1、例項程式碼
在例項搭建文章中，通過 SqlSession 物件查詢資料，可樂寫了兩種方法。
①、常規的需要我們拼接 statement 方式；
②、xxxMapper.interface 介面代理方式；

對應下面兩種方法：

//根據id查詢person表資料
@Test
public void testSelectPersonById() {
    /*這個字串由 PersonMapper.xml 檔案中 兩個部分構成
        <mapper namespace="com.itcoke.mapper.PersonMapper"> 的 namespace 的值
        <select id="selectPersonById" > id 值
    
 
*/
    String namespace = "com.itcoke.mapper.PersonMapper";
    String method = "selectPersonById";
    //根據 sqlSessionFactory 產生 session
    SqlSession sqlSession = sessionFactory.openSession();
    Person person = sqlSession.selectOne(namespace + "." + method, 1L);
    System.out.println(person);
    sqlSession.close();
}

 
//根據id查詢person表資料
//通過介面代理的方式
@Test
public void testInterfaceSelectPersonById() {
    //根據 sqlSessionFactory 產生 session
    SqlSession sqlSession = sessionFactory.openSession();
    PersonMapper mapper = sqlSession.getMapper(PersonMapper.class);
    Person person = mapper.selectPersonById(1L);
    System.out.println(person);
    sqlSession.close();
}
 

2、構建過程圖示

3、程式碼剖析
3.1 Executor
我們通過 DefaultSessionFactory.openSession() 方法獲取 sqlSession

public interface SqlSessionFactory {
  
    SqlSession openSession();
  
    SqlSession openSession(boolean autoCommit);
  
    SqlSession openSession(Connection connection);
  
    SqlSession openSession(TransactionIsolationLevel level);
  
    SqlSession openSession(ExecutorType execType);
  
    SqlSession openSession(ExecutorType execType, boolean autoCommit);
  
    SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
  
    SqlSession openSession(ExecutorType execType, Connection connection);
  
    Configuration getConfiguration();
  
  }

其實是可以通過構造方法指定 Executor 的型別，比如：

SqlSession sqlSession = sessionFactory.openSession(ExecutorType.SIMPLE);

再看生成 Executor 的原始碼：在Configuration類中

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

如果不指定執行器型別，直接預設 openSession() 方法，生成的是 CachingExecutor 執行器，這裡的 cacheEnabled 其實是預設開啟二級快取的配置,在 mybatis-config.xml 檔案中.
並且需要注意的是這裡 new CachingExecutor(executor)，傳進去了一個 SimpleExecutor 物件，後面和資料庫互動的實際上是該物件。