MyBatis主键生成器Jdbc3KeyGenerator（二）

淩亂°似流年 2022-02-28 14:40 258阅读 0赞

上一篇博客MyBatis主键生成器KeyGenerator（一）中我们大体介绍了主键生成器的接口及配置等，接下来我们介绍一下KeyGenerator的实现类Jdbc3KeyGenerator

Jdbc3KeyGenerator实际上实现了processAfter方法，其使用方法如下配置，目的是将主键作为返回值返回，这种用法只能是在有自增主键的数据库中使用：

<insert id="save" useGeneratedKeys="true" keyColumn="i_id" keyProperty="id">
    <span style="white-space:pre">	</span>insert into tbl_log (log_type,log_info) values (#{logType},#{logInfo})
    </insert>

其实现原理就是获得数据库的记录条数，然后加1，作为返回参数的值返回，源码实现如下：

/**
     * @author Clinton Begin
     */
    public class Jdbc3KeyGenerator implements KeyGenerator {
     
      //jdbc模式只能是执行之后返回主键id
      @Override
      public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
        // do nothing
      }
     
      @Override
      public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
        List<Object> parameters = new ArrayList<Object>();
        parameters.add(parameter);
        processBatch(ms, stmt, parameters);
      }
      //
      public void processBatch(MappedStatement ms, Statement stmt, List<Object> parameters) {
        ResultSet rs = null;
        try {
          rs = stmt.getGeneratedKeys();
          final Configuration configuration = ms.getConfiguration();
          final TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    	  //获得返回值的参数名
          final String[] keyProperties = ms.getKeyProperties();
          final ResultSetMetaData rsmd = rs.getMetaData();
          TypeHandler<?>[] typeHandlers = null;
          if (keyProperties != null && rsmd.getColumnCount() >= keyProperties.length) {
            for (Object parameter : parameters) {
              // there should be one row for each statement (also one for each parameter)
              if (!rs.next()) {
                break;
              }
              final MetaObject metaParam = configuration.newMetaObject(parameter);
    		  //获得执行之后返回值的类型
              if (typeHandlers == null) {
                typeHandlers = getTypeHandlers(typeHandlerRegistry, metaParam, keyProperties);
              }
              populateKeys(rs, metaParam, keyProperties, typeHandlers);
            }
          }
        } catch (Exception e) {
          throw new ExecutorException("Error getting generated key or setting result to parameter object. Cause: " + e, e);
        } finally {
          if (rs != null) {
            try {
              rs.close();
            } catch (Exception e) {
              // ignore
            }
          }
        }
      }
      //设置执行sql之后返回参数对应的值
      private TypeHandler<?>[] getTypeHandlers(TypeHandlerRegistry typeHandlerRegistry, MetaObject metaParam, String[] keyProperties) {
        TypeHandler<?>[] typeHandlers = new TypeHandler<?>[keyProperties.length];
        for (int i = 0; i < keyProperties.length; i++) {
          if (metaParam.hasSetter(keyProperties[i])) {
            Class<?> keyPropertyType = metaParam.getSetterType(keyProperties[i]);
            TypeHandler<?> th = typeHandlerRegistry.getTypeHandler(keyPropertyType);
            typeHandlers[i] = th;
          }
        }
        return typeHandlers;
      }
     
      private void populateKeys(ResultSet rs, MetaObject metaParam, String[] keyProperties, TypeHandler<?>[] typeHandlers) throws SQLException {
        for (int i = 0; i < keyProperties.length; i++) {
          TypeHandler<?> th = typeHandlers[i];
          if (th != null) {
    		 //获得执行后对应参数的值
            Object value = th.getResult(rs, i + 1);
    		//设置返回参数的值
            metaParam.setValue(keyProperties[i], value);
          }
        }
      }
     
    }

本文原文地址：[https://blog.csdn.net/qq924862077/article/details/52673625][https_blog.csdn.net_qq924862077_article_details_52673625]

[https_blog.csdn.net_qq924862077_article_details_52673625]: https://blog.csdn.net/qq924862077/article/details/52673625