mybatis中的几个问题

公司里用mybatis，写着写着会有几个问题冒出来，终于有空去看了一下源代码。

  • mybatis 方法签名返回list，LinkedList，如何实现不同的list?
    解：单纯使用mybatis时 List<Object> user = session.selectList(statement); 只能返回list且实际为arraylist org.apache.ibatis.executor.resultset.DefaultResultSetHandler里
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    @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;
        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);
          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);
      }

配合spring时，实际的sqlSession是一个代理，在SqlSessionTemplate能看到sqlSessionProxy这个代理类，并且代理类里有一个实现InvocationHandler的SqlSessionInterceptor。spring读取配置初始化MapperFactoryBean,方法执行完后对返回类型转换。org.apache.ibatis.binding.MapperMethod 里关于方法返回不同类型集合的转换MapperProxy的invoke方法里调mapperMethod.execute()。

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

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private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
  List<E> result;
  Object param = method.convertArgsToSqlCommandParam(args);
  if (method.hasRowBounds()) {
    RowBounds rowBounds = method.extractRowBounds(args);
    result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
  } else {
    result = sqlSession.<E>selectList(command.getName(), param);
  }
  // issue #510 Collections & arrays support
  if (!method.getReturnType().isAssignableFrom(result.getClass())) {
    if (method.getReturnType().isArray()) {
      return convertToArray(result);
    } else {
      return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
    }
  }
  return result;
}
  • resulttype为map时，map里的泛型,如何转化？
    解：配置有 resultType=”java.util.Map” xml配置会组装在类 MappedStatement mybatis 把结果包装成Map<Object,Object>
java
mybatis