Spring data JPA只查询部分字段问题及解决

目录

• 背景
• JPA 2.1以上的解决办法
• 实体中增加named query和result map
• 定义一个新的DTO对象
• repository中定义查询接口
• 其它方案
• 查询中构造新对象
• 自己写convertor
• 使用entityManager的Transformers.aliasToBean
• 使用entityManager的Transforms.ALIAS_TO_ENTITY_MAP
• 总结

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 prefaceandroid;
    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：

@Data
public 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")php;
    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 Nativwww.devze.comeResultProcessUtils {

        /**
         * tuple转实体对象
         * @param source tuple对象
         * @param targetClass 目标实体class
         * @param <T> 目标实体类型
         * @return 目标实体
         */
        p编程客栈ublic static <T> T processResult(Tuple source,Class<T> targetClass) {
            Object instantiate = BeanUtils.instantiate(targetClass);
            convertTupleToBean(source,instantiate,null);
            return (T) instantiate;
        }

        /**
         *
         * tuple转实体对象
         * @param source tuple对象
         * @param targetClass 目标实体class
         * @param <T> 目标实体类型
         * @param ignoreProperties 要忽略的属性
         * @return 目标实体
         */
        public static <T> T processResult(Tuple source,Class<T> targetClass,String... ignoreProperties) {
            Object instantiate = BeanUtils.instantiate(targetClass);
            convertTupleToBean(source,instantiate,ignoreProperties);
            return (T) instantiate;
        }

        /**
         * 把tuple中属性名相同的值复制到实体中
         * @param source tuple对象
         * @param target 目标对象实例
         */
        public static void convertTupleToBean(Tuple source,Object target){
            convertTupleToBean(source,target,null);
        }

        /**
         * 把tuple中属性名相同的值复制到实体中
         * @param source tuple对象
         * @param target 目标对象实例
         * @param ignoreProperties 要忽略的属性
         */
        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 pro编程pertyName = 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private
                            if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers())) {
                                writeMethod.setAccessible(true);
                            }
                            //写入target
                            writeMethod.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

总结

以上为个人经验，希望能给大家一个参考，也希望大家多多支持编程客栈(www.devze.com)。