Could not open JDBC connection for transaction

background

The company recently reported this error in the online project ,Could not open JDBC Connection for transaction, Unable to get the data source connection pool .

analysis

Read the source code , Let's see if the data source connection can be automatically released in all cases .

MyBatis Release the connection

MyBatis Run when running alone SQL Statement will not automatically release the data source connection , But and Spring After consolidation, the data source connection will be automatically released .Spring Changed the MyBatis Of SqlSession, Change to Spring In the integration package SqlSessionTemplate, The key codes are as follows ：

public class SqlSessionTemplate implements SqlSession, DisposableBean {
    
    //...
    // Omit some code 
    private class SqlSessionInterceptor implements InvocationHandler {
    
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    
            SqlSession sqlSession = getSqlSession(
                    SqlSessionTemplate.this.sqlSessionFactory,
                    SqlSessionTemplate.this.executorType,
                    SqlSessionTemplate.this.exceptionTranslator);
            try {
    
                Object result = method.invoke(sqlSession, args);
                if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
    
                    sqlSession.commit(true);
                }
                return result;
            } catch (Throwable t) {
    
                Throwable unwrapped = unwrapThrowable(t);
                if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
    
                    // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
                    closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                    sqlSession = null;
                    Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
                    if (translated != null) {
    
                        unwrapped = translated;
                    }
                }
                throw unwrapped;
            } finally {
    
                if (sqlSession != null) {
    
                    closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                }
            }
        }
    }
}

At the end of the finally in , Will close session, Release data source connection .

Business @Transactional Release the connection

Add comments to methods @Transactional Mark the method as a transaction , The connection will also be automatically released , The key codes are as follows ：

public class DataSourceTransactionManager extends AbstractPlatformTransactionManager
		implements ResourceTransactionManager, InitializingBean {
    
	//...
    // Omit some code 
	@Override
	protected void doCleanupAfterCompletion(Object transaction) {
    
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		if (txObject.isNewConnectionHolder()) {
    
			TransactionSynchronizationManager.unbindResource(this.dataSource);
		}
		Connection con = txObject.getConnectionHolder().getConnection();
		try {
    
			if (txObject.isMustRestoreAutoCommit()) {
    
				con.setAutoCommit(true);
			}
			DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());
		}
		catch (Throwable ex) {
    
			logger.debug("Could not reset JDBC Connection after transaction", ex);
		}
		if (txObject.isNewConnectionHolder()) {
    
			if (logger.isDebugEnabled()) {
    
				logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
			}
			DataSourceUtils.releaseConnection(con, this.dataSource);
		}
		txObject.getConnectionHolder().clear();
	}

Among them ,DataSourceUtils.releaseConnection(con, this.dataSource) Method closes the data source connection .

To find problems

The company project uses Druid data source , The maximum number of connections is set to 50, Follow the above analysis , Under normal circumstances, it is impossible to use up , There must be some code that doesn't release the connection .
I've been looking for it for a long time , Finally locate the following code ：

@Autowired
private SqlSessionFactory sqlSessionFactory;
public void batchInsert(List<TaskCenter> list) {
    
	if(list == null || list.size() == 0){
    
		return;
	}
	try {
    
		SqlSession sqlSession = sqlSessionFactory.openSession(ExecutorType.BATCH);
		TaskCenterMapper mapper = sqlSession.getMapper(TaskCenterMapper.class);
		for(TaskCenter taskCenter : list){
    
			mapper.insertSelective(taskCenter);
		}
		sqlSession.flushStatements();
		sqlSession.commit();
		log.info(" Batch insert succeeded ： " + list.size()+" Data ");
	}catch (Exception ex){
    
		log.error(" Batch insert failed ： ", ex);
	}
}

This code means to use MyBatis Batch insert function of batch insert data , We analyzed above , Use MyBatis Of SqlSession The data source connection will not be closed automatically , Need to use Spring Packed SqlSessionTemplate Will automatically close the data source connection . So every time you execute this batchInsert Method , Will occupy a data source connection without releasing , Eventually, the data source connection pool is full , Unable to open new connection .

solve the problem

Based on the above analysis , There are two solutions to this problem
1、 Add this method to the transaction , Add a comment to the method @Transactional, The code is as follows ：

@Transactional
public void batchInsert(List<TaskCenter> list) {
    
	if(list == null || list.size() == 0){
    
		return;
	}
	//  Omitted below 
	// ...

2、 Use up sqlSession Manual closing after sqlSession, The code is as follows ：

public void batchInsert(List<TaskCenter> list) {
    
	if(list == null || list.size() == 0){
    
		return;
	}
	SqlSession sqlSession = null;
	try {
    
		sqlSession = sqlSessionFactory.openSession(ExecutorType.BATCH);
		TaskCenterMapper mapper = sqlSession.getMapper(TaskCenterMapper.class);
		for(TaskCenter taskCenter : list){
    
			mapper.insertSelective(taskCenter);
		}
		sqlSession.flushStatements();
		sqlSession.commit();
		log.info(" Batch insert succeeded ： " + list.size()+" Data ");
	}catch (Exception ex){
    
		log.error(" Batch insert failed ： ", ex);
	}finally {
    
		if (sqlSession != null) {
    
			SqlSessionUtils.closeSqlSession(sqlSession, sqlSessionFactory);
		}
	}
}

Conclusion

According to a production problem, this article analyzes the source code to understand MyBatis The framework and Spring Transaction management automatically turns off the data source connection pool , Understand the principle to solve the problem .