MyBatis主键生成器SelectKeyGenerator（三）

冷不防 2022-02-27 06:44 269阅读 0赞

前面两篇博客我们介绍了MyBatis主键生成器KeyGenerator（一）和MyBatis主键生成器Jdbc3KeyGenerator（二），接下来我们介绍SelectKeyGenerator，

如在Oraclee中并不提供自增组件，提供了Sequence主键，我们就需要执行它的Sequence主键，如在mysql中如下配置：

<insert id="save">
    	<selectKey resultType="int" keyProperty="id" order="BEFORE">
    		 SELECT LAST_INSERT_ID() AS id 
    	</selectKey>
    	insert into tbl_log (log_type,log_info) values
    	    (#{logType},#{logInfo})
    </insert>

其实现原理就是执行selectKey中的sql语句来获得结果，在SelectKeyGenerator的源码实现中就是执行 select last\_insert\_id() as id这条sql语句，获得结果，并赋值给id，源码实现如下：

/**
     * @author Clinton Begin
     * @author Jeff Butler
     */
    public class SelectKeyGenerator implements KeyGenerator {
      
      public static final String SELECT_KEY_SUFFIX = "!selectKey";
      private boolean executeBefore;
      private MappedStatement keyStatement;
     
      public SelectKeyGenerator(MappedStatement keyStatement, boolean executeBefore) {
        this.executeBefore = executeBefore;
        this.keyStatement = keyStatement;
      }
     
      @Override
      public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
        if (executeBefore) {
          processGeneratedKeys(executor, ms, parameter);
        }
      }
     
      @Override
      public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
        if (!executeBefore) {
          processGeneratedKeys(executor, ms, parameter);
        }
      }
      //执行来获得结果值
      private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
        try {
          if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
            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.
              Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
    		  //执行获得主键的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 {
    			//执行结果
                MetaObject metaResult = configuration.newMetaObject(values.get(0));
    			//返回参数如果是一个
                if (keyProperties.length == 1) {
    				//判断返回参数使用有get方法
                  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 {
    			  //如果有多个返回参数则需要循环赋值
                  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);
        }
      }
      
      //如果有多个返回参数则需要循环赋值
      private void handleMultipleProperties(String[] keyProperties,
          MetaObject metaParam, MetaObject metaResult) {
        String[] keyColumns = keyStatement.getKeyColumns();
        
    	//如果没设置返回参数的值，则使用结果属性的值
        if (keyColumns == null || keyColumns.length == 0) {
          // no key columns specified, just use the property names
          for (int i = 0; i < keyProperties.length; i++) {
            setValue(metaParam, keyProperties[i], metaResult.getValue(keyProperties[i]));
          }
        } else {
          if (keyColumns.length != keyProperties.length) {
            throw new ExecutorException("If SelectKey has key columns, the number must match the number of key properties.");
          }
          for (int i = 0; i < keyProperties.length; i++) {
            setValue(metaParam, keyProperties[i], metaResult.getValue(keyColumns[i]));
          }
        }
      }
      
      //设置返回参数对应的值
      private void setValue(MetaObject metaParam, String property, Object value) {
        if (metaParam.hasSetter(property)) {
          metaParam.setValue(property, value);
        } else {
          throw new ExecutorException("No setter found for the keyProperty '" + property + "' in " + metaParam.getOriginalObject().getClass().getName() + ".");
        }
      }
    }

本文原文地址：[https://blog.csdn.net/qq924862077/article/details/52673683][https_blog.csdn.net_qq924862077_article_details_52673683]

[https_blog.csdn.net_qq924862077_article_details_52673683]: https://blog.csdn.net/qq924862077/article/details/52673683