MyBatis Source Code Execution Flow Explained

1. Obtain Configuration File Stream

The process starts by reading the MyBatis XML configuration as an InputStream :

InputStream input = Resources.getResourceAsStream("SqlSessionConfig.xml");

2. Build SqlSessionFactory

The XML is parsed by XMLConfigBuilder , which creates an SqlSessionFactory using the Builder pattern.

SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(input);

Inside SqlSessionFactoryBuilder , the build(InputStream) method delegates to SqlSessionFactoryBuilder.build(Configuration) , returning a DefaultSqlSessionFactory .

3. Create SqlSession

From the factory, a session is opened:

SqlSession sqlSession = sessionFactory.openSession();

The DefaultSqlSessionFactory creates a transaction, an executor, and finally a DefaultSqlSession instance.

4. Generate Mapper Proxy

The session creates a dynamic proxy for the mapper interface:

TestMapperDao mapper = sqlSession.getMapper(TestMapperDao.class);

The proxy is built by MapperProxyFactory , which stores a MapperProxy that implements InvocationHandler . The proxy is cached in MapperRegistry.knownMappers .

5. Invoke Mapped Method

Calling a mapper method (e.g., selectByPrimaryKey(1) ) triggers the proxy’s invoke method, which retrieves a MapperMethod and calls its execute method.

System.out.println(mapper.selectByPrimaryKey(1));

MapperMethod.execute determines the SQL command type (SELECT, INSERT, UPDATE, DELETE) and delegates to the appropriate SqlSession method. For a SELECT, it eventually calls DefaultSqlSession.selectOne , which uses the executor chain.

Executor Chain

The executor hierarchy consists of CachingExecutor (second‑level cache) wrapping a concrete executor such as SimpleExecutor . The caching executor builds a CacheKey , checks the second‑level cache, and falls back to the delegate executor.

public
List
query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    // ... cache lookup ...
    return delegate.query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

The delegate ( SimpleExecutor ) prepares a PreparedStatement via a StatementHandler and executes it:

Statement stmt = null;
try {
    StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);
} finally {
    closeStatement(stmt);
}

The PreparedStatementHandler finally calls ps.execute() and hands the ResultSet to a ResultSetHandler to build the result objects.

Cache Interaction

Before querying, the executor checks the second‑level cache; if a miss occurs, the result is stored in the local (first‑level) cache and then placed into the second‑level cache after the database call completes.

6. Mapper Registration

During XML parsing, each <mapper> node registers its interface in Configuration via MapperRegistry.addMapper . This adds a MapperProxyFactory to knownMappers , enabling the sqlSession.getMapper call to retrieve the proxy.

The article also notes that the same registration works for annotation‑based mappers, where MapperAnnotationBuilder parses @Select, @Insert, etc., and registers the mapper in the same way.