MyBatis源码分析（四）：SQL执行过程分析

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

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

而调用configuration对象的getMapper方法

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

再次调用mapperRegister，注册mapper的类

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);
    }
  }

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

@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);
  }

然后就这样给UserMapper赋予了一个代理对象

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

user = userMapper.getUser(30L);

对应的mapper映射文件：

<select id="getUser" parameterType="long" resultMap="userMap">
      SELECT user_id as id, user_name as username, sex, user_password as password, email from tb_user WHERE user_id = #{id}
</select>

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

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
}

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

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    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()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          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;
}

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

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

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，

public Object getNamedParams(Object[] args) {
    final int paramCount = names.size();
    if (args == null || paramCount == 0) {
      return null;
    } else if (!hasParamAnnotation && paramCount == 1) {
      return args[names.firstKey()];
    } else {
      final Map<String, Object> param = new ParamMap<>();
      int i = 0;
      for (Map.Entry<Integer, String> entry : names.entrySet()) {
        param.put(entry.getValue(), args[entry.getKey()]);
        // add generic param names (param1, param2, ...)
        final String genericParamName = GENERIC_NAME_PREFIX + String.valueOf(i + 1);
        // ensure not to overwrite parameter named with @Param
        if (!names.containsValue(genericParamName)) {
          param.put(genericParamName, args[entry.getKey()]);
        }
        i++;
      }
      return param;
    }
 }

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

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

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

@Override
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();
    }
}

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；

private Object wrapCollection(final Object object) {
    if (object instanceof Collection) {
      StrictMap<Object> map = new StrictMap<>();
      map.put("collection", object);
      if (object instanceof List) {
        map.put("list", object);
      }
      return map;
    } else if (object != null && object.getClass().isArray()) {
      StrictMap<Object> map = new StrictMap<>();
      map.put("array", object);
      return map;
    }
    return object;
}

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，

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

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，

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

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

      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, 
        boundSql);
      }  

因为我没有自己写的resultHandler类，所以直接执行

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

其方法源码为BaseExecutor抽象类中的queryFromDataBase

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 {
      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;
}

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

@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);
    }
}

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：

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

}

RoutingStatementHandler执行query方法

@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    return delegate.<E>query(statement, resultHandler);
}

PreparedStatementHandler执行query方法

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

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

@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

    final List<Object> multipleResults = new ArrayList<>();

    int resultSetCount = 0;
    ResultSetWrapper rsw = getFirstResultSet(stmt);

    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) { //一个resultMap对应一个结果集，依次遍历resultMap并处理结果集
      ResultMap resultMap = resultMaps.get(resultSetCount);
      handleResultSet(rsw, resultMap, multipleResults, null); // 处理结果集
      rsw = getNextResultSet(stmt);// 获取下一个结果集
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }

    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); //把结果集转化为List
}

DefaultResultSetHandler的getFirstResultSet方法

private ResultSetWrapper getFirstResultSet(Statement stmt) throws SQLException {
    ResultSet rs = stmt.getResultSet();
    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
      // move forward to get the first resultset in case the driver
      // doesn't return the resultset as the first result (HSQLDB 2.1)
      if (stmt.getMoreResults()) { // 尝试再一次获取结果集
        rs = stmt.getResultSet();
      } else {
        if (stmt.getUpdateCount() == -1) {  //表示驱动已经返回，没有更多结果，没有结果集
          // no more results. Must be no resultset
          break;
        }
      }
    }
    return rs != null ? new ResultSetWrapper(rs, configuration) : null; //不为空则返回结果集的包装
}

ResultSetWrapper构造函数（包装结果集）

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

把结果集转化为List

@SuppressWarnings("unchecked")
private List<Object> collapseSingleResultList(List<Object> multipleResults) {
    return multipleResults.size() == 1 ? (List<Object>) multipleResults.get(0) : multipleResults;
}

然后一层层传递回去，最后获得查询结果。（待续）