public static List<Map<String,Object>> queryForList(){      Connection connection = null;      ResultSet rs = null;      PreparedStatement stmt = null;      List<Map<String,Object>> resultList = new ArrayList<Map<String,Object>>();                try {          // 加载JDBC驱动          Class.forName("oracle.jdbc.driver.OracleDriver").newInstance();          String url = "jdbc:oracle:thin:@localhost:1521:ORACLEDB";                        String user = "trainer";           String password = "trainer";                         // 获取数据库连接          connection = DriverManager.getConnection(url,user,password);                         String sql = "select * from userinfo where user_id = ? ";          // 创建Statement对象（每一个Statement为一次数据库执行请求）          stmt = connection.prepareStatement(sql);                        // 设置传入参数          stmt.setString(1, "zhangsan");                        // 执行SQL语句          rs = stmt.executeQuery();                        // 处理查询结果（将查询结果转换成List<Map>格式）          ResultSetMetaData rsmd = rs.getMetaData();          int num = rsmd.getColumnCount();                        while(rs.next()){              Map map = new HashMap();              for(int i = 0;i < num;i++){                  String columnName = rsmd.getColumnName(i+1);                  map.put(columnName,rs.getString(columnName));              }              resultList.add(map);          }                    } catch (Exception e) {          e.printStackTrace();      } finally {          try {              // 关闭结果集              if (rs != null) {                  rs.close();                  rs = null;              }              // 关闭执行              if (stmt != null) {                  stmt.close();                  stmt = null;              }              if (connection != null) {                  connection.close();                  connection = null;              }          } catch (SQLException e) {              e.printStackTrace();          }      }            return resultList;  }

SqlSession sqlSession = factory.openSession();

public <E> List<E> selectList(String statement, Object parameter) {      return this.selectList(statement, parameter, RowBounds.DEFAULT);  }   public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {      try {          //1.根据Statement Id，在mybatis 配置对象Configuration中查找和配置文件相对应的MappedStatement              MappedStatement ms = configuration.getMappedStatement(statement);          //2. 将查询任务委托给MyBatis 的执行器 Executor          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();      }  }

<select id="selectByMinSalary" resultMap="BaseResultMap" parameterType="java.util.Map" >     select          EMPLOYEE_ID, FIRST_NAME, LAST_NAME, EMAIL, SALARY     from LOUIS.EMPLOYEES     <if test="min_salary != null">         where SALARY < #{min_salary,jdbcType=DECIMAL}     </if>  </select>

/**    * BaseExecutor 类部分代码    *    */  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {      // 1. 根据具体传入的参数，动态地生成需要执行的SQL语句，用BoundSql对象表示          BoundSql boundSql = ms.getBoundSql(parameter);        // 2. 为当前的查询创建一个缓存Key        CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);        return query(ms, parameter, rowBounds, resultHandler, key, boundSql);  }   @SuppressWarnings("unchecked")  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 {                 // 3.缓存中没有值，直接从数据库中读取数据                   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;  }
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 {                     //4. 执行查询，返回List 结果，然后    将查询的结果放入缓存之中            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;  }

/** 
   *    * SimpleExecutor类的doQuery()方法实现    *    */  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();            //5. 根据既有的参数，创建StatementHandler对象来执行查询操作            StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);            //6. 创建java.Sql.Statement对象，传递给StatementHandler对象            stmt = prepareStatement(handler, ms.getStatementLog());            //7. 调用StatementHandler.query()方法，返回List结果集            return handler.<E>query(stmt, resultHandler);         } finally {             closeStatement(stmt);         }  }

/**    *    * SimpleExecutor类的doQuery()方法实现    *    */  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);           // 1.准备Statement对象，并设置Statement对象的参数           stmt = prepareStatement(handler, ms.getStatementLog());           // 2. StatementHandler执行query()方法，返回List结果           return handler.<E>query(stmt, resultHandler);       } finally {          closeStatement(stmt);       } }   private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {      Statement stmt;        Connection connection = getConnection(statementLog);        stmt = handler.prepare(connection);        //对创建的Statement对象设置参数，即设置SQL 语句中 ? 设置为指定的参数        handler.parameterize(stmt);        return stmt;  }

/**    * StatementHandler 类的parameterize(statement) 方法实现     */  public void parameterize(Statement statement) throws SQLException {        //使用ParameterHandler对象来完成对Statement的设值          parameterHandler.setParameters((PreparedStatement) statement);  }

/** 
   *     * ParameterHandler类的setParameters(PreparedStatement ps) 实现    * 对某一个Statement进行设置参数    */  public void setParameters(PreparedStatement ps) throws SQLException {        ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());        List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();        if (parameterMappings != null) {            for (int i = 0; i < parameterMappings.size(); i++) {                ParameterMapping parameterMapping = parameterMappings.get(i);                if (parameterMapping.getMode() != ParameterMode.OUT) {                    Object value;                    String propertyName = parameterMapping.getProperty();                    if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params                        value = boundSql.getAdditionalParameter(propertyName);                    } else if (parameterObject == null) {                        value = null;                    } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {                        value = parameterObject;                    } else {                        MetaObject metaObject = configuration.newMetaObject(parameterObject);                        value = metaObject.getValue(propertyName);                    }                             // 每一个Mapping都有一个TypeHandler，根据TypeHandler来对preparedStatement进行设置参数                    TypeHandler typeHandler = parameterMapping.getTypeHandler();                    JdbcType jdbcType = parameterMapping.getJdbcType();                    if (value == null && jdbcType == null) jdbcType = configuration.getJdbcTypeForNull();                    // 设置参数                    typeHandler.setParameter(ps, i + 1, value, jdbcType);                }            }        }  }

 /**     * PreParedStatement类的query方法实现     */  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {        //1.调用preparedStatemnt。execute()方法，然后将resultSet交给ResultSetHandler处理          PreparedStatement ps = (PreparedStatement) statement;        ps.execute();        //2. 使用ResultHandler来处理ResultSet        return resultSetHandler.<E> handleResultSets(ps);  }

/**      * ResultSetHandler类的handleResultSets()方法实现    *     */  public List<Object> handleResultSets(Statement stmt) throws SQLException {        final List<Object> multipleResults = new ArrayList<Object>();         int resultSetCount = 0;        ResultSetWrapper rsw = getFirstResultSet(stmt);         List<ResultMap> resultMaps = mappedStatement.getResultMaps();        int resultMapCount = resultMaps.size();        validateResultMapsCount(rsw, resultMapCount);             while (rsw != null && resultMapCount > resultSetCount) {            ResultMap resultMap = resultMaps.get(resultSetCount);                   //将resultSet            handleResultSet(rsw, resultMap, multipleResults, null);            rsw = getNextResultSet(stmt);            cleanUpAfterHandlingResultSet();            resultSetCount++;        }       String[] resultSets = mappedStatement.getResulSets();        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);  }

String resource = "mybatis-config.xml";  InputStream inputStream = Resources.getResourceAsStream(resource);  SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);  SqlSession sqlSession = sqlSessionFactory.openSession();  List list = sqlSession.selectList("com.foo.bean.BlogMapper.queryAllBlogInfo");

public SqlSessionFactory build(InputStream inputStream)  {        return build(inputStream, null, null);  }  
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties)  {        try  {            //2. 创建XMLConfigBuilder对象用来解析XML配置文件，生成Configuration对象            XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);            //3. 将XML配置文件内的信息解析成Java对象Configuration对象            Configuration config = parser.parse();            //4. 根据Configuration对象创建出SqlSessionFactory对象            return build(config);        } 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.            }        }}
// 从此处可以看出，MyBatis内部通过Configuration对象来创建SqlSessionFactory,用户也可以自己通过API构造好Configuration对象，调用此方法创SqlSessionFactory  public SqlSessionFactory build(Configuration config) {        return new DefaultSqlSessionFactory(config);  }

public Configuration parse() {       if (parsed) {           throw new BuilderException("Each XMLConfigBuilder can only be used once.");       }       parsed = true;       //源码中没有这一句，只有parseConfiguration(parser.evalNode("/configuration"));       //为了让读者看得更明晰，源码拆分为以下两句       XNode configurationNode = parser.evalNode("/configuration");       parseConfiguration(configurationNode);       return configuration;}/**   * 解析 "/configuration"节点下的子节点信息，然后将解析的结果设置到Configuration对象中   */  private void parseConfiguration(XNode root) {       try {           //1.首先处理properties 节点              propertiesElement(root.evalNode("properties")); //issue #117 read properties first           //2.处理typeAliases           typeAliasesElement(root.evalNode("typeAliases"));           //3.处理插件           pluginElement(root.evalNode("plugins"));           //4.处理objectFactory           objectFactoryElement(root.evalNode("objectFactory"));           //5.objectWrapperFactory           objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));           //6.settings           settingsElement(root.evalNode("settings"));           //7.处理environments           environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631           //8.database           databaseIdProviderElement(root.evalNode("databaseIdProvider"));           //9.typeHandlers           typeHandlerElement(root.evalNode("typeHandlers"));           //10.mappers           mapperElement(root.evalNode("mappers"));       } catch (Exception e) {           throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);       }   }

/**   * 解析environments节点，并将结果设置到Configuration对象中   * 注意：创建envronment时，如果SqlSessionFactoryBuilder指定了特定的环境（即数据源）；  *      则返回指定环境（数据源）的Environment对象，否则返回默认的Environment对象；  *      这种方式实现了MyBatis可以连接多数据源   */  private void environmentsElement(XNode context) throws Exception {      if (context != null)      {           if (environment == null)           {               environment = context.getStringAttribute("default");           }           for (XNode child : context.getChildren())           {                String id = child.getStringAttribute("id");                if (isSpecifiedEnvironment(id))                {                    //1.创建事务工厂 TransactionFactory                    TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));                    DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));                    //2.创建数据源DataSource                    DataSource dataSource = dsFactory.getDataSource();                    //3. 构造Environment对象                    Environment.Builder environmentBuilder = new Environment.Builder(id)               .transactionFactory(txFactory)               .dataSource(dataSource);                    //4. 将创建的Envronment对象设置到configuration 对象中                    configuration.setEnvironment(environmentBuilder.build());               }           }      }  }

private boolean isSpecifiedEnvironment(String id)  
{        if (environment == null)        {             throw new BuilderException("No environment specified.");        }        else if (id == null)        {             throw new BuilderException("Environment requires an id attribute.");        }        else if (environment.equals(id))        {            return true;        }        return false;   }

String resource = "mybatis-config.xml";  
InputStream inputStream = Resources.getResourceAsStream(resource);  // 手动创建XMLConfigBuilder，并解析创建Configuration对象  XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, null,null);  Configuration configuration=parse();  // 使用Configuration对象创建SqlSessionFactory  SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);  // 使用MyBatis  SqlSession sqlSession = sqlSessionFactory.openSession();  List list = sqlSession.selectList("com.foo.bean.BlogMapper.queryAllBlogInfo");  

private void environmentsElement(XNode context) throws Exception {      if (context != null) {          if (environment == null) {              environment = context.getStringAttribute("default");          }          for (XNode child : context.getChildren()) {              String id = child.getStringAttribute("id");              //是和默认的环境相同时，解析之              if (isSpecifiedEnvironment(id)) {                  TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));                  DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));                  DataSource dataSource = dsFactory.getDataSource();                    //使用了Environment内置的构造器Builder，传递id 事务工厂和数据源                  Environment.Builder environmentBuilder = new Environment.Builder(id)                  .transactionFactory(txFactory)                  .dataSource(dataSource);                  configuration.setEnvironment(environmentBuilder.build());              }          }      }  }