深入Mybatis源码——执行流程

前言

上一篇分析Mybatis是如何加载解析XML文件的,本篇紧接上文,分析Mybatis的剩余两个阶段:代理封装SQL执行

正文

代理封装

Mybatis有两种方式调用Mapper接口:

private static SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);

// 第一种
try (SqlSession session = sqlMapper.openSession(TransactionIsolationLevel.SERIALIZABLE)) {
  Blog blog = session.selectOne("org.apache.ibatis.domain.blog.mappers.BlogMapper.selectBlogWithPostsUsingSubSelect", 1);
}

// 第二种
try (SqlSession session = sqlMapper.openSession()) {
  AuthorMapper mapper = session.getMapper(AuthorMapper.class);
  Author author = mapper.selectAuthor(101);
}

从上面代码可以看到无论是哪一种首先都要创建SqlSessionFactory对象,然后通过这个对象拿到SqlSession对象。在早期版本中只能通过该对象的增删改调用Mapper接口,很明显这种方式可读性很差,难以维护,写起来也复杂,所以后面谷歌开始维护Mybatis后,重新封装提供了第二种方式直接调用Mapper接口。不过本质上第二种是在第一种的基础之上实现的,所以下面就以第二种为主进行分析,进入到getMapper方法:

  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

  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);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

mapperRegistry对象在上一篇分析过,是在解析xml中的mapper节点时注册进去的,而这个对象中缓存了Mapper接口和对应的代理工厂的映射,所以getMapper的核心就是通过这个工厂去创建代理对象:

  public T newInstance(SqlSession sqlSession) {
	 //每次调用都会创建新的MapperProxy对象
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

然后通过Mapper接口调用时首先就会调用到MapperProxyinvoke方法:

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {//如果是Object本身的方法不增强
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //从缓存中获取mapperMethod对象,如果缓存中没有,则创建一个,并添加到缓存中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //调用execute方法执行sql
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
  }

首先从缓存中拿到MapperMethod对象,这个对象封装了SQL语句的类型、命名空间、入参、返回类型等信息,然后通过它的execute方法调用SqlSession的增删查改方法:

  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //根据sql语句类型以及接口返回的参数选择调用不同的
    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()) {//返回值为void
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {//返回值为集合或者数组
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {//返回值为map
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {//返回值为游标
          result = executeForCursor(sqlSession, args);
        } else {//处理返回为单一对象的情况
          //通过参数解析器解析解析参数
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional() &&
              (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = OptionalUtil.ofNullable(result);
          }
        }
        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;
  }

上文说过SqlSession本质上是门面模式的体现,其本质上是通过Executor执行器组件实现的,在该组件中定义了所有访问数据库的方法:

  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //从configuration中获取要执行的sql语句的配置信息
      MappedStatement ms = configuration.getMappedStatement(statement);
      //通过executor执行语句,并返回指定的结果集
      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对象是在获取SqlSession时创建的:

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

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
    	//获取mybatis配置文件中的environment对象
      final Environment environment = configuration.getEnvironment();
      //从environment获取transactionFactory对象
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //创建事务对象
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //根据配置创建executor
      final Executor executor = configuration.newExecutor(tx, execType);
      //创建DefaultSqlSession
      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();
    }
  }

TransactionFactory是我们在xml中配置的transactionManager属性,可选的属性有JDBC和Managed,然后根据我们的配置创建事务对象,之后才是创建Executor对象。

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    //如果有<cache>节点,通过装饰器,添加二级缓存的能力
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    //通过interceptorChain遍历所有的插件为executor增强,添加插件的功能
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

Executor有三个基本的实现类:

  • BatchExecutor:批处理执行器,执行批量更新、插入等操作。
  • ReuseExecutor:可重用执行器,缓存并重用Statement(Statement、PreparedStatement、CallableStatement)。
  • SimpleExecutor:默认使用的执行器,每次执行都会创建 新的Statement

这三个执行器都继承了自抽象的BaseExecutor,同时如果开启了二级缓存功能,在这里还会装饰一个CachingExecutor为其添加二级缓存的能力。另外还要注意在这段代码的最后还有拦截器进行了包装,也就是扩展插件的实现 ,关于这部分内容在一篇进行分析。

SQL执行

二级缓存的代码很简单,这里直接略过,所以直接进入到BaseExecutor.query方法:

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
	//获取sql语句信息,包括占位符,参数等信息
    BoundSql boundSql = ms.getBoundSql(parameter);
    //拼装缓存的key值
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    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) {//检查当前executor是否关闭
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {//非嵌套查询,并且FlushCache配置为true,则需要清空一级缓存
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;//查询层次加一
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;//查询以及缓存
      if (list != null) {
    	 //针对调用存储过程的结果处理
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
    	 //缓存未命中,从数据库加载数据
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    
    
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {//延迟加载处理
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {//如果当前sql的一级缓存配置为STATEMENT,查询完既清空一集缓存
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

首先从一级缓存localCache里面拿,如果没有,才真正地访问数据库,并将返回结果存入到一级缓存中。

  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);//在缓存中添加占位符
    try {
      //调用抽象方法doQuery,方法查询数据库并返回结果,可选的实现包括:simple、reuse、batch
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);//在缓存中删除占位符
    }
    localCache.putObject(key, list);//将真正的结果对象添加到一级缓存
    if (ms.getStatementType() == StatementType.CALLABLE) {//如果是调用存储过程
      localOutputParameterCache.putObject(key, parameter);//缓存输出类型结果参数
    }
    return list;
  }

这里的doQuery是子类实现的,即模板模式,以SimpleExecutor为例:

  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configuration对象
      //创建StatementHandler对象,
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //StatementHandler对象创建stmt,并使用parameterHandler对占位符进行处理
      stmt = prepareStatement(handler, ms.getStatementLog());
      //通过statementHandler对象调用ResultSetHandler将结果集转化为指定对象返回
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

通读这里的代码我们可以发现,Executor本身是不会访问到数据库,而是作为指挥官,指挥三个小弟干事:

  • StatementHandler:创建PreparedStatementStatementCallableStatement对象。
  • ParameterHandler:在StatementHandler构造函数中创建,对预编译的 SQL 语句进行参数设置。
  • ResultSetHandler:在StatementHandler构造函数中创建,对数据库返回的结果集(ResultSet)进行封装,返回用户指定的实体类型。

上面三个对象都是在configuration.newStatementHandler方法中创建的,然后调用prepareStatement拿到合适的Statement,如果是预编译的还会进行参数设置:

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    //获取connection对象的动态代理,添加日志能力;
    Connection connection = getConnection(statementLog);
    //通过不同的StatementHandler,利用connection创建(prepare)Statement
    stmt = handler.prepare(connection, transaction.getTimeout());
    //使用parameterHandler处理占位符
    handler.parameterize(stmt);
    return stmt;
  }

如果在DEBUG模式下拿到的Connection对象是ConnectionLogger,这就和第一篇的内容串联起来了。之后再通过query方法调用execute执行SQL语句,并使用ResultSetHandler处理结果集:

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

    int resultSetCount = 0;
    //statment可能返回多个结果集对象,这里先取出第一个结果集
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    //获取结果集对应resultMap,本质就是获取字段与java属性的映射规则
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);//结果集和resultMap不能为空,为空抛出异常
    while (rsw != null && resultMapCount > resultSetCount) {
     //获取当前结果集对应的resultMap
      ResultMap resultMap = resultMaps.get(resultSetCount);
      //根据映射规则(resultMap)对结果集进行转化,转换成目标对象以后放入multipleResults中
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);//获取下一个结果集
      cleanUpAfterHandlingResultSet();//清空nestedResultObjects对象
      resultSetCount++;
    }
    //获取多结果集。多结果集一般出现在存储过程的执行,存储过程返回多个resultset,
    //mappedStatement.resultSets属性列出多个结果集的名称,用逗号分割;
    //多结果集的处理不是重点,暂时不分析
    String[] resultSets = mappedStatement.getResultSets();
    if (resultSets != null) {
      while (rsw != null && resultSetCount < resultSets.length) {
        ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
        if (parentMapping != null) {
          String nestedResultMapId = parentMapping.getNestedResultMapId();
          ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
          handleResultSet(rsw, resultMap, null, parentMapping);
        }
        rsw = getNextResultSet(stmt);
        cleanUpAfterHandlingResultSet();
        resultSetCount++;
      }
    }

    return collapseSingleResultList(multipleResults);
  }

这里最终就是通过反射模块以及Configuration类中的result相关配置进行结果映射:

  private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
    try {
      if (parentMapping != null) {//处理多结果集的嵌套映射
        handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
      } else {
        if (resultHandler == null) {//如果resultHandler为空,实例化一个人默认的resultHandler
          DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
          //对ResultSet进行映射,映射结果暂存在resultHandler中
          handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
          //将暂存在resultHandler中的映射结果,填充到multipleResults
          multipleResults.add(defaultResultHandler.getResultList());
        } else {
          //使用指定的rusultHandler进行转换
          handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
        }
      }
    } finally {
      // issue #228 (close resultsets)
      //调用resultset.close()关闭结果集
      closeResultSet(rsw.getResultSet());
    }
  }

  public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
    if (resultMap.hasNestedResultMaps()) {//处理有嵌套resultmap的情况
      ensureNoRowBounds();
      checkResultHandler();
      handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    } else {//处理没有嵌套resultmap的情况
      handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    }
  }

  private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
      throws SQLException {
	//创建结果上下文,所谓的上下文就是专门在循环中缓存结果对象的
    DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
    //1.根据分页信息,定位到指定的记录
    skipRows(rsw.getResultSet(), rowBounds);
    //2.shouldProcessMoreRows判断是否需要映射后续的结果,实际还是翻页处理,避免超过limit
    while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
      //3.进一步完善resultMap信息,主要是处理鉴别器的信息
      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
      //4.读取resultSet中的一行记录并进行映射,转化并返回目标对象
      Object rowValue = getRowValue(rsw, discriminatedResultMap);
      //5.保存映射结果对象
      storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
  }

  private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
    final ResultLoaderMap lazyLoader = new ResultLoaderMap();
    //4.1 根据resultMap的type属性,实例化目标对象
    Object rowValue = createResultObject(rsw, resultMap, lazyLoader, null);
    if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
      //4.2 对目标对象进行封装得到metaObjcect,为后续的赋值操作做好准备
      final MetaObject metaObject = configuration.newMetaObject(rowValue);
      boolean foundValues = this.useConstructorMappings;//取得是否使用构造函数初始化属性值
      if (shouldApplyAutomaticMappings(resultMap, false)) {//是否使用自动映射
    	 //4.3一般情况下 autoMappingBehavior默认值为PARTIAL,对未明确指定映射规则的字段进行自动映射
        foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
      }
       //4.4 映射resultMap中明确指定需要映射的列
      foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
      foundValues = lazyLoader.size() > 0 || foundValues;
      //4.5 如果没有一个映射成功的属性,则根据<returnInstanceForEmptyRow>的配置返回null或者结果对象
      rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
    }
    return rowValue;
  }
  • 自动映射
  private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
	//获取resultSet中存在的,但是ResultMap中没有明确映射的列,填充至autoMapping中
    List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
    boolean foundValues = false;
    if (!autoMapping.isEmpty()) {
      //遍历autoMapping,通过自动匹配的方式为属性复制
      for (UnMappedColumnAutoMapping mapping : autoMapping) {
    	//通过typeHandler从resultset中拿值
        final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
        if (value != null) {
          foundValues = true;
        }
        if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
          // gcode issue #377, call setter on nulls (value is not 'found')
          //通过metaObject给属性赋值
          metaObject.setValue(mapping.property, value);
        }
      }
    }
    return foundValues;
  }
  • 指定映射
  private boolean applyPropertyMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, ResultLoaderMap lazyLoader, String columnPrefix)
      throws SQLException {
	//从resultMap中获取明确需要转换的列名集合
    final List<String> mappedColumnNames = rsw.getMappedColumnNames(resultMap, columnPrefix);
    boolean foundValues = false;
    //获取ResultMapping集合
    final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
    for (ResultMapping propertyMapping : propertyMappings) {
      String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);//获得列名,注意前缀的处理
      if (propertyMapping.getNestedResultMapId() != null) {
        // the user added a column attribute to a nested result map, ignore it
    	//如果属性通过另外一个resultMap映射,则忽略
        column = null;
      }
      if (propertyMapping.isCompositeResult()//如果是嵌套查询,column={prop1=col1,prop2=col2}
          || (column != null && mappedColumnNames.contains(column.toUpperCase(Locale.ENGLISH)))//基本类型映射
          || propertyMapping.getResultSet() != null) {//嵌套查询的结果
    	//获得属性值
        Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);
        // issue #541 make property optional
        //获得属性名称
        final String property = propertyMapping.getProperty();
        if (property == null) {//属性名为空跳出循环
          continue;
        } else if (value == DEFERED) {//属性名为DEFERED,延迟加载的处理
          foundValues = true;
          continue;
        }
        if (value != null) {
          foundValues = true;
        }
        if (value != null || (configuration.isCallSettersOnNulls() && !metaObject.getSetterType(property).isPrimitive())) {
          // gcode issue #377, call setter on nulls (value is not 'found')
          //通过metaObject为目标对象设置属性值
          metaObject.setValue(property, value);
        }
      }
    }
    return foundValues;
  }

反射实例化对象的代码比较长,但逻辑都比较清晰,上面的关键流程代码也都加上了注释,读者可自行参照源码阅读。

总结

Mybatis核心原理就分析完了,相比较Spring源码简单了很多,但代码的优雅度和优秀的设计思想一点也不亚于Spring,也是非常值得我们好好学习掌握的。不过这3篇只是分析了Mybaits的核心执行原理,另外还有插件怎么扩展、拦截器会拦截哪些方法以及Mybatis和Spring的整合又是怎么实现的呢?读者们可以好好思考下,答案将在下一篇揭晓。

posted @ 2020-07-07 22:47  夜勿语  阅读(178)  评论(0编辑  收藏