在做项目的过程中，有时候一个数据源是不够，那么就需要配置多个数据源。本例介绍mybatis多数据源配置
前言
一般项目单数据源，使用流程如下：
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
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：
1 public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean
2、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：
1 public Connection getConnection() throws SQLException { 2 return determineTargetDataSource().getConnection(); 3 } 4 5 public Connection getConnection(String username, String password) throws SQLException { 6 return determineTargetDataSource().getConnection(username, password); 7 }
3、 获取连接的方法中，重点是determineTargetDataSource()方法，看源码：
1 /** 2 * Retrieve the current target DataSource. Determines the 3 * {@link #determineCurrentLookupKey() current lookup key}, performs 4 * a lookup in the {@link #setTargetDataSources targetDataSources} map, 5 * falls back to the specified 6 * {@link #setDefaultTargetDataSource default target DataSource} if necessary. 7 * @see #determineCurrentLookupKey() 8 */ 9 protected DataSource determineTargetDataSource() { 10 Assert.notNull(this.resolvedDataSources, "DataSource router not initialized"); 11 Object lookupKey = determineCurrentLookupKey(); 12 DataSource dataSource = this.resolvedDataSources.get(lookupKey); 13 if (dataSource == null && (this.lenientFallback || lookupKey == null)) { 14 dataSource = this.resolvedDefaultDataSource; 15 } 16 if (dataSource == null) { 17 throw new IllegalStateException("Cannot determine target DataSource for lookup key [" + lookupKey + "]"); 18 } 19 return dataSource; 20 }
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
案例
1、搭建一个Springmvc + Spring + Mybatis maven项目，POM文件中引入AOP相关依赖，参考：【Mybatis】MyBatis之整合Spring（八）
1 <project xmlns="http://maven.apache.org/POM/4.0.0" 2 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 3 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"> 4 <modelVersion>4.0.0</modelVersion> 5 <groupId>com.test</groupId> 6 <artifactId>test-spring-mybatis</artifactId> 7 <packaging>war</packaging> 8 <version>1.0.0-SNAPSHOT</version> 9 <url>http://maven.apache.org</url> 10 11 <!-- 定义maven变量 --> 12 <properties> 13 <!-- spring --> 14 <spring.version>5.1.4.RELEASE</spring.version> 15 16 <!-- Mybatis --> 17 <mybatis.version>3.5.0</mybatis.version> 18 <!-- Mybatis 整合 Spring --> 19 <mybatis-spring.version>2.0.0</mybatis-spring.version> 20 21 <!-- mysql --> 22 <mysql.version>8.0.13</mysql.version> 23 24 <!-- c3p0 连接池 --> 25 <c3p0.version>0.9.5.4</c3p0.version> 26 27 <!-- logback --> 28 <slf4j-api.version>1.7.5</slf4j-api.version> 29 <logback.version>0.9.30</logback.version> 30 31 <!-- Servlet --> 32 <servlet.version>3.0.1</servlet.version> 33 <jsp-api.version>2.2</jsp-api.version> 34 35 <!-- jstl --> 36 <jstl.version>1.2</jstl.version> 37 <standard.version>1.1.2</standard.version> 38 39 <!-- test junit --> 40 <junit.version>3.8.1</junit.version> 41 42 <!-- jdk --> 43 <jdk.version>1.8</jdk.version> 44 <maven.compiler.plugin.version>2.3.2</maven.compiler.plugin.version> 45 </properties> 46 47 48 <dependencies> 49 50 <!-- Spring IOC 核心容器 --> 51 <dependency> 52 <groupId>org.springframework</groupId> 53 <artifactId>spring-core</artifactId> 54 <version>${spring.version}</version> 55 </dependency> 56 57 <dependency> 58 <groupId>org.springframework</groupId> 59 <artifactId>spring-beans</artifactId> 60 <version>${spring.version}</version> 61 </dependency> 62 63 <dependency> 64 <groupId>org.springframework</groupId> 65 <artifactId>spring-context</artifactId> 66 <version>${spring.version}</version> 67 </dependency> 68 69 <dependency> 70 <groupId>org.springframework</groupId> 71 <artifactId>spring-expression</artifactId> 72 <version>${spring.version}</version> 73 </dependency> 74 75 <!-- Spring AOP 切面 模块 --> 76 <dependency> 77 <groupId>org.springframework</groupId> 78 <artifactId>spring-aop</artifactId> 79 <version>${spring.version}</version> 80 </dependency> 81 82 <dependency> 83 <groupId>org.aspectj</groupId> 84 <artifactId>aspectjrt</artifactId> 85 <version>1.9.2</version> 86 </dependency> 87 88 <dependency> 89 <groupId>org.aspectj</groupId> 90 <artifactId>aspectjweaver</artifactId> 91 <version>1.9.2</version> 92 </dependency> 93 94 <!-- Spring WEB MVC 模块 --> 95 <dependency> 96 <groupId>org.springframework</groupId> 97 <artifactId>spring-web</artifactId> 98 <version>${spring.version}</version> 99 </dependency> 100 101 <dependency> 102 <groupId>org.springframework</groupId> 103 <artifactId>spring-webmvc</artifactId> 104 <version>${spring.version}</version> 105 </dependency> 106 107 <!-- Spring 事物 模块 --> 108 <dependency> 109 <groupId>org.springframework</groupId> 110 <artifactId>spring-tx</artifactId> 111 <version>${spring.version}</version> 112 </dependency> 113 114 <!-- Spring ORM 对象关系映射 模块 --> 115 <dependency> 116 <groupId>org.springframework</groupId> 117 <artifactId>spring-orm</artifactId> 118 <version>${spring.version}</version> 119 </dependency> 120 121 <!-- Spring JDBC 模块 --> 122 <dependency> 123 <groupId>org.springframework</groupId> 124 <artifactId>spring-jdbc</artifactId> 125 <version>${spring.version}</version> 126 </dependency> 127 128 <!-- Mybatis --> 129 <dependency> 130 <groupId>org.mybatis</groupId> 131 <artifactId>mybatis</artifactId> 132 <version>${mybatis.version}</version> 133 </dependency> 134 135 <!-- Mybatis 整合 Spring --> 136 <dependency> 137 <groupId>org.mybatis</groupId> 138 <artifactId>mybatis-spring</artifactId> 139 <version>${mybatis-spring.version}</version> 140 </dependency> 141 142 <!-- mysql --> 143 <dependency> 144 <groupId>mysql</groupId> 145 <artifactId>mysql-connector-java</artifactId> 146 <version>${mysql.version}</version> 147 </dependency> 148 149 <!-- c3p0 连接池 --> 150 <!-- https://mvnrepository.com/artifact/c3p0/c3p0 --> 151 <dependency> 152 <groupId>com.mchange</groupId> 153 <artifactId>c3p0</artifactId> 154 <version>${c3p0.version}</version> 155 </dependency> 156 157 158 <!-- logback --> 159 <dependency> 160 <groupId>org.slf4j</groupId> 161 <artifactId>slf4j-api</artifactId> 162 <version>${slf4j-api.version}</version> 163 <type>jar</type> 164 <scope>compile</scope> 165 </dependency> 166 167 <dependency> 168 <groupId>ch.qos.logback</groupId> 169 <artifactId>logback-core</artifactId> 170 <version>${logback.version}</version> 171 <type>jar</type> 172 </dependency> 173 174 <dependency> 175 <groupId>ch.qos.logback</groupId> 176 <artifactId>logback-classic</artifactId> 177 <version>${logback.version}</version> 178 <type>jar</type> 179 </dependency> 180 181 <dependency> 182 <groupId>ch.qos.logback</groupId> 183 <artifactId>logback-access</artifactId> 184 <version>${logback.version}</version> 185 </dependency> 186 187 188 <!-- Servlet --> 189 <dependency> 190 <groupId>javax.servlet</groupId> 191 <artifactId>javax.servlet-api</artifactId> 192 <version>${servlet.version}</version> 193 <scope>provided</scope> 194 </dependency> 195 <dependency> 196 <groupId>javax.servlet.jsp</groupId> 197 <artifactId>jsp-api</artifactId> 198 <version>${jsp-api.version}</version> 199 <scope>provided</scope> 200 </dependency> 201 202 <!-- jstl --> 203 <dependency> 204 <groupId>javax.servlet</groupId> 205 <artifactId>jstl</artifactId> 206 <version>${jstl.version}</version> 207 </dependency> 208 209 <dependency> 210 <groupId>taglibs</groupId> 211 <artifactId>standard</artifactId> 212 <version>${standard.version}</version> 213 </dependency> 214 215 <!-- test --> 216 <dependency> 217 <groupId>junit</groupId> 218 <artifactId>junit</artifactId> 219 <version>${junit.version}</version> 220 <scope>test</scope> 221 </dependency> 222 223 </dependencies> 224 225 <build> 226 <plugins> 227 <!-- define the project compile level --> 228 <plugin> 229 <groupId>org.apache.maven.plugins</groupId> 230 <artifactId>maven-compiler-plugin</artifactId> 231 <version>${maven.compiler.plugin.version}</version> 232 <configuration> 233 <source>${jdk.version}</source> 234 <target>${jdk.version}</target> 235 </configuration> 236 </plugin> 237 </plugins> 238 <finalName>test_spring_mybatis</finalName> 239 </build> 240 </project>
2、编辑一个扩展AbstractRoutingDataSource类，DynamicDataSource.java
1 package com.test.datasource; 2 3 import org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource; 4 5 /** 6 * 动态数据源（依赖于spring） 7 * @author chenheng 8 * @date 2019-08-03 17:27:35 9 * 10 */ 11 public class DynamicDataSource extends AbstractRoutingDataSource { 12 13 @Override 14 protected Object determineCurrentLookupKey() { 15 return DataSourceHolder.getDataSource(); 16 } 17 18 }
3、 封装一个的对数据源进行操作的类，DataSourceHolder.java
1 package com.test.datasource; 2 3 public class DataSourceHolder { 4 5 // 线程本地环境 6 private static final ThreadLocal<String> dataSources = new ThreadLocal<String>(); 7 8 // 设置数据源 9 public static void setDataSource(String customerType) { 10 dataSources.set(customerType); 11 } 12 13 // 获取数据源 14 public static String getDataSource() { 15 return (String) dataSources.get(); 16 } 17 18 // 清除数据源 19 public static void clearDataSource() { 20 dataSources.remove(); 21 } 22 }
4、当需要切换数据源的时候执行啦。手动在代码中调用写死吗？调用setDataSource方法
但是这种方法比较死板，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）
1 @TargetDataSource(name=TargetDataSource.SLAVE) 2 public List<Employee> getEmpsFromSalve()
编辑注解标签TargetDataSource.java
1 package com.test.annotation; 2 3 import java.lang.annotation.*; 4 5 @Target({ElementType.METHOD, ElementType.TYPE}) 6 @Retention(RetentionPolicy.RUNTIME) 7 @Documented 8 public @interface TargetDataSource { 9 10 String name() default TargetDataSource.MASTER; 11 12 public static String MASTER = "dataSource1"; 13 14 public static String SLAVE = "dataSource2"; 15 16 }
5、编辑切面的Bean，DataSourceExchange.java
1 package com.test.datasource; 2 3 import java.lang.reflect.Method; 4 5 import org.springframework.aop.AfterReturningAdvice; 6 import org.springframework.aop.MethodBeforeAdvice; 7 8 import com.test.annotation.TargetDataSource; 9 10 public class DataSourceExchange implements MethodBeforeAdvice, AfterReturningAdvice { 11 12 @Override 13 public void afterReturning(Object returnValue, Method method, Object[] args, Object target) throws Throwable { 14 DataSourceHolder.clearDataSource(); 15 } 16 17 @Override 18 public void before(Method method, Object[] args, Object target) throws Throwable { 19 // 这里TargetDataSource是自定义的注解 20 if (method.isAnnotationPresent(TargetDataSource.class)) { 21 TargetDataSource datasource = method.getAnnotation(TargetDataSource.class); 22 DataSourceHolder.setDataSource(datasource.name()); 23 } else { 24 if(target.getClass().isAnnotationPresent(TargetDataSource.class)) 25 { 26 TargetDataSource datasource = target.getClass().getAnnotation(TargetDataSource.class); 27 DataSourceHolder.setDataSource(datasource.name()); 28 } 29 } 30 31 } 32 }
6、配置文件
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans xmlns="http://www.springframework.org/schema/beans" 3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 4 xmlns:aop="http://www.springframework.org/schema/aop" 5 xmlns:context="http://www.springframework.org/schema/context" 6 xmlns:mybatis-spring="http://mybatis.org/schema/mybatis-spring" 7 xmlns:tx="http://www.springframework.org/schema/tx" 8 xsi:schemaLocation="http://www.springframework.org/schema/beans 9 http://www.springframework.org/schema/beans/spring-beans.xsd 10 http://mybatis.org/schema/mybatis-spring 11 http://mybatis.org/schema/mybatis-spring.xsd 12 http://www.springframework.org/schema/aop 13 http://www.springframework.org/schema/aop/spring-aop.xsd 14 http://www.springframework.org/schema/tx 15 http://www.springframework.org/schema/tx/spring-tx-4.0.xsd 16 http://www.springframework.org/schema/context 17 http://www.springframework.org/schema/context/spring-context-4.0.xsd"> 18 19 <!-- 引入数据库的配置文件 --> 20 <context:property-placeholder location="classpath:dbconfig.properties" /> 21 22 23 <bean id="dataSource1" class="com.mchange.v2.c3p0.ComboPooledDataSource"> 24 <property name="jdbcUrl" value="${datasource1.jdbc.url}"></property> 25 <property name="driverClass" value="${datasource1.jdbc.driver}"></property> 26 <property name="user" value="${datasource1.jdbc.username}"></property> 27 <property name="password" value="${datasource1.jdbc.password}"></property> 28 </bean> 29 30 <bean id="dataSource2" class="com.mchange.v2.c3p0.ComboPooledDataSource"> 31 <property name="jdbcUrl" value="${datasource2.jdbc.url}"></property> 32 <property name="driverClass" value="${datasource2.jdbc.driver}"></property> 33 <property name="user" value="${datasource2.jdbc.username}"></property> 34 <property name="password" value="${datasource2.jdbc.password}"></property> 35 </bean> 36 37 38 39 <!-- 数据源：Spring用来控制业务逻辑。数据源、事务控制、aop --> 40 <bean id="dataSource" class="com.test.datasource.DynamicDataSource"> 41 <property name="targetDataSources"> 42 <map key-type="java.lang.String"> 43 <entry key="dataSource1" value-ref="dataSource1"></entry> 44 <entry key="dataSource2" value-ref="dataSource2"></entry> 45 </map> 46 </property> 47 <!-- 默认目标数据源为你主库数据源 --> 48 <property name="defaultTargetDataSource" ref="dataSource1"/> 49 </bean> 50 51 52 <!-- spring事务管理 --> 53 <bean id="dataSourceTransactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> 54 <property name="dataSource" ref="dataSource"></property> 55 </bean> 56 57 <!-- 开启基于注解的事务 --> 58 <tx:annotation-driven transaction-manager="dataSourceTransactionManager" order="2"/> 59 60 <!-- 61 整合mybatis 62 目的：1、spring管理所有组件。mapper的实现类。 63 service==>Dao @Autowired:自动注入mapper； 64 2、spring用来管理事务，spring声明式事务 65 --> 66 <!--创建出SqlSessionFactory对象 --> 67 <bean id="sqlSessionFactoryBean" class="org.mybatis.spring.SqlSessionFactoryBean"> 68 <property name="dataSource" ref="dataSource"></property> 69 <!-- configLocation指定全局配置文件的位置 --> 70 <property name="configLocation" value="classpath:mybatis-config.xml"></property> 71 <!--mapperLocations: 指定mapper文件的位置--> 72 <property name="mapperLocations" value="classpath:mybatis/mapper/*.xml"></property> 73 </bean> 74 75 <!--配置一个可以进行批量执行的sqlSession --> 76 <bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate"> 77 <constructor-arg name="sqlSessionFactory" ref="sqlSessionFactoryBean"></constructor-arg> 78 <constructor-arg name="executorType" value="BATCH"></constructor-arg> 79 </bean> 80 81 <!-- 扫描所有的mapper接口的实现，让这些mapper能够自动注入； 82 base-package：指定mapper接口的包名 83 --> 84 <mybatis-spring:scan base-package="com.test.dao"/> 85 86 87 <!-- 配置切面的Bean --> 88 <bean id="dataSourceExchange" class="com.test.datasource.DataSourceExchange"/> 89 90 91 <!-- 配置AOP --> 92 <aop:config> 93 <!-- 配置切点表达式 --> 94 <aop:pointcut id="servicePointcut" expression="execution(* com.test.service.*.*(..))"/> 95 <!-- 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"/> --> 96 <aop:advisor advice-ref="dataSourceExchange" pointcut-ref="servicePointcut" order="1"/> 97 </aop:config> 98 99 </beans>
注意：Spring中的事务是通过aop来实现的，当我们自己写aop拦截的时候，会遇到跟spring的事务aop执行的先后顺序问题，比如说动态切换数据源的问题，如果事务在前，数据源切换在后，会导致数据源切换失效，所以就用到了Order（排序）这个关键字
1 <aop:advisor advice-ref="dataSourceExchange" pointcut-ref="servicePointcut" order="1"/>
1 <!-- 开启基于注解的事务 --> 2 <tx:annotation-driven transaction-manager="dataSourceTransactionManager" order="2"/>
7、在service上加上注解即可使用
1 @Transactional 2 @TargetDataSource(name=TargetDataSource.SLAVE) 3 public int addEmployeeFromSalve(Employee employee) { 4 5 return employeeMapper.insert(employee); 6 }
数据流转顺序：
1.xml<aop>拦截到数据源名称
2.执行切面DataSourceExchange中的before方法，将数据源名称放入 DataSourceHolder中
3.Spring 调用determineCurrentLookupKey()方法<DynamicDataSource中重写AbstractRoutingDataSource类中的方法> ，从DataSourceHolder取出当前的数据库名称，并返回
4.AbstractRoutingDataSource类中determineTargetDataSource()方法调用determineCurrentLookupKey()匹配到指定的数据库，并建立链接，即为切换到相应的数据库；
5.在指定的数据库中执行相应的sql
