Mybatis源码分析-整体设计(一)

SSM是目前常见的构建Web项目的方案，Mybatis是其中重要的一环，假如能深刻的了解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有肯定的了解，里面相对简单，就不多说了

Resources.getResourceAsStream("mybatis.xml");

2. 创立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来创立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);  }

3. 通过SqlSessionFactory创立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();    }  }

4. 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，而后通过Executor查询MapperStatement。

在解析配置的时候建立key与MappedStatement映射关系的

@Override  public <E> List<E> 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();    }  }

5. 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 假如没有缓存从数据库中进行查询，重点语句

  @Override  public <E> List<E> 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 <E> List<E> 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<E> list;    try {      queryStack++;      list = resultHandler == null ? (List<E>) 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;  }

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

• 新建StatementHandler，语句解决器
• 通过StatementHandler再进行query
• Statement为JDK提供的SQL接口，获取Statement经历了StatementHandler.prepare，而后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解决获取到的结果

@Override  public <E> List<E> 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.<E>query(stmt, resultHandler);    } finally {      closeStatement(stmt);    }  }

7. StatementHandler内部的调用，结果集解决使用的是DefaultResultSetHandler,内部ResultSet的解决属于JDBC的知识，想看懂Mybatis，对JDBC也要有肯定的了解

8. 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 语句解决器
• ResultHandler 返回的结果解决器
• Executor Mybatis中的语句执行器

创立SqlSessionFactory -> 获取SqlSession -> 获取->MappedStatement -> 获取StatementHandler同时创立Statement -> 执行Statement -> 使用ResultSet解决执行的结果，解决结果根据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更直观少量，不过需要熟习JDBC的知识，内部还有少量细节，后续继续研究