关于后端:mybatis-selectKey-赋值未生效为什么

Statement配置如下：

 <insert id="insertStatemnet" parameterType="User">
       insert into user ( user_name )  
       values  (#{user#userName})
       <selectKey resultType="long" order="AFTER"
             keyProperty="id">
          SELECT LAST_INSERT_ID() 
      </selectKey>
  </insert>

Dao 层代码如下

public interface UserMapper{
   int insert(@Param(user) User user);
}

class User{
     long id;
     String userName;

     ...getter/setter
}


User user = userMapper.insert(user);

user.getId() 获取的后果等于0

应用Mybatis Insert User表，应用selectKey获取LAST_INSERT_ID() ,赋值给user的id属性，发现id属性值未被set进去，user.getId() 获取的后果等于0，会话的LAST_INSERT_ID() 曾经到远远超过0了，返回0显著是不对的。

对于LAST_INSERT_ID() 能够参考 LAST_INSERT_ID 文章。

开始探索为什么？

属性值获取到的是0，要么是SELECT LAST_INSERT_ID() sql未执行，应用了id long类型的默认值0，要么是执行了但获取到的值是0，或者是Mybatis set对象id属性值的时候没set进去。

Mybatis应用SelectKeyGenerator解决selectKey标签，从这里开始动手

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) {
          Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
          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));
            ...
            setValue(metaParam, keyProperties[0], values.get(0));
            ....
          }
        }
      }
    } catch (ExecutorException e) {
      throw e;
    } catch (Exception e) {
      throw new ExecutorException("Error selecting key or setting result to parameter object. Cause: " + e, e);
    }
  }

验证发现keyExecutor.query获取到的values是[100]，阐明SELECT LAST_INSERT_ID() 查问没问题，那问题必然呈现在上面的setValue办法外面。

持续跟进setValue，setValue最终走的是ObjectWrapper的实现类MapWrapper#set，

class MapWrapper{
      @Override
      public void set(PropertyTokenizer prop, Object value) {
        if (prop.getIndex() != null) {
          Object collection = resolveCollection(prop, map);
          setCollectionValue(prop, collection, value);
        } else {
          map.put(prop.getName(), value);
        }
      }
}

该办法set只是在map外面put了一个kv，实践上Mybatis要对字段赋值的话，应该反射调用对象Filed的set办法。

于此同时看到ObjectWrapper有一个实现类BeanWrapper，其中的set办法是反射set字段值，刚好和咱们预期的想法统一。

class BeanWrapper{
  @Override
  public void set(PropertyTokenizer prop, Object value) {
    if (prop.getIndex() != null) {
      Object collection = resolveCollection(prop, object);
      setCollectionValue(prop, collection, value);
    } else {
      setBeanProperty(prop, object, value);
    }
  }
}

此预计就是决策ObjectWrapper的时候呈现问题导致的属性值为set进去，上面是获取ObjectWrapper实现的代码片段：

private MetaObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory, ReflectorFactory reflectorFactory) {
    this.originalObject = object;
    this.objectFactory = objectFactory;
    this.objectWrapperFactory = objectWrapperFactory;
    this.reflectorFactory = reflectorFactory;

    if (object instanceof ObjectWrapper) {
      this.objectWrapper = (ObjectWrapper) object;
    } else if (objectWrapperFactory.hasWrapperFor(object)) {
      this.objectWrapper = objectWrapperFactory.getWrapperFor(this, object);
    } else if (object instanceof Map) {
      this.objectWrapper = new MapWrapper(this, (Map) object);
    } else if (object instanceof Collection) {
      this.objectWrapper = new CollectionWrapper(this, (Collection) object);
    } else {
      this.objectWrapper = new BeanWrapper(this, object);
    }
  }

实践上要走到else逻辑的，理论object类型是MapperMethod.ParamMap类型，走到了Map分支。

MapperMethod.ParamMap类型是在MapperMethod执行过程中转换java对象参数到sql命令行参数生成的，具体参考ParamNameResolver#getNamedParams

 public Object getNamedParams(Object[] args) {
    final int paramCount = names.size();
    if (args == null || paramCount == 0) {
      return null;
    } else if (!hasParamAnnotation && paramCount == 1) {
      return args[names.firstKey()];
    } else {
      final Map<String, Object> param = new ParamMap<Object>();
      int i = 0;
      for (Map.Entry<Integer, String> entry : names.entrySet()) {
        param.put(entry.getValue(), args[entry.getKey()]);
        // add generic param names (param1, param2, ...)
        final String genericParamName = GENERIC_NAME_PREFIX + String.valueOf(i + 1);
        // ensure not to overwrite parameter named with @Param
        if (!names.containsValue(genericParamName)) {
          param.put(genericParamName, args[entry.getKey()]);
        }
        i++;
      }
      return param;
    }
  }

因为Mapper办法参数外面标注了@Param注解，导致生成的是MapperMethod.ParamMap。

回过头来理下，因为Mapper办法参数外面标注了@Param注解，导致生成的sql参数类型是MapperMethod.ParamMap，继而导致获取MetaObject的时候ObjectWrapper被谬误决策成MapWrapper，导致setValue属性值未set进去。

实际上还是应用不标准导致的问题，去掉办法上的@Param注解即可失常运行，

参考资料

https://blog.csdn.net/wt_better/article/details/128851775

https://blog.csdn.net/m0_46205920/article/details/122103024

Mybatis selectKey 采坑笔记

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