获取sqlSession流程

• new SqlSessionFactoryBuilder对象
• sqlSessionFactoryBuilder对象new出一个XMLConfigBuilder对象，读取mybatis-config.xml配置文件信息，返回configuration
• sqlsessionFactoryBuilder调用build(configuration方法)，返回defaultSqlSessionFactory
• sqlSessionFactory调用opensession方法，返回sqlSession

获取sqlSession流程.png

• openSessionFromDataSource源码
• • 从配置中获取Environment；
• • 从Environment中取得DataSource；
• • 从Environment中取得TransactionFactory；
• • 从DataSource里获取数据库连接对象Connection；
• • 在取得的数据库连接上创建事务对象Transaction；
• • 创建Executor对象（该对象非常重要，事实上sqlsession的所有操作都是通过它完成的）；
• • 创建sqlsession对象；

/**
    * 通常一系列openSession方法最终都会调用本方法
    * @param execType 
    * @param level
    * @param autoCommit
    * @return
    */
 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
     Transaction tx = null;
     try {
         //通过Confuguration对象去获取Mybatis相关配置信息, Environment对象包含了数据源和事务的配置
         final Environment environment = configuration.getEnvironment();
         final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
         tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
         //之前说了，从表面上来看，咱们是用sqlSession在执行sql语句，通过excutor执行， excutor是对于Statement的封装
         final Executor executor = configuration.newExecutor(tx, execType);
         //创建了一个DefaultSqlSession对象
         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();
     }
 }

• MapperProxy分析

  
  
  mapperProxy获取流程.png

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //获取代理工厂，在解析配置文件时，会在knownMappers中放入key为Class<T>,value为代理工厂类对象
    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);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

  protected T newInstance(MapperProxy<T> mapperProxy) {
    //jdk动态代理
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

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

//代理类实现 jdk动态代理接口
public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;

  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    //通过MapperMethod来真正的实现增删改查功能
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //执行增删改查
    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;
  }

}

• excutor分析

  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //根据不同的sqlCommandType类型，以及不同的返回类型，调用不同的方法
    if (SqlCommandType.INSERT == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
    } else if (SqlCommandType.SELECT == command.getType()) {
      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 {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else if (SqlCommandType.FLUSH == command.getType()) {
        result = sqlSession.flushStatements();
    } else {
      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;
  }

 @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //获取mapperStatement信息
      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();
    }
  }

  //最终会调用这个方法中
  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();
      //获取statement代理对象   并且或创建resultehander代理对象
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }