获取SqlSession对象

SqlSession session = sqlSessionFactory.openSession();

首先通过SqlSessionFactory的openSession方法获取SqlSession接口的实现类DefaultSqlSession对象。

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();

}

SqlSessionFactory接口提供一系列重载的openSession方法，其参数如下：

• boolean autoCommit：是否开启JDBC事务的自动提交，默认为false。
• Connection：提供连接。
• TransactionIsolationLevel：定义事务隔离级别。
• ExecutorType：定义执行器类型。
  DefaultSqlSessionFactory对象调用覆写的openSession方法：

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

得到一个定义了ExecutorType为configuration的默认执行器SIMPLE,事务隔离级别为null,JDBC事务自动提交为false的DefaultSqlSession对象。

获取MapperProxy代理对象

有了DefaultSqlSession对象，以查询一条数据为例，来看一下整个处理过程。

For example:

SqlSession session = sqlSessionFactory.openSession();
try {
  BlogMapper mapper = session.getMapper(BlogMapper.class);
  Blog blog = mapper.selectBlog(101);
} finally {
  session.close();
}

Mybatis时序图

Mybatis时序图

根据MyBatis文档推荐的方法，调用Mapper接口中的方法实现对数据库的操作，上述例子中根据blog ID获取Blog对象。
通过DefaultSqlSession对象的getMapper方法获取的是一个MapperProxy代理对象，这也是Mapper接口不用实现类的原因。当调用BlogMapper中的方法时，由于BlogMapper是一个JDK动态代理对象，它会运行invoke方法，代码如下：

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      //判断代理对象是否是一个类
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //生成MapperMethod对象
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //执行execute方法
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }
...

invoke方法判断代理的对象是否是一个类，由于代理对象是一个接口，所以通过cachedMapperMethod生成一个MappedMethod对象，然后执行execute方法，execute方法代码如下：

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
      Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

因为这里是根据ID查询一个对象，所以最终调用了DefaultSqlSession的selectOne方法，selectOne方法又调用自身selectList方法，最终将查询操作委托给Executor：

@Override
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.<T>selectList(statement, parameter);
    if (list.size() == 1) {
      return list.get(0);
    } else if (list.size() > 1) {
      throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
      return null;
    }
  }

public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }

  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //根据id获取MappedStatement对象
      MappedStatement ms = configuration.getMappedStatement(statement);
      //wrapCollection方法处理集合参数
      //委托Exector执行SQL
      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();
    }
  }

Executor

Executor

Executor在MyBatis加载全局配置文件时初始化，我们可以在全局配置文件settings元素中配置Executor类型，MyBatis默认使用SimpleExecutor，如果开启了二级缓存，则再用CachingExecutor进行包装。

如果开启了二级缓存，SqlSession调用CachingExecutor执行器的query方法，先从二级缓存获取数据，当无法从二级缓存获取数据时，则委托给BaseExecutor的子类进行操作，CachingExecutor执行过程代码如下：

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    //判断是否有二级缓存  
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")

        //从二级缓存获取数据
        List<E> list = (List<E>) tcm.getObject(cache, key);
        //如果二级缓存没有数据则委托给BaseExcutor的子类进行操作
        if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    //如果没有二级缓存则委托给BaseExcutor的子类进行操作
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

MyBatis默认使用SimpleExecutor，调用父类BaseExecutor的query方法：

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++;
      //从一级缓存获取数据
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        //如果一级缓存没有数据，则从数据库获取
        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;
  }

最后，queryFromDatabase方法则调用SimpleExecutor的doQuery方法，通过Configuration构建StatementHandler对SQL和参数编译，parameterize()方法通过ParameterHandler对参数进行设置，使用TypeHandler转换参数类型，执行查询后再通过ResultSetHandler封装结果并返回，其方法代码如下：

 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();
      //根据Configuration构建StatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //编译SQL
      stmt = prepareStatement(handler, ms.getStatementLog());
      //ResultSetHandler处理结果，并返回处理后的结果
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    //处理SQL参数
    handler.parameterize(stmt);
    return stmt;
  }

通过以上流程发现，MyBatis核心工作实际上是由Executor、StatementHandler、ParameterHandler和ResultSetHandler四个接口完成的，掌握这四个接口的工作原理，对理解MyBatis底层工作原理有很大帮助。

StatementHandler

StatementHandler

StatementHandler接口设计采用了适配器模式， 实现类RoutingStatementHandler根据上下文来选择适配器生成相应的StatementHandler。三个适配器分别是SimpleStatementHandler、PreparedStatementHandler和CallableStatementHandler。StatementHandler初始化过程如下：

//在Configuration中构建StatementHandler
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

RoutingStatementHandler构建过程：

public class RoutingStatementHandler implements StatementHandler {

  private final StatementHandler delegate;

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    //根据我们映射配置文件中的SQL选择适配器，MyBatis默认使用PreparedStatement
    switch (ms.getStatementType()) {
      case STATEMENT:
        //SimpleStatementHandler对应JDBC中的Statement
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        //PreparedStatementHandler对应JDBC中PreparedStatement
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        //CallableStatementHandler对应JDBC中CallableStatement
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

通常我们使用PreparedStatementHandler,调用父类prepare方法，对SQL预编译：

@Override
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      statement = instantiateStatement(connection);
      setStatementTimeout(statement, transactionTimeout);
      setFetchSize(statement);
      return statement;
    } catch (SQLException e) {
      closeStatement(statement);
      throw e;
    } catch (Exception e) {
      closeStatement(statement);
      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
    }
  }

通过PreparedStatementHandler的instantiateStatement方法可以看到，这里实际就是在调用JDBC中的prepareStatement方法进行SQL的预编译，query方法则是调用JDBC的execute方法来执行编译好的SQL和返回结果：

@Override
  protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      //JDBC方法创建PrepareStatement对象
      if (keyColumnNames == null) {
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
                return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() != null) {
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      return connection.prepareStatement(sql);
    }
  }
  
  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    //JDBC中PreparedStatement对象的execute()方法执行SQL
    ps.execute();
    //ResultSetHandler对结果进行封装和返回
    return resultSetHandler.<E> handleResultSets(ps);
  }

ParameterHandler

ParameterHandler接口作用就是设置预编译SQL的参数：

public interface ParameterHandler {

  Object getParameterObject();

  void setParameters(PreparedStatement ps)
      throws SQLException;

}

接口提供getParameterObject和setParameters方法，前者作用获取参数对象，后者作用是设置预编译SQL的参数，由实现类DefaultParameterHandler执行，使用TypeHandler将参数对象类型转换成jdbcType，完成预编译SQL的参数设置。

  @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          }
          //验证参数类型
          else if
 (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          //设置参数类型为jdbcType
          try {
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          } catch (SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }

ResultSetHandler

ResultSetHandler接口根据配置文件中定义的规则将结果映射成相应对象，接口定义了三个方法，源码如下：

public interface ResultSetHandler {
  // 处理结果集，映射成对应的对象集合
  <E> List<e> handleResultSets(Statement stmt) throws SQLException;
  // 处理结果集，返回相应的游标对象
  <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;
  // 处理储存过程输出参数
  void handleOutputParameters(CallableStatement cs) throws SQLException;
}

ResultSetHandler接口的具体实现是DefaultResultSetHandler，我们通过SELECT语句执行得到的结果集由其handleResultSets方法处理，方法如下：

@Override
  public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    // 生成ArrayList用于保存结果集映射的对象
    final List<Object> multipleResults = new ArrayList<Object>();

    int resultSetCount = 0;
    // 获取第一个ResultSet对象
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    // 获取MyBatis初始化时解析映射器配置文件中的resultMap节点生成的ResultMap对象
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    // 验证resultMap是否存在
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      // 根据映射规则将查询结果映射成ResultMap并放入multipleResults集合中
      handleResultSet(rsw, resultMap, multipleResults, null);
      // 获取下一个结果
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      //递增直到所有结果映射完成
      resultSetCount++;
    }
    //此处省略resultSets多结果集处理方法
    //...
    return collapseSingleResultList(multipleResults);
  }

通过对4个核心功能接口作用的简单分析，我们对MyBatis底层工作原理就有了初步认识，但是要进一步深入理解MyBatis框架原理还需要对MyBatis其他组件的原理进行探究