一、获取Mapper接口的代理

根据上一节，Mybatis初始化之后，利用sqlSession(defaultSqlSession)的getMapper方法获取Mapper接口

1 @Override2 public 
     
       T getMapper(Class
      
        type) {3     return configuration.
       
        getMapper(type, this);4 }
       
      
     

而调用configuration对象的getMapper方法

1 public 
     
       T getMapper(Class
      
        type, SqlSession sqlSession) {2     return mapperRegistry.getMapper(type, sqlSession);3 }
      
     

再次调用mapperRegister，注册mapper的类

1 public 
     
       T getMapper(Class
      
        type, SqlSession sqlSession) { 2     final MapperProxyFactory
       
         mapperProxyFactory = (MapperProxyFactory
        
         ) knownMappers.get(type); 3     if (mapperProxyFactory == null) { 4       throw new BindingException("Type " + type + " is not known to the MapperRegistry."); 5     } 6     try { 7       return mapperProxyFactory.newInstance(sqlSession); 8     } catch (Exception e) { 9       throw new BindingException("Error getting mapper instance. Cause: " + e, e);10     }11   }
        
       
      
     

而mapperRegister根据传进来的mapper接口来创建MapperProxyFactory代理工厂对象，再用sqlSession参数创建Mapper的代理对象，这里运用的是JDK的动态代理，Proxy.newProxyInstance方法绑定mapper接口，第一个参数是类加载器，第二个参数是需要实现的接口数组，第三个是InvocationHandler接口，也就是交由InvocationHandler接口实现类MapperProxy里的invoke()方法去处理

1 @SuppressWarnings("unchecked")2   protected T newInstance(MapperProxy
     
       mapperProxy) {3     return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);4   }5 6   public T newInstance(SqlSession sqlSession) {7     final MapperProxy
      
        mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);8     return newInstance(mapperProxy);9   }
      
     

然后就这样给UserMapper赋予了一个代理对象

1 UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

二、使用Mapper代理对象进行查询操作

主代码调用代理对象查询，方法里面的参数为数据库字段的长整型id

1 user = userMapper.getUser(30L);

对应的mapper映射文件：

1 2       SELECT user_id as id, user_name as username, sex, user_password as password, email from tb_user WHERE user_id = #{id}3 

使用Mapper代理对象，首先调用的是MapperProxy里面的invoke方法，传进三个主要的参数，分别是：代理对象、被调用的方法、方法的参数

1 @Override 2 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 3     try { 4       if (Object.class.equals(method.getDeclaringClass())) { 5         return method.invoke(this, args); 6       } else if (isDefaultMethod(method)) { 7         return invokeDefaultMethod(proxy, method, args); 8       } 9     } catch (Throwable t) {10       throw ExceptionUtil.unwrapThrowable(t);11     }12     final MapperMethod mapperMethod = cachedMapperMethod(method);13     return mapperMethod.execute(sqlSession, args);14 }

上面这段代码首先检查当前这个method是哪个类的方法，然后再判断有无默认方法，如果都没有则对方法进行缓存，最后对 SqlSession 进行的包装调用。

MapperMethod对SqlSession的操作进行了封装，来看其中的一段execute方法源码

1 public Object execute(SqlSession sqlSession, Object[] args) { 2     Object result; 3     switch (command.getType()) { 4       case INSERT: { 5         Object param = method.convertArgsToSqlCommandParam(args); 6         result = rowCountResult(sqlSession.insert(command.getName(), param)); 7         break; 8       } 9       case UPDATE: {10         Object param = method.convertArgsToSqlCommandParam(args);11         result = rowCountResult(sqlSession.update(command.getName(), param));12         break;13       }14       case DELETE: {15         Object param = method.convertArgsToSqlCommandParam(args);16         result = rowCountResult(sqlSession.delete(command.getName(), param));17         break;18       }19       case SELECT:20         if (method.returnsVoid() && method.hasResultHandler()) {21           executeWithResultHandler(sqlSession, args);22           result = null;23         } else if (method.returnsMany()) {24           result = executeForMany(sqlSession, args);25         } else if (method.returnsMap()) {26           result = executeForMap(sqlSession, args);27         } else if (method.returnsCursor()) {28           result = executeForCursor(sqlSession, args);29         } else {30           Object param = method.convertArgsToSqlCommandParam(args);31           result = sqlSession.selectOne(command.getName(), param);32           if (method.returnsOptional() &&33               (result == null || !method.getReturnType().equals(result.getClass()))) {34             result = Optional.ofNullable(result);35           }36         }37         break;38       case FLUSH:39         result = sqlSession.flushStatements();40         break;41       default:42         throw new BindingException("Unknown execution method for: " + command.getName());43     }44     if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {45       throw new BindingException("Mapper method '" + command.getName()46           + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");47     }48     return result;49 }

调用的mapper的查询操作，先看看上面这段的SELECT这一段代码。首先是看方法的返回值类型是否为空并且结果处理器resultHandler，有的话则执行实现的ResultHandler的方法；之后也是检查方法的参数和返回类型，有的话执行各种情况下的方法；都没有的话，把参数传进SQL命令中

1 public Object convertArgsToSqlCommandParam(Object[] args) {2       return paramNameResolver.getNamedParams(args);3 }

可以看到参数传递利用了ParamNameResolver，处理接口形式的参数，最后会把参数处放在一个map中，

1 public Object getNamedParams(Object[] args) { 2     final int paramCount = names.size(); 3     if (args == null || paramCount == 0) { 4       return null; 5     } else if (!hasParamAnnotation && paramCount == 1) { 6       return args[names.firstKey()]; 7     } else { 8       final Map
     
       param = new ParamMap<>(); 9       int i = 0;10       for (Map.Entry
      
        entry : names.entrySet()) {11         param.put(entry.getValue(), args[entry.getKey()]);12         // add generic param names (param1, param2, ...)13         final String genericParamName = GENERIC_NAME_PREFIX + String.valueOf(i + 1);14         // ensure not to overwrite parameter named with @Param15         if (!names.containsValue(genericParamName)) {16           param.put(genericParamName, args[entry.getKey()]);17         }18         i++;19       }20       return param;21     }22  }
      
     

参数解析完后，MapperMethod使用sqlSession，执行一条操作：

1 result = sqlSession.selectOne(command.getName(), param);

1 @Override 2 public 
     
       T selectOne(String statement, Object parameter) { 3     // Popular vote was to return null on 0 results and throw exception on too many. 4     List
      
        list = this.selectList(statement, parameter); 5     if (list.size() == 1) { 6       return list.get(0); 7     } else if (list.size() > 1) { 8       throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size()); 9     } else {10       return null;11     }12 }
      
     

1 @Override 2 public 
     
       List
      
        selectList(String statement, Object parameter, RowBounds rowBounds) { 3     try { 4       MappedStatement ms = configuration.getMappedStatement(statement); 5       return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); 6     } catch (Exception e) { 7       throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e); 8     } finally { 9       ErrorContext.instance().reset();10     }11 }
      
     

sqlSession的selectList最后使用到MappedStatement，这个MappedStatement是保存Mapper中一个SQL语句的结点。利用执行器进行查询，第二个参数是为了检查参数是不是一个集合；

1 private Object wrapCollection(final Object object) { 2     if (object instanceof Collection) { 3       StrictMap

第三个参数是rowBounds逻辑分页方式，这里使用的是默认的；第四个是执行器的参数，这里是null。

然后跳到了CacheExecutor的query方法，它根据传进的MappedStatement参数获取BoundSql对象，ms中有mapper中的sql语句，放在SqlSource，然后根据传进来的参数组装成boundSql；之后生成一个对应二级缓存的key，

1 @Override2 public 
     
       List
      
        query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {3     BoundSql boundSql = ms.getBoundSql(parameterObject);4     CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);5     return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);6 }
      
     

但是Mybatis默认只开启了一级缓存，本例中并没有开启二级缓存，所以直接执行最后一个父类delegate.query方法，

1 public 
     
       List
      
        query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) 2       throws SQLException { 3     Cache cache = ms.getCache(); 4     if (cache != null) { 5       flushCacheIfRequired(ms); 6       if (ms.isUseCache() && resultHandler == null) { 7         ensureNoOutParams(ms, boundSql); 8         @SuppressWarnings("unchecked") 9         List
       
         list = (List
        
         ) tcm.getObject(cache, key);10         if (list == null) {11           list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);12           tcm.putObject(cache, key, list); // issue #578 and #11613         }14         return list;15       }16     }17     return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);18 }
        
       
      
     

上面调用的是BaseExecutor中的query方法，此方法中的最重要的一段代码

1       list = resultHandler == null ? (List
     
      ) localCache.getObject(key) : null;2       if (list != null) {3         handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);4       } else {5         list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, 6         boundSql);7       }
     

因为我没有自己写的resultHandler类，所以直接执行

1 　　　　list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);

其方法源码为BaseExecutor抽象类中的queryFromDataBase

1 private 
     
       List
      
        queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { 2     List
       
         list; 3     localCache.putObject(key, EXECUTION_PLACEHOLDER); 4     try { 5       list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); 6     } finally { 7       localCache.removeObject(key); 8     } 9     localCache.putObject(key, list);10     if (ms.getStatementType() == StatementType.CALLABLE) {11       localOutputParameterCache.putObject(key, parameter);12     }13     return list;14 }
       
      
     

queryFromDataBase中在深入，主要是第5行的doQuery方法：

1 @Override 2 public 
     
       List
      
        doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException { 3     Statement stmt = null; 4     try { 5       Configuration configuration = ms.getConfiguration(); 6       StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); 7       stmt = prepareStatement(handler, ms.getStatementLog()); 8       return handler.query(stmt, resultHandler); 9     } finally {10       closeStatement(stmt);11     }12 }
      
     

StatementHadler是四大核心对象之一，它的任务就是和数据库对话。上面这段代码configuration.newStatementHandler方法使用了RoutingStatementHandler(采用的适配器模式)创建StatementHandler：

1 public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 2  3     switch (ms.getStatementType()) { 4       case STATEMENT: 5         delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql); 6         break; 7       case PREPARED: 8         delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql); 9         break;10       case CALLABLE:11         delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);12         break;13       default:14         throw new ExecutorException("Unknown statement type: " + ms.getStatementType());15     }16 17 }

RoutingStatementHandler执行query方法

1 @Override2 public 
     
       List
      
        query(Statement statement, ResultHandler resultHandler) throws SQLException {3     return delegate.
       
        query(statement, resultHandler);4 }
       
      
     

PreparedStatementHandler执行query方法

1 @Override2 public 
     
       List
      
        query(Statement statement, ResultHandler resultHandler) throws SQLException {3     PreparedStatement ps = (PreparedStatement) statement;4     ps.execute();5     return resultSetHandler.handleResultSets(ps);6  }
      
     

DefaultResultSetHandler执行handleResultSets方法，getFirstResultSet获取第一个结果集在于知道sql语句要操作到哪些元素数据（表的列），会获取到元数据名称、Java数据类型、JDBC数据类型，之后getResultMaps获取执行的sql配置的resultMap，之后一个resultMap对应一个结果集，依次遍历resultMap并处理结果集

1 @Override 2 public List

DefaultResultSetHandler的getFirstResultSet方法

1 private ResultSetWrapper getFirstResultSet(Statement stmt) throws SQLException { 2     ResultSet rs = stmt.getResultSet(); 3     while (rs == null) { // 没有结果集，也许是数据库驱动还没有返回第一个结果集 4       // move forward to get the first resultset in case the driver 5       // doesn't return the resultset as the first result (HSQLDB 2.1) 6       if (stmt.getMoreResults()) { // 尝试再一次获取结果集 7         rs = stmt.getResultSet(); 8       } else { 9         if (stmt.getUpdateCount() == -1) {  //表示驱动已经返回，没有更多结果，没有结果集10           // no more results. Must be no resultset11           break;12         }13       }14     }15     return rs != null ? new ResultSetWrapper(rs, configuration) : null; //不为空则返回结果集的包装16 }

ResultSetWrapper构造函数（包装结果集）

1 public ResultSetWrapper(ResultSet rs, Configuration configuration) throws SQLException { 2     super(); 3     this.typeHandlerRegistry = configuration.getTypeHandlerRegistry(); 4     this.resultSet = rs; 5     final ResultSetMetaData metaData = rs.getMetaData(); 6     final int columnCount = metaData.getColumnCount(); 7     for (int i = 1; i <= columnCount; i++) { // 设置结果集的元数据 8       columnNames.add(configuration.isUseColumnLabel() ? metaData.getColumnLabel(i) : metaData.getColumnName(i)); 9       jdbcTypes.add(JdbcType.forCode(metaData.getColumnType(i)));10       classNames.add(metaData.getColumnClassName(i));11     }12 }

把结果集转化为List

1 @SuppressWarnings("unchecked")2 private List

然后一层层传递回去，最后获得查询结果。（待续）