现在有一个Mysql数据源和一个Postgresql数据源,使用Mybatis对两个数据源进行操作:
1. 注入多数据源
可以对两个数据源分别实现其Service层和Mapper层,以及Mybatis的配置类:
@Configuration
// 这里需要配置扫描包路径,以及sqlSessionTemplateRef
@MapperScan(basePackages = "com.example.mybatisdemo.mapper.mysql", sqlSessionTemplateRef = "mysqlSqlSessionTemplate")
public class MysqlMybatisConfigurer {
@Bean
@ConfigurationProperties(prefix = "spring.datasource.mysql")
public DataSource mysqlDatasource() {
return new DruidDataSource();
}
@Bean
public SqlSessionFactory mysqlSqlSessionFactory(DataSource mysqlDatasource) throws Exception {
SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();
factoryBean.setDataSource(mysqlDatasource);
// 设置对应的mapper文件
factoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath:" +
"/mappers/MysqlMapper.xml"));
return factoryBean.getObject();
}
@Bean
public SqlSessionTemplate mysqlSqlSessionTemplate(SqlSessionFactory mysqlSqlSessionFactory) {
return new SqlSessionTemplate(mysqlSqlSessionFactory);
}
@Bean
public DataSourceTransactionManager mysqlTransactionalManager(DataSource mysqlDatasource) {
return new DataSourceTransactionManager(mysqlDatasource);
}
}
@Configuration
// 这里需要配置扫描包路径,以及sqlSessionTemplateRef
@MapperScan(basePackages = "com.example.mybatisdemo.mapper.postgresql", sqlSessionTemplateRef = "postgresqlSqlSessionTemplate")
public class PostgresqlMybatisConfigurer {
@Bean
@ConfigurationProperties(prefix = "spring.datasource.postgresql")
public DataSource postgresqlDatasource() {
return new DruidDataSource();
}
@Bean
public SqlSessionFactory postgresqlSqlSessionFactory(DataSource postgresqlDatasource) throws Exception {
SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();
factoryBean.setDataSource(postgresqlDatasource);
// 设置对应的mapper文件
factoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath:" +
"/mappers/PostgresqlMapper.xml"));
return factoryBean.getObject();
}
@Bean
public SqlSessionTemplate postgresqlSqlSessionTemplate(SqlSessionFactory postgresqlSqlSessionFactory) {
return new SqlSessionTemplate(postgresqlSqlSessionFactory);
}
@Bean
public DataSourceTransactionManager postgresqlTransactionalManager(DataSource postgresqlDatasource) {
return new DataSourceTransactionManager(postgresqlDatasource);
}
}
在配置类中,分别注入了一个事务管理器TransactionManager,这个和事务管理是相关的。在使用@Transactional注解时,需要配置其value属性指定对应的事务管理器。
2. 动态数据源
Spring中提供了AbstractRoutingDataSource抽象类,可以用于动态地选择数据源。
public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {
@Nullable
private Map<Object, Object> targetDataSources;
@Nullable
private Object defaultTargetDataSource;
private boolean lenientFallback = true;
private DataSourceLookup dataSourceLookup = new JndiDataSourceLookup();
@Nullable
private Map<Object, DataSource> resolvedDataSources;
@Nullable
private DataSource resolvedDefaultDataSource;
// 略
}
通过源码可以看到,该抽象类实现了InitializingBean接口,并在其afterPropertiesSet方法中将数据源以<lookupkey, dataSource>的形式放入一个Map中。
public void afterPropertiesSet() {
if (this.targetDataSources == null) {
throw new IllegalArgumentException("Property 'targetDataSources' is required");
} else {
this.resolvedDataSources = CollectionUtils.newHashMap(this.targetDataSources.size());
this.targetDataSources.forEach((key, value) -> {
Object lookupKey = this.resolveSpecifiedLookupKey(key);
DataSource dataSource = this.resolveSpecifiedDataSource(value);
// 将数据源以<lookupkey, dataSource>的形式放入Map中
this.resolvedDataSources.put(lookupKey, dataSource);
});
if (this.defaultTargetDataSource != null) {
this.resolvedDefaultDataSource = this.resolveSpecifiedDataSource(this.defaultTargetDataSource);
}
}
}
该类中还有一个determineTargetDataSource方法,是根据lookupkey从Map中获取对应的数据源,如果没有获取到,则使用默认的数据源。
protected DataSource determineTargetDataSource() {
Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");
Object lookupKey = this.determineCurrentLookupKey();
// 根据lookupkey从Map中获取对应的数据源
DataSource 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 + "]");
} else {
return dataSource;
}
}
lookupkey是通过determineTargetDataSource方法获取到的,而它是一个抽象方法,我们要做的就是通过实现这个方法,来控制获取到的数据源。
@Nullable
protected abstract Object determineCurrentLookupKey();
(1) 创建并注入动态数据源
创建AbstractRoutingDataSource的子类,实现determineCurrentLookupKey方法
public class RoutingDataSource extends AbstractRoutingDataSource {
@Override
protected Object determineCurrentLookupKey() {
return DataSourceContextHolder.get();
}
}
这里的DataSourceContextHolder是一个操作ThreadLocal对象的工具类
public class DataSourceContextHolder {
private static final ThreadLocal<DataSourceType> contextHolder = new ThreadLocal<>();
public static void set(DataSourceType type) {
contextHolder.set(type);
}
public static DataSourceType get() {
return contextHolder.get();
}
public static void mysql() {
set(DataSourceType.MYSQL);
}
public static void postgresql() {
set(DataSourceType.POSTGRESQL);
}
public static void remove() {
contextHolder.remove();
}
}
通过调用DataSourceContextHolder.mysql()或者DataSourceContextHolder.postgresql()就能修改contextHolder的值,从而在动态数据源的determineTargetDataSource方法中就能获取到对应的数据源。
在数据源配置类中,将mysql和postgresql的数据源设置到动态数据源的Map中,并注入容器。
@Configuration
public class DataSourceConfigurer {
@Bean
@ConfigurationProperties(prefix = "spring.datasource.mysql")
public DataSource mysqlDatasource() {
return new DruidDataSource();
}
@Bean
@ConfigurationProperties(prefix = "spring.datasource.postgresql")
public DataSource postgresqlDatasource() {
return new DruidDataSource();
}
@Bean
public RoutingDataSource routingDataSource(DataSource mysqlDatasource, DataSource postgresqlDatasource) {
Map<Object, Object> dataSources = new HashMap<>();
dataSources.put(DataSourceType.MYSQL, mysqlDatasource);
dataSources.put(DataSourceType.POSTGRESQL, postgresqlDatasource);
RoutingDataSource routingDataSource = new RoutingDataSource();
routingDataSource.setDefaultTargetDataSource(mysqlDatasource);
// 设置数据源
routingDataSource.setTargetDataSources(dataSources);
return routingDataSource;
}
}
(2) Mybatis配置类
由于使用了动态数据源,所以只需要编写一个配置类即可。
@Configuration
@MapperScan(basePackages = "com.example.mybatisdemo.mapper", sqlSessionTemplateRef = "sqlSessionTemplate")
public class MybatisConfigurer {
// 注入动态数据源
@Resource
private RoutingDataSource routingDataSource;
@Bean
public SqlSessionFactory sqlSessionFactory() throws Exception {
SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
sqlSessionFactoryBean.setDataSource(routingDataSource);
// 这里可以直接设置所有的mapper.xml文件
sqlSessionFactoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath" +
":mappers/*.xml"));
return sqlSessionFactoryBean.getObject();
}
@Bean
public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
return new SqlSessionTemplate(sqlSessionFactory);
}
@Bean
public DataSourceTransactionManager transactionalManager(DataSource mysqlDatasource) {
return new DataSourceTransactionManager(mysqlDatasource);
}
}
(3) 使用注解简化数据源切换
我们虽然可以使用DataSourceContextHolder类中的方法进行动态数据源切换,但是这种方式有些繁琐,不够优雅。可以考虑使用注解的形式简化数据源切换。
我们先定义两个注解,表示使用Mysql数据源或Postgresql数据源:
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Mysql {
}
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Postgresql {
}
再定义一个切面,当使用了注解时,会先调用切换数据源的方法,再执行后续逻辑。
@Component
@Aspect
public class DataSourceAspect {
@Pointcut("@within(com.example.mybatisdemo.aop.Mysql) || @annotation(com.example.mybatisdemo.aop.Mysql)")
public void mysqlPointcut() {
}
@Pointcut("@within(com.example.mybatisdemo.aop.Postgresql) || @annotation(com.example.mybatisdemo.aop.Postgresql)")
public void postgresqlPointcut() {
}
@Before("mysqlPointcut()")
public void mysql() {
DataSourceContextHolder.mysql();
}
@Before("postgresqlPointcut()")
public void postgresql() {
DataSourceContextHolder.postgresql();
}
}
在使用动态数据源的事务操作时有两个需要注意的问题:
问题一 同一个事务操作两个数据源
Mybatis使用Executor执行SQL时需要获取连接,BaseExecutor类中的getConnection方法调用了SpringManagedTransaction中的getConnection方法,这里优先从connection字段获取连接,如果connection为空,才会调用openConnection方法,并把连接赋给connection字段。
也就是说,如果你使用的是同一个事务来操作两个数据源,那拿到的都是同一个连接,会导致数据源切换失败。
protected Connection getConnection(Log statementLog) throws SQLException {
Connection connection = this.transaction.getConnection();
return statementLog.isDebugEnabled() ? ConnectionLogger.newInstance(connection, statementLog, this.queryStack) : connection;
}
public Connection getConnection() throws SQLException {
if (this.connection == null) {
this.openConnection();
}
return this.connection;
}
private void openConnection() throws SQLException {
this.connection = DataSourceUtils.getConnection(this.dataSource);
this.autoCommit = this.connection.getAutoCommit();
this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);
LOGGER.debug(() -> {
return "JDBC Connection [" + this.connection + "] will" + (this.isConnectionTransactional ? " " : " not ") + "be managed by Spring";
});
}
问题二 两个独立事务分别操作两个数据源
(1) 在开启事务的时候,DataSourceTransactionManager中的doBegin方法会先获取Connection,并保存到ConnectionHolder中,将数据源和ConnectionHolder的对应关系绑定到TransactionSynchronizationManager中。
protected void doBegin(Object transaction, TransactionDefinition definition) {
DataSourceTransactionManager.DataSourceTransactionObject txObject = (DataSourceTransactionManager.DataSourceTransactionObject)transaction;
Connection con = null;
try {
if (!txObject.hasConnectionHolder() || txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
// 获取连接
Connection newCon = this.obtainDataSource().getConnection();
if (this.logger.isDebugEnabled()) {
this.logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
}
// 保存到ConnectionHolder中
txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
}
txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
// 从ConnectionHolder获取连接
con = txObject.getConnectionHolder().getConnection();
// 略
// 将数据源和ConnectionHolder的关系绑定到TransactionSynchronizationManager中
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.bindResource(this.obtainDataSource(), txObject.getConnectionHolder());
}
// 略
}
(2) TransactionSynchronizationManager的bindResource方法将数据源和ConnectionHolder的对应关系存入线程变量resources中。
public abstract class TransactionSynchronizationManager {
// 线程变量
private static final ThreadLocal<Map<Object, Object>> resources =
new NamedThreadLocal<>("Transactional resources");
// 略
// 绑定数据源和ConnectionHolder的对应关系
public static void bindResource(Object key, Object value) throws IllegalStateException {
Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
Assert.notNull(value, "Value must not be null");
Map<Object, Object> map = resources.get();
// set ThreadLocal Map if none found
if (map == null) {
map = new HashMap<>();
resources.set(map);
}
Object oldValue = map.put(actualKey, value);
// Transparently suppress a ResourceHolder that was marked as void...
if (oldValue instanceof ResourceHolder && ((ResourceHolder) oldValue).isVoid()) {
oldValue = null;
}
if (oldValue != null) {
throw new IllegalStateException(
"Already value [" + oldValue + "] for key [" + actualKey + "] bound to thread");
}
}
// 略
}
(3) 上边提到的openConnection方法,其实最终也是从TransactionSynchronizationManager的resources中获取连接的
public static Connection doGetConnection(DataSource dataSource) throws SQLException {
Assert.notNull(dataSource, "No DataSource specified");
// 获取ConnectionHolder
ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(dataSource);
if (conHolder == null || !conHolder.hasConnection() && !conHolder.isSynchronizedWithTransaction()) {
logger.debug("Fetching JDBC Connection from DataSource");
Connection con = fetchConnection(dataSource);
if (TransactionSynchronizationManager.isSynchronizationActive()) {
try {
ConnectionHolder holderToUse = conHolder;
if (conHolder == null) {
holderToUse = new ConnectionHolder(con);
} else {
conHolder.setConnection(con);
}
holderToUse.requested();
TransactionSynchronizationManager.registerSynchronization(new DataSourceUtils.ConnectionSynchronization(holderToUse, dataSource));
holderToUse.setSynchronizedWithTransaction(true);
if (holderToUse != conHolder) {
TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
}
} catch (RuntimeException var4) {
releaseConnection(con, dataSource);
throw var4;
}
}
return con;
} else {
conHolder.requested();
if (!conHolder.hasConnection()) {
logger.debug("Fetching resumed JDBC Connection from DataSource");
conHolder.setConnection(fetchConnection(dataSource));
}
// 从ConnectionHolder中获取连接
return conHolder.getConnection();
}
}
也就是说,如果修改了数据源,那么resources中就找不到对应的连接,就可以重新获取连接,从而达到切换数据源的目的。然而我们数据源的只有一个,就是动态数据源,因此即使使用两个独立事务,也不能成功切换数据源。
3. 结语
如果想要使用动态数据源的事务处理,可能需要考虑使用多线程分布式的事务处理机制;
如果使用直接注入多个数据源的方式实现事务处理,实现简单,但是各数据源事务是独立的;
应该根据具体情况进行选择。
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