SSM是目前常见的构建Web项目的方案，Mybatis是其中重要的一环，如果能深刻的理解Mybatis的内部原理，对我们会有极大的帮助，接下来一起看看Mybatis的内部设计。

准备

1. 搭建Mybatis的基本运行环境，参考Mybatis入门

2. 贴上自己的代码

public static void main(String[] args) {
        SqlSession sqlSession = null;
        try {
            InputStream inputStream = Resources.getResourceAsStream("mybatis.xml");
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

            // 查询数据库内容
            sqlSession = sqlSessionFactory.openSession();
            User user = sqlSession.selectOne("me.aihe.dao.UserMapper.selectUser",1);
            System.out.println(user);

            // 插入数据库内容

        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if (sqlSession != null) {
                sqlSession.close();
            }
        }
    }

分析

1. 获取Mybatis的配置文件，内部通过ClassLoader加载文件流，这一步需要对Classloader有一定的理解，里面相对简单，就不多说了

   Resources.getResourceAsStream("mybatis.xml");

2. 创建SqlSessionFactory, 通过JDK内部的w3c解析配置文件的内容，封装到Configration对象中，最后通过Configuration来创建DefaultSqlSessionFactory.

  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

1. 通过SqlSessionFactory创建SqlSession对象

• 获取配置的env参数，在解析配置的时候根据配置生成了TransactionFactory
• TransactionFactory对象，在设置env后，默认是JDBCTransactionFactory
• 通过配置生成默认的executor,executor是很重要的组件,可以看到executor封装了很多操作数据库相关的东西
• 生成默认的DefaultSqlSession，内部有executor,TransactionFactory，configuration

 @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      // 获取配置的
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      final Executor executor = configuration.newExecutor(tx, execType);
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

1. SqlSession通过key获取到与key绑定的sql语句，并且执行，最后获取到结果。这步内部可以细分

sqlSession.selectOne("me.aihe.dao.UserMapper.selectUser",1);

selectOne内部调用的是selectList的函数，selectList函数内部，首先获取到key对应的MappedStatement，然后通过Executor查询MapperStatement。

在解析配置的时候建立key与MappedStatement映射关系的

@Override
  public  List selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

1. 不同的executor内部的查询方法不同，后面我们详细讲解不同的executor。这次先看SimpleExecutor.

• queryStack判断当前的SQL执行栈，可能会连续执行多条sql语句
• localCache 缓存当前Sqlsession查询到的对象
• queryFromDatabase 如果没有缓存从数据库中进行查询，重点语句

  @Override
  public  List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }

 @SuppressWarnings("unchecked")
  @Override
  public  List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List list;
    try {
      queryStack++;
      list = resultHandler == null ? (List) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

1. query方法中真正的查询交给子类的doQuery，而SimpleExecutor的doQuery如下

• 新建StatementHandler，语句处理器
• 通过StatementHandler再进行query
• Statement为JDK提供的SQL接口，获取Statement经历了StatementHandler.prepare，然后parameterize设置参数。
• query方法利用MySQL内部的query
• 获取到结果之后，通过resultHandler处理获取到的结果

@Override
  public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

1. StatementHandler内部的调用，结果集处理使用的是DefaultResultSetHandler,内部ResultSet的处理属于JDBC的知识，想看懂Mybatis，对JDBC也要有一定的理解

1. 获取到结果返回给我们

   User user = sqlSession.selectOne("me.aihe.dao.UserMapper.selectUser",1);

回顾

这次大致看了下Mybatis的基本执行流程，涉及到了几个关键的类

• SqlSessionFactory ，SqlSession
• MappedStatement 封装我们写的SQL语句
• StatementHandler 语句处理器
• ResultHandler 返回的结果处理器
• Executor Mybatis中的语句执行器

创建SqlSessionFactory -> 获取SqlSession -> 获取->MappedStatement -> 获取StatementHandler同时创建Statement -> 执行Statement -> 使用ResultSet处理执行的结果，处理结果根据Mapperr.xml文件中指定的类型映射最终实现ORM功能

总结

Mybatis的过程相比Spring MVC更直观一些，不过需要熟悉JDBC的知识，内部还有一些细节，后续继续研究