Mybatis源码解析 KeyGenerator

怼烎@ 2021-06-11 15:10 539阅读 0赞

KeyGenerator接口定义了2个函数：

//执行insert之前
    void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter);
    
    //执行insert之后
    void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter);

目前Mybatis給了3个实现：

![1240][]

image.png

### NoKeyGenerator ###

没有执行任何操作，并且是Mybatis的默认实现

### Jdbc3KeyGenerator ###

适用于可以自动生成主键的sql(自增等)，由于Statement.execute执行后返回的是操作行数，并不会返回主键，当配置了

后，该函数会自动将id赋值到keyProperty对应的javabean属性中  
具体代码如：

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();
          //获得配置的属性字段名
          final String[] keyProperties = ms.getKeyProperties();
          //返回的id值
          final ResultSetMetaData rsmd = rs.getMetaData();
          TypeHandler<?>[] typeHandlers = null;
          if (keyProperties != null && rsmd.getColumnCount() >= keyProperties.length) {
            for (Object parameter : parameters) {//循环用于批量insert操作
              if (!rs.next()) break; // there should be one row for each statement (also one for each parameter)
              //将javabean 包装
              final MetaObject metaParam = configuration.newMetaObject(parameter);
              //类型转换，一般为java基础类型转换
              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
            }
          }
        }
      }
    
    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) {
            Object value = th.getResult(rs, i + 1);//获得db->java的值
            //设值到javabean对应的字段属性中
            metaParam.setValue(keyProperties[i], value);
          }
        }
      }

### SelectKeyGenerator： ###

通过自定义sql手动获取主键值，有2种配置，before和after  
before既是在insert之前设置到pojo中作为参数一起insert到db  
after即为在insert之后，通过自定义sql获取并设置到pojo中

<selectKey resultType="java.lang.Integer" keyProperty="id" order="BEFORE">  
          select max(id) from TB_USER  
    </selectKey>

核心代码：

private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
        try {
          if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
            //获取配置的属性字段名
            String[] keyProperties = keyStatement.getKeyProperties();
            final Configuration configuration = ms.getConfiguration();
            final MetaObject metaParam = configuration.newMetaObject(parameter);
            if (keyProperties != null) {
              // Do not close keyExecutor.
              // The transaction will be closed by parent executor.
              //这里使用SimpleStatement
              Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
              //执行selectKey对应的sql语句
              List<Object> values = keyExecutor.query(keyStatement, parameter, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER);
              if (values.size() == 0) {
                throw new ExecutorException("SelectKey returned no data.");            
              } else if (values.size() > 1) {
                throw new ExecutorException("SelectKey returned more than one value.");
              } else {
                MetaObject metaResult = configuration.newMetaObject(values.get(0));
                if (keyProperties.length == 1) {//设值
                  if (metaResult.hasGetter(keyProperties[0])) {
                    setValue(metaParam, keyProperties[0], metaResult.getValue(keyProperties[0]));
                  } else {
                    // no getter for the property - maybe just a single value object
                    // so try that
                    setValue(metaParam, keyProperties[0], values.get(0));
                  }
                } else {
                  handleMultipleProperties(keyProperties, metaParam, metaResult);
                }
              }
            }
          }
        } catch (ExecutorException e) {
          throw e;
        } catch (Exception e) {
          throw new ExecutorException("Error selecting key or setting result to parameter object. Cause: " + e, e);
        }
      }

[1240]: /images/20210527/df82aa654f634a4c92d28a72169fe8ee.png