jpa关联查询left join（jpa join 子查询）

1、IndexTagController.java

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 @GetMapping("/tags/{id}") public String types(@PageableDefault(size = 3,sort = {"updateTime"},direction = Sort.Direction.DESC)Pageable pageable, @PathVariable long id, Model model, HttpSession session){ //找到所有的标签，并且按照标签新闻量排序 List<Tag> tags = tagService.listTagTop(50); if(id == -1){ //得到最大数据量的分类 id = tags.get(0).getId(); } model.addAttribute("tags",tags); model.addAttribute("page",newsService.listNews(id,pageable)); model.addAttribute("activeId",id); session.setAttribute("query",""); return "tags"; }

newService.listNews(id,pgeable)中id为标签的id，这个方法要做的就是查询出标签中包含id为参数id的所有新闻。

2、业务层代码

NewService.java是一个接口，其中存在以下方法

1 2	`//根据标签Id查找符合条件的新闻` `Page<News> listNews(long` `id,Pageable pageable);`

NewServiceImpl.java为实现NewService接口的类，实现listNews方法

1 2 3 4 5 6 7 8 9 10 11 12 @Override public Page<News> listNews(long id, Pageable pageable) { return newsRepository.findAll(new Specification() { @Override public Predicate toPredicate(Root root, CriteriaQuery cq, CriteriaBuilder cb) { //外连接查询 Join Join join =root.join("tags"); return cb.equal(join.get("id"),id); } },pageable); }

NewsRepository.java 继承了JpaSpecificationExecutor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 public interface NewsRepository extends JpaRepository<News,Long>, JpaSpecificationExecutor { @Query("select n from News n where n.recommend = true ") List<News> findTop(Pageable pageable); @Query("select n from News n where n.title like ?1 or n.content like ?1") Page<News> findByQuery(String query,Pageable pageable); @Query("select function('date_format',n.updateTime,'%Y') as year1 from News n group by year1 order by year1 desc ") List<String> findGroupYear(); @Query("select n from News n where function('date_format',n.updateTime,'%Y') = ?1 ") List<News> findByYear(String year); }