• mybatis源码(四)SqlSession执行mapper的过程下篇


      

    mybatis源码(四)SqlSession执行mapper的过程下篇

    代码案例:以SqlSession的selectList为例

    <!DOCTYPE mapper
            PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
            "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="mapper">
        <select id="selectUsers" resultType="com.forest.owl.entity.User">
            SELECT id, account, passwd, nickname, phone, email, create_time as createTime, update_time as updateTime
            FROM user
        </select>
    </mapper>
    
    public class TestUserMapper {
        private static SqlSessionFactory sqlSessionFactory;
        @BeforeClass
        public static void init(){
            try {
                Reader reader = Resources.getResourceAsReader("mybatis.config.xml");
                sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
                reader.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        @Test
        public void TestMybatis(){
            SqlSession sqlSession = sqlSessionFactory.openSession();
            List<User> userList = sqlSession.selectList("selectUsers");
            System.out.println(userList.size());
            System.out.println(userList.get(0).getNickname());
            System.out.println(userList.get(0).getCreateTime());
        }
    }
    

    首先SqlSession是一个接口,有一个DefaultSqlSession的实现类,实现类实现该接口的代码如下:

      @Override
      public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
        try {
          // 根据Mapper的Id,获取对应的MappedStatement对象
          MappedStatement ms = configuration.getMappedStatement(statement);
          // 以MappedStatement对象作为参数,调用Executor的query()方法
          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();
        }
      }
    

    configuration的mapperstatement的对象的值 是在SqlSession创建的时候,通过XmlConfigBuilder解析mybatis-config主配置文件的时候,添加进去的 configration中有一个map集合如下图所示

    protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");
    该属性是一个Map对象,它的key为mapperSql配置的id。如果sql是通过xml注释的。则id为命名空间加上<select|update|delete|insert标签的id> ,如果sql通过java注解配置。则id为mapper接口的全限定名称加上方法名称

    通过上述源码可以看到,实际操作数据库的事Excutor执行器,这里用到了一个外观模式,为了对用户提供友好的访问接口,Excutor执行器是一个接口,有几个实现类

    进入实现类BaseExecutor中,查看query的源码

    queryFromDatabase 这个地方由于缓存中没有我们要找的数据,所以从数据库中进行数据的读取,这个doQuery方法用到了模板方法设计模式根据上述的类图,可以看到有几个实现类,分别提供了不同的实现
     @Override
        public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
            // 获取BoundSql对象,BoundSql是对动态SQL解析生成的SQL语句和参数映射信息的封装
            BoundSql boundSql = ms.getBoundSql(parameter);
            // 创建CacheKey,用于缓存Key
            CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
            // 调用重载的query()方法
            return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    
        @SuppressWarnings("unchecked")
        @Override
        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++;
                // 从缓存中获取结果
                list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
                if (list != null) {
                    handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                    // 缓存中获取不到,则调用queryFromDatabase()方法从数据库中查询
                    list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
                queryStack--;
            }
            if (queryStack == 0) {
                for (DeferredLoad deferredLoad : deferredLoads) {
                    deferredLoad.load();
                }
                // issue #601
                deferredLoads.clear();
                if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                    // issue #482
                    clearLocalCache();
                }
            }
            return list;
        }
    
        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 {
                // 调用doQuery()方法查询
                list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
            } finally {
                localCache.removeObject(key);
            }
            // 缓存查询结果
            localCache.putObject(key, list);
            if (ms.getStatementType() == StatementType.CALLABLE) {
                localOutputParameterCache.putObject(key, parameter);
            }
            return list;
        }
    
        protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
                throws SQLException;
    
     实现类1:simpleExecutor 的实现方式
    @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();
        // 获取StatementHandler对象
        StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        // 调用prepareStatement()方法,创建Statement对象,并进行设置参数等操作
        stmt = prepareStatement(handler, ms.getStatementLog());
        // 调用StatementHandler对象的query()方法执行查询操作
        return handler.<E>query(stmt, resultHandler);
      } finally {
        closeStatement(stmt);
      }
    }
    

     statementhandler的创建,默认是PREPARED 类型的

    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));

    实现类2:BatchExecutor

    @Override
    public <E> List<E> doQuery(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
        throws SQLException {
      Statement stmt = null;
      try {
        flushStatements();
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
        Connection connection = getConnection(ms.getStatementLog());
        stmt = handler.prepare(connection, transaction.getTimeout());
        handler.parameterize(stmt);
        return handler.<E>query(stmt, resultHandler);
      } finally {
        closeStatement(stmt);
      }
    }
    

    实现类3:ReuseExecutor

    @Override
    public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      Statement stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    }
    Statement 操作数据库就是JDBC提供的操作数据的实现
    

    从三述三种实现类中可以看出StatementHandler是操作Statement的。

    StatementHandler也是一个接口,通过具体的实现类,来操作数据。StatementHandler的实现类有SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler
     
    实现类1:SimpleStatementHandler中的代码实现
      @Override
      public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        String sql = boundSql.getSql();
        statement.execute(sql);
        return resultSetHandler.<E>handleResultSets(statement);
      }
    

    实现类2:PreparedStatementHandler

      @Override
      public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        PreparedStatement ps = (PreparedStatement) statement;
        // 调用PreparedStatement对象的execute()方法,执行SQL语句
        ps.execute();
        // 调用ResultSetHandler的handleResultSets()方法处理结果集
        return resultSetHandler.<E> handleResultSets(ps);
      }
    

    实现类3:CallableStatementHandler

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

      

    通过上述三个statementHandler可以看出,对结果集的处理,是通过ResultSetHandler进行处理的。即:return resultSetHandler.<E>handleResultSets(statement);
    ResultSetHandler 是一个接口实现,只有一个实现类
    public interface ResultSetHandler {
    
      <E> List<E> handleResultSets(Statement stmt) throws SQLException;
    
      <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;
    
      void handleOutputParameters(CallableStatement cs) throws SQLException;
    
    }
    

      

    DefaultResultSetHandler

     //
      // 处理结果集
      //
      @Override
      public List<Object> handleResultSets(Statement stmt) throws SQLException {
        ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
        final List<Object> multipleResults = new ArrayList<Object>();
        int resultSetCount = 0;
        // 1、获取ResultSet对象,將ResultSet对象包装为ResultSetWrapper
        ResultSetWrapper rsw = getFirstResultSet(stmt);
        // 2、获取ResultMap信息,一般只有一个ResultMap
        List<ResultMap> resultMaps = mappedStatement.getResultMaps();
        int resultMapCount = resultMaps.size();
        // 校验ResultMap,如果该ResultMap名称没有配置,则抛出异常
        validateResultMapsCount(rsw, resultMapCount);
        // 如果指定了多个ResultMap,则对每个ResultMap进行处理
        while (rsw != null && resultMapCount > resultSetCount) {
          ResultMap resultMap = resultMaps.get(resultSetCount);
          // 3、调用handleResultSet方法处理结果集
          handleResultSet(rsw, resultMap, multipleResults, null);
          // 获取下一个结果集对象,需要JDBC驱动支持多结果集
          rsw = getNextResultSet(stmt);
          cleanUpAfterHandlingResultSet();
          resultSetCount++;
        }
        // 如果JDBC驱动支持多结果集,可以通过<select>标签resultSets属性指定多个ResultMap
        // 处理<select>标签resultSets属性,该属性一般情况不会指定
        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);
              //调用handleResultSet方法处理结果集
              handleResultSet(rsw, resultMap, null, parentMapping);
            }
            rsw = getNextResultSet(stmt);
            cleanUpAfterHandlingResultSet();
            resultSetCount++;
          }
        }
        // 对multipleResults进行处理,如果只有一个结果集,则返回结果集中的元素,否则返回多个结果集
        return collapseSingleResultList(multipleResults);
      }
      @SuppressWarnings("unchecked")
      private List<Object> collapseSingleResultList(List<Object> multipleResults) {
        return multipleResults.size() == 1 ? (List<Object>) multipleResults.get(0) : multipleResults;
      }
    

    至此,就完成了SqlSession.selectList的 流程,源码涉及的东西比较多,这就是个大致的流程  

     

    session.selectList("XXX"); 执行流程
    1.调用SqlSession接口的selectList()方法
    2.默认执行的是DefaultSqlSession的<E> List<E> selectList(String statement)方法
    2.1.根据Mapper的Id,从configuration对象中获取对应的MappedStatement对象
      2.2 通过Executor执行器,执行query方法(<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;)
      2.3 有2个实现类,选择一个进去BaseExecutor.query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException
        2.3.1 获取BoundSql对象,BoundSql是对动态SQL解析生成的SQL语句和参数映射信息的封装
        2.3.2 创建CacheKey,用于缓存Key
        2.3.3 根据缓存key从缓存中localCache(PerpetualCache是mybatis的以及缓存对象)获取结果
        2.3.4 如果结果不为空,从缓存中取数据
        2.3.5 如果数据为空,从数据库中查询数据 private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
          2.3.5.1 调用子类进行查询(代码中了用了模板方法) public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
          2.3.5.2 获取StatementHandler对象
          2.3.5.3 调用prepareStatement()方法,创建Statement对象,并进行设置参数等操作
            2.3.5.3.1 获取JDBC中的Connection对象
            2.3.5.3.2 调用StatementHandler的prepare()方法创建Statement对象 stmt = handler.prepare(connection, transaction.getTimeout());
            2.3.5.3.3 调用StatementHandler对象的parameterize()方法设置参数
                2.3.5.3.3.1 PreparedStatementHandler.parameterize(Statement statement) throws SQLException 进行赋值操作
                2.3.5.3.3.2 调用ParameterHandler的实现类DefaultParameterHandler进行赋值操作。 // 获取所有参数映射信息List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
          2.3.5.4 调用StatementHandler对象的query()方法执行查询操作
          2.3.5.6 调用StatementHandler有多个子类,选择一个,这里选择PreparedStatementHandler。调用其中女的query方法
          2.3.5.7 用PreparedStatement对象的execute()方法,执行SQL语句
          2.3.5.8 调用ResultSetHandler的handleResultSets()方法处理结果集.ResultSetHandler是一个接口,有一个默认的实现类来完成该功能
              实现类为DefaultResultSetHandler
        2.3.6 数据库中查询出的结果放入到缓存中

     
     
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  • 原文地址:https://www.cnblogs.com/yingxiaocao/p/13552474.html
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