MyBatis主键生成器Jdbc3KeyGenerator（二）

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);
      }
    }
  }

}