文章目录
背景
在JPA查询中，有时只需要查部分字段，这时jpa repository查出的是map，无法映射到Entity类。会提示错误:
org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [org.springframework.data.jpa.repository.query.AbstractJpaQuery$TupleConverter$TupleBackedMap] to type网上搜索有多种解决方案。这里列举一下。经过验证，本人采取了第一种方案，证明是可行的。
JPA 2.1以上的解决办法
实体中增加named query和result map
@SqlResultSetMapping(name = "EBookInfo", classes = @ConstructorResult(targetClass = EBookInfo.class, columns = { @ColumnResult(name = "book_id", type = Long.class), @ColumnResult(name = "book_name", type = String.class), @ColumnResult(name = "file_type", type = String.class) }))@NamedNativeQuery(name = "listExpressEbooks", query = "select book_id, book_name, file_type from ebook order by update_date desc", resultSetMapping = "EBookInfo")@Entity@Table(name = "ebook")public class Ebook { private Long bookId; private Integer authorId; private String authorName; private Integer categoryId; private String bookName; private String subTitle; private String tags; private String isbn; private String edition; private Byte bookType; private Integer star; private Integer downloadCount; private Byte status; private String fileType; private String outline; private String introduction; private String preface; private String cover; private Float price; private String publisher; private String bgColor; private String foreColor; private String titleColor; private String coverBackgroundId; private String coverPictureId; private Integer coverTemplateId; private String coverPictureMode; private Integer pageMode; private Timestamp requestDate; private Timestamp publishDate; private Timestamp updateDate;定义一个新的DTO对象
字段和查询的字段对应，需要提供构造函数:
@Datapublic class EBookInfo { private Long bookId; private String bookName; private String fileType; public EBookInfo(Long bookId, String bookName, String fileType) { this.bookId = bookId; this.bookName = bookName; this.fileType = fileType; }}repository中定义查询接口
@Query(name = "listExpressEbooks", nativeQuery = true) public List<EBookInfo> listExpressEbooks();其它方案
查询中构造新对象
public List<Blog> selectByYearMonth(String year, String month, int status) { String sql = String.format("select new Blog(blog.id, blog.title, blog.abs, blog.createtime) from Blog blog where blog.status = %d and YEAR(createtime) = %s and MONTH(createtime) = %s order by blog.createtime desc", status, year, month); //Query query = this.em.createNativeQuery(sql, "ExpressedResult"); Query query = this.em.createQuery(sql); List results = query.getResultList(); return results;}上述方法是之前我项目中代码库里的写法，Blog需要提供相应的构造函数。
自己写convertor
repository 返回 Tuple 对象，自己写代码手动转换为指定对象，repository层使用native查询。
这里要借助辅助类:
class NativeResultProcessUtils { public static <T> T processResult(Tuple source,Class<T> targetClass) { Object instantiate = BeanUtils.instantiate(targetClass); convertTupleToBean(source,instantiate,null); return (T) instantiate; } public static <T> T processResult(Tuple source,Class<T> targetClass,String... ignoreProperties) { Object instantiate = BeanUtils.instantiate(targetClass); convertTupleToBean(source,instantiate,ignoreProperties); return (T) instantiate; } public static void convertTupleToBean(Tuple source,Object target){ convertTupleToBean(source,target,null); } public static void convertTupleToBean(Tuple source,Object target, String... ignoreProperties){ //目标class Class<?> actualEditable = target.getClass(); //获取目标类的属性信息 PropertyDescriptor[] targetPds = BeanUtils.getPropertyDescriptors(actualEditable); //忽略列表 List<String> ignoreList = (ignoreProperties != null ? Arrays.asList(ignoreProperties) : null); //遍历属性节点信息 for (PropertyDescriptor targetPd : targetPds) { //获取set方法 Method writeMethod = targetPd.getWriteMethod(); //判断字段是否可以set if (writeMethod != null && (ignoreList == null || !ignoreList.contains(targetPd.getName()))) { //获取source节点对应的属性 String propertyName = targetPd.getName(); Object value = source.get(propertyName); if(value!=null && ClassUtils.isAssignable(writeMethod.getParameterTypes()[0], value.getClass())) { try {//判断target属性是否privateif (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers())) { writeMethod.setAccessible(true);}//写入targetwriteMethod.invoke(target, value); } catch (Throwable ex) {throw new FatalBeanException( "Could not copy property '" + targetPd.getName() + "' from source to target", ex); } } } } } }使用entityManager的Transformers.aliasToBean
未验证，Spring data jpa未必支持
使用entityManager的Transforms.ALIAS_TO_ENTITY_MAP
未验证
