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MyBatis測(cè)試報(bào)錯(cuò):Cannot?determine?value?type?from?string?'xxx'的解決辦法

更新時(shí)間：2023年02月17日 09:16:35 作者：大先生哈哈哈

這篇文章主要給大家介紹了關(guān)于MyBatis測(cè)試報(bào)錯(cuò):Cannot?determine?value?type?from?string?'xxx'的解決辦法,文中通過(guò)實(shí)例代碼介紹的非常詳細(xì),需要的朋友可以參考下

1、產(chǎn)生

實(shí)體類Student中屬性有id,name,email,age，未使用無(wú)參構(gòu)造器

在mapper.xml中只查詢name,email,age

測(cè)試時(shí)報(bào)錯(cuò)

Cannot determine value type from string '張三'

2、解決

實(shí)體類Student中添加無(wú)參構(gòu)造器

得到結(jié)果

Student{id=null, name='張三', email='zhangsan@126.com', age=22}

3、探究

跟蹤源碼

org.apache.ibatis.executor.resultset.DefaultResultSetHandler

這個(gè)類有個(gè)方法createResultObject

private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, String columnPrefix)
      throws SQLException {
    final Class<?> resultType = resultMap.getType();
    final MetaClass metaType = MetaClass.forClass(resultType, reflectorFactory);
    final List<ResultMapping> constructorMappings = resultMap.getConstructorResultMappings();
    if (hasTypeHandlerForResultObject(rsw, resultType)) {
      return createPrimitiveResultObject(rsw, resultMap, columnPrefix);
    } else if (!constructorMappings.isEmpty()) {
      return createParameterizedResultObject(rsw, resultType, constructorMappings, constructorArgTypes, constructorArgs, columnPrefix);
    } else if (resultType.isInterface() || metaType.hasDefaultConstructor()) {
      return objectFactory.create(resultType);
    } else if (shouldApplyAutomaticMappings(resultMap, false)) {
      return createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs);
    }
    throw new ExecutorException("Do not know how to create an instance of " + resultType);
  }

這個(gè)方法是根據(jù)結(jié)果集返回值的類型創(chuàng)建出相應(yīng)的bean字段對(duì)象

當(dāng)實(shí)體使用無(wú)參構(gòu)造器時(shí)

mybatis會(huì)調(diào)用createResultObject方法中objectFactory.create(resultType)，使用無(wú)參構(gòu)造器創(chuàng)建對(duì)象

private  <T> T instantiateClass(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    try {
      Constructor<T> constructor;
      if (constructorArgTypes == null || constructorArgs == null) {
        constructor = type.getDeclaredConstructor();
        try {
          return constructor.newInstance();
        } catch (IllegalAccessException e) {
          if (Reflector.canControlMemberAccessible()) {
            constructor.setAccessible(true);
            return constructor.newInstance();
          } else {
            throw e;
          }
        }
      }
      constructor = type.getDeclaredConstructor(constructorArgTypes.toArray(new Class[0]));
      try {
        return constructor.newInstance(constructorArgs.toArray(new Object[0]));
      } catch (IllegalAccessException e) {
        if (Reflector.canControlMemberAccessible()) {
          constructor.setAccessible(true);
          return constructor.newInstance(constructorArgs.toArray(new Object[0]));
        } else {
          throw e;
        }
      }
    } catch (Exception e) {
      String argTypes = Optional.ofNullable(constructorArgTypes).orElseGet(Collections::emptyList)
          .stream().map(Class::getSimpleName).collect(Collectors.joining(","));
      String argValues = Optional.ofNullable(constructorArgs).orElseGet(Collections::emptyList)
          .stream().map(String::valueOf).collect(Collectors.joining(","));
      throw new ReflectionException("Error instantiating " + type + " with invalid types (" + argTypes + ") or values (" + argValues + "). Cause: " + e, e);
    }
  }

當(dāng)實(shí)體使用有參構(gòu)造參數(shù)

mybatis會(huì)調(diào)用createResultObject方法中createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs);

private Object createUsingConstructor(ResultSetWrapper rsw, Class<?> resultType, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, Constructor<?> constructor) throws SQLException {
    boolean foundValues = false;
    for (int i = 0; i < constructor.getParameterTypes().length; i++) {
      Class<?> parameterType = constructor.getParameterTypes()[i];
      String columnName = rsw.getColumnNames().get(i);
      TypeHandler<?> typeHandler = rsw.getTypeHandler(parameterType, columnName);
      Object value = typeHandler.getResult(rsw.getResultSet(), columnName);
      constructorArgTypes.add(parameterType);
      constructorArgs.add(value);
      foundValues = value != null || foundValues;
    }
    return foundValues ? objectFactory.create(resultType, constructorArgTypes, constructorArgs) : null;
  }

? 這個(gè)代碼片段里面有個(gè)TypeHandler，這個(gè)是mybatis的類型處理器，用來(lái)處理 JavaType 與 JdbcType 之間的轉(zhuǎn)換。

由代碼我們看出，當(dāng)實(shí)體使用有參構(gòu)造函數(shù)時(shí)，會(huì)遍歷有參構(gòu)造參數(shù)個(gè)數(shù)，根據(jù)有參構(gòu)造參數(shù)下標(biāo) 查找相應(yīng)的數(shù)據(jù)庫(kù)字段名稱，根據(jù)有參構(gòu)造字段類型以及數(shù)據(jù)庫(kù)字段名稱找類型處理器。然后使用TypeHandler來(lái)處理JavaType 與 JdbcType 之間的轉(zhuǎn)換。當(dāng)轉(zhuǎn)換異常，就會(huì)報(bào)錯(cuò)。

4、總結(jié)

解決Cannot determine value type from string 'xxx’的方法有2種

實(shí)體加無(wú)參構(gòu)造參數(shù)
mapper.xml中查詢的數(shù)據(jù)庫(kù)字段屬性的類型要和有參構(gòu)造器的字段類型一一匹配；查詢字段的個(gè)數(shù)要和有參構(gòu)造器個(gè)數(shù)一樣

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