MyBatis源码学习笔记（十）SQL执行流程分析

SQL执行流程分析

前面几篇文章，主要讲了对mybatis配置文件的解析，并且把他set到configuration这个对象中去，所以之后就要开始建立SqlSessionFactory了。

@Test    public void findStudentById() {        //首先通过getSessionFactory()方法获取SqlSessionFactory        SqlSessionFactory sqlSessionFactory = getSessionFactory();        //其次通过SqlSessionFactory中的openSession方法获取SqlSession        SqlSession sqlSession = sqlSessionFactory.openSession();        StudentDao studentDao = sqlSession.getMapper(StudentDao.class);        Student student = studentDao.findStudentById("20140101");        List<Course> courseList = student.getCourseList();        for (Course course: courseList) {            System.out.println(course.getId() + "   " + course.getName());        }    }    //Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互    private static SqlSessionFactory getSessionFactory() {        SqlSessionFactory sessionFactory = null;        String resource = "xml/configuration.xml";        try {        //而SqlSessionFactory是通过SqlSessionFactoryBuilder中的build方法去建立            sessionFactory = new SqlSessionFactoryBuilder().build(Resources                    .getResourceAsReader(resource));        } catch (IOException e) {            e.printStackTrace();        }        return sessionFactory;    }

1、SqlSessionFactoryBuilder中的build方法:

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 而parser.parse()方法返回的就是configuration对象 而configuration对象中存储的就是mybatis配置文件的信息      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);  }

2、当我们创建到SqlSessionFactory之后，就可以通过SqlSessionFactory的openSession方法去获取SqlSession对象。

public interface SqlSessionFactory {//这里可以根据不同的参数获取SqlSession 而在例子中使用的是第一种  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();}//然后实现方式选DefaultSqlSessionFactory，因为当初我们创建的也是DefaultSqlSessionFactory。 public SqlSession openSession() {    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);  }//通常一系列openSession方法最终都会调用本方法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);      //创建了一个DefaultSqlSession对象(关键)      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();    }  }

3、当拿到SqlSession之后，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。但在例子中，我们是用dao+xml映射文件的方式集中管理sql，所以要用到mybatis的MapperProxy，动态代理我们自己的dao。

 public void findStudentById() {        SqlSessionFactory sqlSessionFactory = getSessionFactory();        SqlSession sqlSession = sqlSessionFactory.openSession();        //通过sqlSession中的getMapper方法通过传入一个Class 获取对应的dao        StudentDao studentDao = sqlSession.getMapper(StudentDao.class);        Student student = studentDao.findStudentById("20140101");        List<Course> courseList = student.getCourseList();        for (Course course: courseList) {            System.out.println(course.getId() + "   " + course.getName());        }    } //直接去configuration中找 public <T> T getMapper(Class<T> type) {    return configuration.<T>getMapper(type, this);  } //configuration扔给了mapperRegistry去处理 public <T> T getMapper(Class<T> type, SqlSession sqlSession) {    return mapperRegistry.getMapper(type, sqlSession);  } @SuppressWarnings("unchecked")  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {  //mapperRegistry交给MapperProxyFactory去处理    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);    if (mapperProxyFactory == null)      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");    try {    //关键方法      return mapperProxyFactory.newInstance(sqlSession);    } catch (Exception e) {      throw new BindingException("Error getting mapper instance. Cause: " + e, e);    }  } @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);  }

4、通过MapperProxy动态代理dao， 也就是说， 当执行自己写的dao里面的方法的时候，其实是对应的mapperProxy在代理，MapperProxy是怎么做的？

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);    //主要交给MapperMethod自己去管    return mapperMethod.execute(sqlSession, args);  }//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 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;  } //回到SqlSession 选择了selectList来分析: 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      List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);      return result;    } catch (Exception e) {      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);    } finally {      ErrorContext.instance().reset();    }  } // Excetor的query方法 这里我们选择BeaseExcetor: 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);    //执行查询方法    return query(ms, parameter, rowBounds, resultHandler, key, boundSql); } 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 {      //这里从数据库中去获取        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);      }    } finally {      queryStack--;    }    if (queryStack == 0) {      for (DeferredLoad deferredLoad : deferredLoads) {        deferredLoad.load();      }      deferredLoads.clear(); // issue #601      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {        clearLocalCache(); // issue #482      }    }    return list;  } //queryFromDatabase方法:  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);    try {      //通过一层一层的调用，最终会来到doQuery 而这个方法就是关键      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);    } finally {      localCache.removeObject(key);    }    localCache.putObject(key, list);    if (ms.getStatementType() == StatementType.CALLABLE) {      localOutputParameterCache.putObject(key, parameter);    }    return list;  } //这里我们随便选择一个 例：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);    }  } //看看StatementHandler 的一个实现类 PreparedStatementHandler（这也是我们最常用的，封装的是PreparedStatement） public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {    PreparedStatement ps = (PreparedStatement) statement;    ps.execute();    //结果交给了ResultSetHandler 去处理    return resultSetHandler.<E> handleResultSets(ps);  }