MyBatis系列之浅谈SQL执行流程分析

- 日理万妓 2023-03-12 07:30 8阅读 0赞

### 目录 ###

*  独立使用Mybatis
 *  Mybatis执行流程
 *   *  SqlSessionFactory\\SqlSession
     *  MapperProxy
     *  Excutor

# 独立使用Mybatis #

这篇文章主要以分析Mybatis框架执行SQL的流程。  
回忆曾经独立使用Mybatis半自动化框架时，我们需要执行以下步骤：

1.  读取配置文件（mybatis-config.xml）,初始化配置类即configuration;
2.  创建SQLSessionFactory;
3.  创建SqlSession;
4.  执行SQL，处理结果集  
    对应如下代码：

public class MyBatisUtils {
        private final static SqlSessionFactory SQL_SESSION_FACTORY;
        static {
            String resource = "mybatis-config.xml";
            Reader reader = null;
            try {
                reader = Resources.getResourceAsReader(resource);
            } catch (IOException e) {
                e.printStackTrace();
            }
            SQL_SESSION_FACTORY = new SqlSessionFactoryBuilder().build(reader);
        }
        public static SqlSessionFactory getSqlSessionFactory() {
            return SQL_SESSION_FACTORY;
        }
    }
    
    //单元测试
    public class MapperTest {
        static SqlSessionFactory sqlSessionFactory = null;
        static {
            sqlSessionFactory = MyBatisUtils.getSqlSessionFactory();
        }
    
        @Test
        public void testAdd() {
            SqlSession sqlSession = sqlSessionFactory.openSession();
            try {
                UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
                User user = new User();
                user.setUsername("hello");
                user.setAge(5);
                userMapper.insert(user);
                sqlSession.commit();    // 这里一定要提交，不然数据进不去数据库中
            } finally {
                sqlSession.close();
            }
        }
    
        @Test
        public void getUser() {
            SqlSession sqlSession = sqlSessionFactory.openSession();
            try {
                UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
                User user = userMapper.selectByPrimaryKey(32L);
                System.out.println("name: " + user.getUsername() + "| age: " + user.getAge());
            } finally {
            }
        }
    }

# Mybatis执行流程 #

基于上面说明，我们基本了解了执行流程，下面我们从源码层面解释一下流程。

## SqlSessionFactory\\SqlSession ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1YW5fbHU_size_16_color_FFFFFF_t_70_pic_center]  
独立使用Mybatis时，第一步要读取配置文件，因此，我们将从读取配置文件开始。

1.  SqlSessionFactoryBuilder读取mybatis的配置文件，调用build方法创建DefaultSqlSessionFactory

/**通过XMLConfigBuilder解析mybatis配置，创建SQLSessionFactory对象*/
        public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
            try {
                XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
                //parse()方法XMLConfigBuilder类解析xml文件,同时完成configuration属性的创建
                return build(parser.parse());
            } catch (Exception e) {
                throw ExceptionFactory.wrapException("Error building SqlSession.", e);
            } finally {
                ErrorContext.instance().reset();
                try {
                    inputStream.close();
                } catch (IOException e) {
                    // Intentionally ignore. Prefer previous error.
                }
            }
        }
    
        /**
         * 以上build()方法默认都是调用此方法，创建DefaultSqlSessionFactory对象
         * 解析mybatis配置文件xml后，生成Configuration对象，然后生成SQLSessionFactory对象
         * @param config
         * @return
         */
        public SqlSessionFactory build(Configuration config) {
            return new DefaultSqlSessionFactory(config);
        }

1.  获取SqlsessionFactory之后，调用openSession()方法创建SQLSession，这里说明一下SQLSession仅仅**向外提供了对数据库操作的支持**，真正执行对数据库的操作是execute的职责。

DefaultSqlSessionFactory类
    /**
       * 通常一系列openSession方法都会调用该方法
       * @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);
          //创建SqlSession
          return new DefaultSqlSession(configuration, executor, autoCommit);
        } catch (Exception e) {
          // may have fetched a connection so lets call close()
          closeTransaction(tx);
          throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }

获取SQLSession之后，对数据进行CRUD操作的准备工作就正式结束。

## MapperProxy ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1YW5fbHU_size_16_color_FFFFFF_t_70_pic_center 1]  
主要处理我们Mybatis的映射文件。该类主要负责代理开发中的mapper。那么思考一下该代理对象如何获取呢？  
下面我们从SQLSession中跟踪。

DefaultSQLSession类中
    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);
      }
    
    
    /**
         * 获取Mapper接口的代理对象
         */
        @SuppressWarnings("unchecked")
        public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
            //从缓存中获取该Mapper接口的代理工厂对象
            final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
            //如果该Mapper接口没有注册过，则抛异常
            if (mapperProxyFactory == null) {
                throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
            }
            try {
                //**使用代理工厂创建Mapper接口的代理对象**重点部分
                return mapperProxyFactory.newInstance(sqlSession);
            } catch (Exception e) {
                throw new BindingException("Error getting mapper instance. Cause: " + e, e);
            }
        }
    
    MapperProxyFactory类
     /**
         * 创建mapperInterface的代理对象
         */
        @SuppressWarnings("unchecked")
        protected T newInstance(MapperProxy<T> mapperProxy) {
            return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);
        }
    
        public T newInstance(SqlSession sqlSession) {
            final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
            return newInstance(mapperProxy);
        }

一路跟踪下来，我们发现MapperProxy对象是在MapperProxyFactory里创建完成。

## Excutor ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1YW5fbHU_size_16_color_FFFFFF_t_70_pic_center 2]  
上面我们提到Excutor的职责是负责对数据的crud操作，上面的时序图，详细地说明了SQL的执行过程。  
对于**每一个MapperProxy对应开发人员自定的Mapper(dao)接口**,下面我们将从源码追踪，如何实现的。

*  MappProxy:

/**代理对象执行的方法，代理以后，所有Mapper的方法调用时，都会调用这个invoke方法*/
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            try {
                //判断是否为通用类，mybatis使用JDK代理方式，即面向接口，false
                //method.getDeclaringClass()返回底层的类对象的class
                if (Object.class.equals(method.getDeclaringClass())) {
                    //如果是类，则利用反射机制，返回目标对象
                    return method.invoke(this, args);
                } else if (isDefaultMethod(method)) {
                    //如果是默认方法，则执行默认方法，java1.8提供；
                    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);
            //执行方法对应的SQL语句，返回查询的结果
            /**
             * 1、先判断SQL的方法类型，insert、select、update
             * 2、根据参数列表的值（value），转成参数名称和参数值对应关系的map对象
             * 3、根据方法名和参数列表的对应关系，查询出结果列表
             */
            return mapperMethod.execute(sqlSession, args);
        }

*  MapperMethod:

/**
         * 核心方法，用于执行对应的SQL语句
         * @param sqlSession
         * @param args
         * @return
         */
        public Object execute(SqlSession sqlSession, Object[] args) {
            Object result;
            switch (command.getType()) {
                case INSERT: {
                    //param 为参数名和参数值的对应关系，即map集合
                    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()) {
                        //void类型且方法中有ResultHandler参数（特殊参数），调用sqlSession.select执行
                        executeWithResultHandler(sqlSession, args);
                        result = null;
                    } else if (method.returnsMany()) {
                        //返回类型是集合或者数组，调用sqlSession.<E>selectList执行
                        result = executeForMany(sqlSession, args);
                    } else if (method.returnsMap()) {
                        //返回 map ,调用 sqlSession.<K, V>selectMap
                        result = executeForMap(sqlSession, args);
                    } else if (method.returnsCursor()) {
                        result = executeForCursor(sqlSession, args);
                    } else {
                        Object param = method.convertArgsToSqlCommandParam(args);
                        result = sqlSession.selectOne(command.getName(), param);
                        if (method.returnsOptional() &&
                                (result == null || !method.getReturnType().equals(result.getClass()))) {
                            result = Optional.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;
        }

本篇文章主要是浅析SQL的执行流程，因此我们建议以selectList为例，带领大家熟悉一下流程，后面会专门介绍  
各个步骤的流程。  
回归到SQLSession

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
        try {
          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();
        }
      }

*  BaseExecutor:

/**
         * 查询方法，专门提供select执行的方法
         */
        @Override
        public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
            //获取查询SQL
            BoundSql boundSql = ms.getBoundSql(parameter);
            //创建缓存的key，即作为HashMap中的key
            CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
            //执行查询
            return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }

*  SimpleExecutor:

@Override
      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());
          return handler.query(stmt, resultHandler);
        } finally {
          closeStatement(stmt);
        }
      }

*  preparedStatement:

@Override
      public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        PreparedStatement ps = (PreparedStatement) statement;
        ps.execute();
        return resultSetHandler.handleResultSets(ps);
      }

参考资料：Spring源码深度解析第二版

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