JPA(hibernate实现）学习笔记

最新推荐文章于 2025-10-18 09:00:00 发布

最新推荐文章于 2025-10-18 09:00:00 发布 · 170 阅读

文章标签：

#Hibernate #JPA #SQL

本文介绍了使用ORM进行数据库操作的最佳实践，包括如何避免不必要的数据库查询、如何重新加载对象状态、如何进行分页查询以及如何利用预定义查询提高效率。此外还讨论了不同场景下使用merge方法的注意事项。

无查询获取reference

child = new Child();
child.SetName("Henry");
Parent parent = em.getReference(Parent.class, parentId); //no query to the DB
child.setParent(parent);
em.persist(child);

重新获取对象实例及其相关对象

em.persist(cat);
em.flush(); // force the SQL insert and triggers to run
em.refresh(cat); //re-read the state (after the trigger executes)

当确信只有一个对象会被返回时可用

getSingleResult()

投影，一次返回多个对象

Iterator kittensAndMothers = sess.createQuery(
            "select kitten, mother from Cat kitten join kitten.mother mother")
            .getResultList()
            .iterator();

while ( kittensAndMothers.hasNext() ) {
    Object[] tuple = (Object[]) kittensAndMothers.next();
    Cat kitten  = tuple[0];
    Cat mother  = tuple[1];
    ....
}

Scalar results

Iterator results = em.createQuery(
        "select cat.color, min(cat.birthdate), count(cat) from Cat cat " +
        "group by cat.color")
        .getResultList()
        .iterator();

while ( results.hasNext() ) {
    Object[] row = results.next();
    Color type = (Color) row[0];
    Date oldest = (Date) row[1];
    Integer count = (Integer) row[2];
    .....
}

分页

Query q = em.createQuery("select cat from DomesticCat cat");
q.setFirstResult(20);
q.setMaxResults(10);
List cats = q.getResultList(); //return cats from the 20th position to 29th

预先定义的查询

@javax.persistence.NamedQuery(name="eg.DomesticCat.by.name.and.minimum.weight", 
  query="select cat from eg.DomesticCat as cat  where cat.name = ?1 and cat.weight > ?2")

Query q = em.createNamedQuery("eg.DomesticCat.by.name.and.minimum.weight");
q.setString(1, name);
q.setInt(2, minWeight);
List cats = q.getResultList();

使用merge的情景