MyBatis主键生成器SelectKeyGenerator（三）

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() + ".");
    }
  }
}