Why SqlSession Is Thread‑Unsafe and How SqlSessionTemplate Ensures Safety

1. Why SqlSession Is Not Thread‑Safe

(1) SqlSession is built on JDBC Connection, which is not thread‑safe, so SqlSession is also not thread‑safe.

(2) SqlSession holds a database connection and transaction operations; sharing a single SqlSession across threads can cause data inconsistency or transaction chaos.

(3) The caching mechanism of SqlSession is thread‑local; sharing it among threads may lead to cache corruption or inconsistency.

2. How to Make SqlSession Safe

(1) Use a separate SqlSession instance per thread, typically created via a factory pattern or MyBatis’s thread‑safe SqlSessionFactory.

(2) Employ ThreadLocal to ensure each thread gets its own SqlSession.

3. How SqlSessionTemplate Guarantees Thread Safety

When Spring integrates MyBatis, it uses SqlSessionTemplate to perform CRUD operations.

SqlSessionTemplate defines a SqlSessionProxy that uses dynamic proxy to execute CRUD.

The dynamic proxy’s interceptor SqlSessionInterceptor obtains a SqlSession via getSqlSession.

Inside getSqlSession, the SqlSession is stored in a SqlSessionHolder, which is kept in the ThreadLocal variable resources of TransactionSynchronizationManager.

Each thread has its own resources, isolated from others, so a singleton SqlSessionTemplate can be safely used across multiple threads and transactions.