mybatis源码学习三

SqlSessionFactory 与 SqlSession
sqlSession操作流程
Excutor真正执行的调用

开始学习mybatis整个运行流程，SQL执行流程分析

SqlSessionFactory 与 SqlSession

获取SqlSession的流程图

获取SqlSession的流程图

首先，SqlSessionFactoryBuilder去读取mybatis的配置文件，然后build一个DefaultSqlSessionFactory

/**
   * 一系列的构造方法最终都会调用本方法（配置文件为Reader时会调用本方法，还有一个InputStream方法与此对应）
   * @param reader
   * @param environment
   * @param properties
   * @return
   */
  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      //通过XMLConfigBuilder解析配置文件，解析的配置相关信息都会封装为一个Configuration对象
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      //这儿创建DefaultSessionFactory对象
      return build(parser.parse());
    } 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);
  }

当我们获取到SqlSessionFactory之后，就可以通过SqlSessionFactory去获取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， 实际呢，其实是通过excutor执行， excutor是对于Statement的封装
      final Executor executor = configuration.newExecutor(tx, execType);
      //关键看这儿，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();
    }
  }

得到SqlSession对象了。接下来就是该干嘛干嘛去了（话说还能干嘛，当然是执行sql语句咯）。看了上面，咱们也回想一下之前写的Demo:

SqlSessionFactory sessionFactory = null;  
String resource = "mybatis-conf.xml";  
try {
     //SqlSessionFactoryBuilder读取配置文件
    sessionFactory = new SqlSessionFactoryBuilder().build(Resources  
              .getResourceAsReader(resource));
} catch (IOException e) {  
    e.printStackTrace();  
}    
//通过SqlSessionFactory获取SqlSession
SqlSession sqlSession = sessionFactory.openSession();

SqlSession咱们也拿到了，咱们可以调用SqlSession中一系列的select…, insert…, update…, delete…方法轻松自如的进行CRUD操作了。

sqlSession操作流程

利器之MapperProxy:

通过MapperProxy动态代理咱们的dao，也就是说，当咱们执行自己写的dao里面的方法的时候，其实是对应的mapperProxy在代理。那么，咱们就看看怎么获取MapperProxy对象：

通过SqlSession从Configuration中获取

/**
   * 什么都不做，直接去configuration中找， 哥就是这么任性
   */
  @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

SqlSession把包袱甩给了Configuration, 接下来就看看Configuration。源码如下：

/**
   * 烫手的山芋，俺不要，你找mapperRegistry去要
   * @param type
   * @param sqlSession
   * @return
   */
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

Configuration不要这烫手的山芋，接着甩给了MapperRegistry，那咱看看MapperRegistry。源码如下：

/**
   * 烂活净让我来做了，没法了，下面没人了，我不做谁来做
   * @param type
   * @param sqlSession
   * @return
   */
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //能偷懒的就偷懒，俺把粗活交给MapperProxyFactory去做
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      //关键在这儿
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

MapperProxyFactory是个苦B的人，粗活最终交给它去做了。咱们看看源码：

/**
   * 别人虐我千百遍，我待别人如初恋
   * @param mapperProxy
   * @return
   */
  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    //动态代理我们写的dao接口
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

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

通过以上的动态代理，咱们就可以方便地使用dao接口啦，就像之前咱们写的demo那样：

UserDao userMapper = sqlSession.getMapper(UserDao.class);  
 User insertUser = new User();

Excutor真正执行的调用

Excutor

拿到了MapperProxy, 每个MapperProxy对应一个dao接口，那么咱们在使用的时候，MapperProxy是怎么做的呢？源码奉上： MapperProxy:

/**
   * MapperProxy在执行时会触发此方法
   */
  @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);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //二话不说，主要交给MapperMethod自己去管
    return mapperMethod.execute(sqlSession, args);
  }

MapperMethod:

/**
   * 看着代码不少，不过其实就是先判断CRUD类型，然后根据类型去选择到底执行sqlSession中的哪个方法，绕了一圈，又转回sqlSession了
   * @param sqlSession
   * @param args
   * @return
   */
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    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 {
      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;
  }

如果是选择了selectList方法：

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      //CRUD实际上是交给Excetor去处理， excutor其实也只是穿了个马甲而已，小样，别以为穿个马甲我就不认识你嘞！
      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();
    }
  }

然后，通过一层一层的调用，最终会来到doQuery方法，这儿咱们就随便找个Excutor看看doQuery方法的实现吧，我这儿选择了SimpleExecutor:

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());
      //StatementHandler封装了Statement, 让 StatementHandler 去处理
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

接下来，咱们看看StatementHandler 的一个实现类 PreparedStatementHandler（这也是我们最常用的，封装的是PreparedStatement）, 看看它使怎么去处理的：

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
     //到此，原形毕露， PreparedStatement, 这个大家都已经滚瓜烂熟了吧
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    //结果交给了ResultSetHandler 去处理
    return resultSetHandler.<E> handleResultSets(ps);
  }

到此，一次sql的执行流程就完了。

mybatis源码学习三

SqlSessionFactory 与 SqlSession

sqlSession操作流程

Excutor真正执行的调用

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