mybatis的执行流程

1.SqlSessionFactoryBuilder与SqlSessionFactory

　　我们一般在使用mybatis是都会通过new SqlSessionFactoryBuilder.build(...)来获取SqlSessionFactory，那么这条语句发生了什么，我们来看一看源码

　　(1).通过将配置文件传递给SqlSessionFactoryBuilder调用build()方法来获取SessionSessionFactory.

public SqlSessionFactory build(Reader reader) {
    return build(reader, null, null); //我们跳到这个方法去看
}

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);  //通过这个方法来加载我们mybatis的xml文件，解析成为XMLConfigBuilder对象。
      return build(parser.parse());　　　　　　　　　　　//我们转到这个方法
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
}

public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);    //通过将Configuration对象最为参数来新建一个DefaultSqlSessionFactory.
}

2.SqlSessionFactory与SqlSession

　（1）我们一般通过SqlSqlSessionFactory.oppenSession()来获取一个SqlSession.我们来看看源码都发生了什么。

@Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); //我们调到这个方法，会以默认的Executor来执行我们的操作。
  }

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment(); //通过Configuration对象去获取我们的配置信息。
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      final Executor executor = configuration.newExecutor(tx, execType);  //新建默认的Executor,默认为Simple
      return new DefaultSqlSession(configuration, executor, autoCommit);  //将Configuration，我们得到的executor作为参数来新建一个DefaultSqlSession.
    } 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();
    }
  }

　　(2)我们也可以通过自定义的ExecutorType来创建我们的SqlSessionFactory

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();

}

　　(3)SqlSession介绍

　　　sqlSession作为顶层的接口，为我们提供的数据库访问的接口方法。

3.Executor

　　(1)在sqlSession中实际上并没有实际的数据库操作而是交给下层的Executor来进行，Executor这一层主要负责mybatis中缓存的查询。

　　举个例子：

　　

@Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);    //获取MapperStatement对象，在mybatis中我们标签<select/>,<update/>,等这些CRUD标签都会被解析为一个个的MappedStatement对象。
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); //调用executor的方法来查询
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

　　(2)我们转到executor(BaseExecutor)的方法中,注意在BaseExecutor中存储的是我们的一级缓存，关于一二级缓存在下一篇中提及，此处只讨论执行过程

@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);  //查找缓存的key
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql); //转到这个方法
 }

@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;  //查询缓存是否存在
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);  //若缓存不存在则去查询数据库
      }
    } 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;
  }

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);   //将当前的查找key放入到缓存中
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); //执行子类中查询数据库的方法
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);         //将结果放入到缓存中
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

我们转到其中一个子类中的查询数据库方法(SimpleExecutor)

@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);  //通过configuration来获取StatementHandler对象
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);  //将查询数据库的操作交给StatementHandler去实现
    } finally {
      closeStatement(stmt);
    }
  }

4.StatementHandler

statementHandler用来执行原始的Jdbc操作

@Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();      //通过preparedStatement来执行原始的数据库操作
    return resultSetHandler.<E> handleResultSets(ps); //返回结果并封装为ResultHandler对象
  }

这样mybatis的一次执行就完成了。

5.最后我们在使用SqlSession时一般会使用SqlSession.getMapper来获取我们的代理类

@Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this); //通过configuration来获取Mapper
  }

//configuration类中的方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession); //在configuration对象中的mapperRegistry来获取mapper，在mapperRegistry中保存了我们的接口信息。
  }

//mapperRegistry类中的方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);    //通过mapperProxyFactory来获取mapper实例
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  

//MapperProxyFactory类中的方法
protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);  //通过jdk动态代理来获取代理对象
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

通过以上来获取我们的Mapper对象。

对于一些细节，可以去查看mybatis的源码进行学习