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jpa EntityManager复杂查询
概念
EntityManager：EntityManager是JPA中用于增删改查的接口，它的作用相当于一座桥梁，连接内存中的java对象和数据库的数据存储。可以用getCriteriaBuilder()的方式获取CriteriaBuilder对象。CriteriaBuilder接口：用于构造标准查询、复合条件、表达式、排序等。可以通过createQuery的方式获取CriteriaQuery实例。CriteriaQuery接口：代表一个specific的顶层查询对象，它包含着查询的各个部分，比如：select 、from、where、group by、order by。Root接口：代表Criteria查询的根对象，定义了实体类型，能为将来导航获得想要的结果，它与SQL查询中的FROM子句类似
接下来上代码啦！
1. 注入entitymanager
@PersistenceContextprivate EntityManager em;2. 建立连接，执行查询操作
CriteriaBuilder cb = em.getCriteriaBuilder();CriteriaQuery<ADTO> q = cb.createQuery(ADTO.class);Root<A> root = q.from(A.class);Join<A, B> b = root.join("b", JoinType.LEFT);Join<A, C> c = root.join("c", JoinType.LEFT);Join<A, D> d = root.join("d", JoinType.LEFT);Join<A, E> e = root.join("e", JoinType.LEFT);Join<A, F> f = root.join("f", JoinType.LEFT);q.multiselect(//备注：ADTO要有对应构造函数哦 root.get("id"), root.get("version"), root.get("code"), root.get("name"), root.get("isAlterable"), root.get("mustCharge"), root.get("isConsignment"), root.get("memo"), b.get("id"), b.get("name"), c.get("id"), c.get("description"), d.get("id"), d.get("name"), e.get("id"), e.get("name"), f.get("id"), f.get("name"), root.get("standardPrice"), root.get("upperPrice"), root.get("lowerPrice"), root.get("isActive"));//还可以进行where.order by等操作哦List<ADTO> rs = em.createQuery(q).getResultList();到此就完成啦！！！
拓展
where条件
List<Predicate> predicates = new ArrayList<>();CriteriaBuilder.In<String> in = cb.in(root.get("code").as(String.class));for (String code: codes) { in.value(code);}predicates.add(in);if (startDate != null && !"".equals(startDate)) { predicateList.add(cb.greaterThan(birthdayPath, startDate));}Predicate[] pre = new Predicate[predicates.size()];q.where(predicates.toArray(pre));or 条件拼接
public List<Predicate> getOrSearchPredicates(CriteriaBuilder cb, Root<T> root, String keywords){ List<Predicate> predicates = new ArrayList<>(); Predicate predicate = null; Predicate predicate1 = null; try { UUID uuid = UUID.fromString(keywords); predicate1 = cb.equal(root.get("id"), uuid); }catch (Exception e){ log.info(e.getMessage()); }finally { Predicate predicate2 = cb.like(root.get("code"), "%"+keywords+"%"); Predicate predicate3 = cb.like(root.get("name"), "%"+keywords+"%"); if (predicate1 != null){ predicate = cb.or(predicate1, cb.or(predicate2,predicate3)); } else { predicate = cb.or(predicate2, predicate3); } } predicates.add(predicate); return predicates;}JPA 动态更新
CriteriaBuilder cb=entityManager.getCriteriaBuilder();CriteriaUpdate<T> op=cb.createCriteriaUpdate(clazz);Root<T> root=op.from(clazz);op.set(fieldName, value);op.where(cb.equal(root.get(keyName), delta.get(keyName)));entityManager.createQuery(op).executeUpdate();基于EntityManager的使用
1、最基础的查询
CriteriaBuilder cb = entityManager.getCriteriaBuilder();CriteriaQuery<User> cq = cb.createQuery(User.class);Root<User> root = cq.from(User.class); //from Usercq.select(root); //select * from Userjavax.persistence.criteria.Predicate pre = cb.equal(root.get("id").as(Integer.class),id);//id=1cq.where(pre);//where id=1Query query = entityManager.createQuery(cq);//select u from User u where u.id = 1System.out.println(query.getResultList());2、spring data jpa的toPredicate方法
public Predicate toPredicate(Root<RoleEntity> root, CriteriaQuery<?> query, CriteriaBuilder cb) { List<Predicate> list = new ArrayList<Predicate>(); list.add(cb.equal(root.get("dr").as(Integer.class), delete)); if (StringUtils.isNotBlank(searchParam)) { list.add(cb.or(cb.like(root.get("roleName").as(String.class), cb.literal("%" + searchParam + "%")), cb.like(root.get("roleCode").as(String.class), cb.literal("%" + searchParam + "%")))); } // 角色id if (roleId != null && !("".equals(roleId.trim()))) { list.add(cb.equal(root.get("id").as(String.class), roleId)); } // 管理员角色标识 if (category != null && category > 0) { list.add(cb.equal(root.get("category").as(Integer.class), category)); } //多角色Id if (roleids != null && roleids.size() > 0) { list.add(root.get("id").as(String.class).in(roleids)); } Predicate[] predicates = new Predicate[list.size()]; query.where(list.toArray(predicates)); query.orderBy(cb.asc(root.get("roleType").as(String.class)), cb.asc(root.get("roleProperty").as(String.class)), cb.asc(root.get("roleCode").as(String.class))); return query.getRestriction();}参考 org.springframework.data.jpa.repository.support.SimpleJpaRepository 的 findAll方法，如下
public List<T> findAll(Specification<T> spec) { return getQuery(spec, (Sort) null).getResultList();}protected TypedQuery<T> getQuery(Specification<T> spec, Sort sort) { CriteriaBuilder builder = em.getCriteriaBuilder(); CriteriaQuery<T> query = builder.createQuery(getDomainClass()); Root<T> root = applySpecificationToCriteria(spec, query); query.select(root); if (sort != null) { query.orderBy(toOrders(sort, root, builder)); } return applyRepositoryMethodMetadata(em.createQuery(query));}private TypedQuery<T> applyRepositoryMethodMetadata(TypedQuery<T> query) { if (metadata == null) { return query; } LockModeType type = metadata.getLockModeType(); TypedQuery<T> toReturn = type == null ? query : query.setLockMode(type); applyQueryHints(toReturn); return toReturn;}感谢各位的阅读，以上就是“jpa EntityManager复杂查询怎么实现”的内容了，经过本文的学习后，相信大家对jpa EntityManager复杂查询怎么实现这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是编程网，小编将为大家推送更多相关知识点的文章，欢迎关注！
