Mybatis plus的多數據源@DS切換為什么不起作用了

由於使用了微服務，會有多個數據庫的情況，有時業務需要，需要切換數據源，所以使用了Mybatis plus的@DS來切換多數據源

yml數據庫配置如下：

spring:
  datasource:
    dynamic:
      primary: master
      datasource:
        master:
          driverClassName: com.mysql.cj.jdbc.Driver
          url: jdbc:mysql://user
          username: root
          password: root
        common:
          driverClassName: com.mysql.cj.jdbc.Driver
          url: jdbc:mysql://book
          username: root
          password: root

service如下，默認是master數據源

@Service
@Slf4j
public class MasterService {
    @Autowired
    UserService userService;
    @Autowired
    BookService bookService;

    /**必須master庫方法先執行，才能回滾，達到事務效果*/
    @Transactional(rollbackFor = Exception.class)
    public void upload(ReqDto reqDto){
        userService.save(reqDto);
        bookService.save(reqDto);
    }
}

　userService如下：

@Service
@Log4j2
public class UserService extends ServiceImpl<UserMapper, User> {
    @Resource
    private UserMapper userMapper;
    
    public void save(ReqDto reqDto) {
        userMapper.save(reqDto);
    }
}

bookService如下：

@Service
@Log4j2
@DS("common")
public class BookService extends ServiceImpl<BookMapper, Book> {
    @Resource
    private BookMapper bookMapper;
    
    public void save(ReqDto reqDto) {
        bookMapper.save(reqDto);
    }
}

　

但是神奇的事發生的，bookService的數據庫應該是common，但是卻是master的，也就是說@DS切換數據源沒有起作用
於是開始排查

去除MasterService.upload上面的@Transactional，數據源切換正常，但是事務無效
BookService的save上面加@Transactional，數據源沒有切換
BookService的save上面加@Transactional(propagation = Propagation.REQUIRES_NEW)，數據源切換，且事務有效
原因：

開啟事務的同時，會從數據庫連接池獲取數據庫連接；
如果內層的service使用@DS切換數據源，只是又做了一層攔截，但是並沒有改變整個事務的連接;
在這個事務內的所有數據庫操作，都是在事務連接建立之后，所以會產生數據源沒有切換的問題;
為了使@DS起作用，必須替換數據庫連接，也就是改變事務的傳播機智，產生新的事務，獲取新的數據庫連接;
所以bookService的save方法上除了加@Transactional外，還需要設置propagation = Propagation.REQUIRES_NEW
使得代碼走以下邏輯：

private TransactionStatus handleExistingTransaction(
        .....省略.....
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction, creating new transaction with name [" +
						definition.getName() + "]");
			}
			SuspendedResourcesHolder suspendedResources = suspend(transaction);
			try {
				return startTransaction(definition, transaction, debugEnabled, suspendedResources);
			}
			catch (RuntimeException | Error beginEx) {
				resumeAfterBeginException(transaction, suspendedResources, beginEx);
				throw beginEx;
			}
		}
		.....省略.....)

　　在走startTransaction，再走doBegin，重新創建新事務，獲取新的數據庫連接，從而得到@DS的數據源

startTransaction(definition, transaction, debugEnabled, suspendedResources);

　　

protected void doBegin(Object transaction, TransactionDefinition definition) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
			if (!txObject.hasConnectionHolder() ||
					txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
				Connection newCon = obtainDataSource().getConnection();//獲取數據庫連接
				if (logger.isDebugEnabled()) {
					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
				}
				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
			}

			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
			con = txObject.getConnectionHolder().getConnection();

			Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
			txObject.setPreviousIsolationLevel(previousIsolationLevel);
			txObject.setReadOnly(definition.isReadOnly());

			// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
			// so we don't want to do it unnecessarily (for example if we've explicitly
			// configured the connection pool to set it already).
			if (con.getAutoCommit()) {
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
				con.setAutoCommit(false);
			}
			.....省略.....)

　　最終代碼如下，只需要修改的是bookService
bookService

@Service
@Log4j2
@DS("common")
public class BookService extends ServiceImpl<BookMapper, Book> {
    @Resource
    private BookMapper bookMapper;
    
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void save(ReqDto reqDto) {
        bookMapper.save(reqDto);
    }
}

@DS數據源切換生效
@Transaction事務生效
需要注意：
master：userService
common：bookService

common數據庫的操作，需要在master之后，這樣當bookService.save失敗，會使得userService回滾；
如果common的操作先，那當userService失敗，無法使bookService回滾

會回滾

    @Transactional(rollbackFor = Exception.class)
    public void upload(ReqDto respDto){
        userService.save(respDto);
        bookService.save(respDto);
    }

　　不會回滾

    @Transactional(rollbackFor = Exception.class)
    public void upload(ReqDto respDto){
        bookService.save(respDto);
        userService.save(respDto);
    }