPrice() {
      return price;
   }

   public String toString() {
      return name + " (" + price + ")";
   }

}

接着定义一个枚举类型, 用来表示不同的用户级别, 通过该类型将映射到不同的数据源 

public enum CustomerType {
   BRONZE,
   SILVER,
   GOLD
}

下面是DataSource定义: 

<bean id="parentDataSource"
         class="org.springframework.jdbc.datasource.DriverManagerDataSource"
         abstract="true">
   <property name="driverClassName" value="org.hsqldb.jdbcDriver"/>
   <property name="username" value="sa"/>
</bean>

<bean id="goldDataSource" parent="parentDataSource">
   <property name="url" value="jdbc:hsqldb:hsql://localhost:${db.port.gold}/blog"/>
</bean>

<bean id="silverDataSource" parent="parentDataSource">
   <property name="url" value="jdbc:hsqldb:hsql://localhost:${db.port.silver}/blog"/>
</bean>

<bean id="bronzeDataSource" parent="parentDataSource">
   <property name="url" value="jdbc:hsqldb:hsql://localhost:${db.port.bronze}/blog"/>
</bean>

<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
   <property name="location" value="classpath:/blog/datasource/db.properties"/>
</bean>

AbstractRoutingDataSource 实现类 

package blog.datasource;

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

public class CustomerRoutingDataSource extends AbstractRoutingDataSource {

   @Override
   protected Object determineCurrentLookupKey() {
      return CustomerContextHolder.getCustomerType();
   }
}

CustomerContextHolder 是一个和LocalThread绑定的类, 定义如下: 

public class CustomerContextHolder {

   private static final ThreadLocal<CustomerType> contextHolder =
            new ThreadLocal<CustomerType>();

   public static void setCustomerType(CustomerType customerType) {
      Assert.notNull(customerType, "customerType cannot be null");
      contextHolder.set(customerType);
   }

   public static CustomerType getCustomerType() {
      return (CustomerType) contextHolder.get();
   }

   public static void clearCustomerType() {
      contextHolder.remove();
   }
}

将dao bean和datasource bean结合起来, 至于dao和真正的datasource如何关联这个可以根据需要指定相关的策略和规则来实现: 

<bean id="catalog" class="blog.datasource.Catalog">
   <property name="dataSource" ref="dataSource"/>
</bean>

<bean id="dataSource" class="blog.datasource.CustomerRoutingDataSource">
   <property name="targetDataSources">
      <map key-type="blog.datasource.CustomerType">
         <entry key="GOLD" value-ref="goldDataSource"/>
         <entry key="SILVER" value-ref="silverDataSource"/>
      </map>
   </property>
   <property name="defaultTargetDataSource" ref="bronzeDataSource"/>
</bean>

下面通过一个TestCase来看看如何使用: 

public class CatalogTests extends AbstractDependencyInjectionSpringContextTests {

   private Catalog catalog;

   public void setCatalog(Catalog catalog) {
      this.catalog = catalog;
   }

   public void testDataSourceRouting() {
      CustomerContextHolder.setCustomerType(CustomerType.GOLD);
      List<Item> goldItems = catalog.getItems();
      assertEquals(3, goldItems.size());
      System.out.println("gold items: " + goldItems);

      CustomerContextHolder.setCustomerType(CustomerType.SILVER);
      List<Item> silverItems = catalog.getItems();
      assertEquals(2, silverItems.size());
      System.out.println("silver items: " + silverItems);

      CustomerContextHolder.clearCustomerType();
      List<Item> bronzeItems = catalog.getItems();
      assertEquals(1, bronzeItems.size());
      System.out.println("bronze items: " + bronzeItems);
   }

   protected String[] getConfigLocations() {
      return new String[] {"/blog/datasource/beans.xml"};
   }
}