Why is DefaultSqlSession Not Thread‑Safe and How Does SqlSessionTemplate Ensure Safety?

On Saturday I posted a collection of interview questions titled “2018 Complete Java Interview Guide”. In the comments I added a question about the thread‑safety of SqlSession, and this article explores that issue.

1. Thread‑Unsafety of DefaultSqlSession

In MyBatis, SqlSession is the top‑level interface exposed to callers. Implementations include DefaultSqlSession, SqlSessionManager, and the MyBatis‑Spring provided SqlSessionTemplate. The default implementation is DefaultSqlSession, which is not thread‑safe.

2. How SqlSessionTemplate Uses DefaultSqlSession

When integrating MyBatis with Spring, developers typically inject a singleton SqlSessionTemplate. The source code of SqlSessionTemplate shows that it implements the SqlSession interface, acting as a proxy for DefaultSqlSession while ensuring thread safety.

Because DefaultSqlSession is not thread‑safe, it cannot be used as a singleton. Instead, SqlSessionTemplate delegates calls to a thread‑local SqlSession instance, guaranteeing safe concurrent access.

3. How SqlSessionTemplate Guarantees Thread Safety

SqlSessionTemplate creates a proxy that implements SqlSession. Method calls are intercepted by SqlSessionInterceptor, whose invoke method contains the core logic.

The interceptor uses two helper methods, getSqlSession and closeSqlSession, to obtain and release a thread‑bound SqlSession.

4. What Is SqlSessionManager?

SqlSessionManager is another implementation of SqlSession. It has a private constructor and is instantiated via the static newInstance methods, which are overloaded to provide various configuration options.

Its openSession methods delegate to the underlying SqlSessionFactory to create SqlSession instances. Internally, it uses a ThreadLocal variable ( localSqlSession) to bind a SqlSession to the current thread, avoiding repeated creation and improving performance.

5. Summary

In summary:

DefaultSqlSession does not provide thread‑local handling like SqlSessionManager.

SqlSessionManager uses a ThreadLocal variable to bind a SqlSession to each thread, preventing performance loss from repeated creation.

When developing without Spring, you must create a new SqlSession for each operation because DefaultSqlSession is not thread‑safe.

6. Additional Interview Questions

1. Why does the mybatis‑spring framework use the non‑thread‑safe DefaultSqlSession with dynamic proxies instead of a thread‑safe SqlSessionManager?

2. How does DefaultSqlSession employ the Strategy pattern via its Executor component?