JPA EntiityManager.find方法

Posted 2020-12-19 wumingoo1

find 方法

• 立即加载：在调用find方法的时候，就会查找缓存，找不到立即发送sql语句查询数据库。
• 找到返回实体类对象，可以用于remove，setter，remove之后还可以调用persist保存。
• 找不到返回null。

find 方法测试

find 之后 setter 实现更新

@Test
public void test1() {
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
    EntityManager em = factory.createEntityManager();
    EntityTransaction tx = em.getTransaction();
    tx.begin();

    Student s = em.find(Student.class, 1L);
    /*

    Hibernate:
        select
            student0_.sid as sid1_0_0_,
            student0_.sage as sage2_0_0_,
            student0_.sname as sname3_0_0_
        from
            Student student0_
        where
            student0_.sid=?

     */
    s.setSage(999); // ORM思想：操作Managed状态的实体类对象就相当于操作表中的记录

    tx.commit();
    /*

    Hibernate:
        update
            Student
        set
            sage=?,
            sname=?
        where
            sid=?

     */
    em.close();
    factory.close();
}

find 之后 remove 实现删除

@Test
public void test2() {
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
    EntityManager em = factory.createEntityManager();
    EntityTransaction tx = em.getTransaction();
    tx.begin();

    Student s = em.find(Student.class, 1L);
    /*

    Hibernate:
        select
            student0_.sid as sid1_0_0_,
            student0_.sage as sage2_0_0_,
            student0_.sname as sname3_0_0_
        from
            Student student0_
        where
            student0_.sid=?

     */
    em.remove(s); // ORM思想：操作Managed状态的实体类对象就相当于操作表中的记录

    tx.commit();
    /*

    Hibernate:
        delete
        from
            Student
        where
            sid=?

     */
    em.close();
    factory.close();
}

find 之后 remove 再 persist 相当于删除后又保存

@Test
public void test() {
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
    EntityManager em = factory.createEntityManager();
    EntityTransaction tx = em.getTransaction();
    tx.begin();

    Student s = em.find(Student.class, 1L);
    /*

    Hibernate:
        select
            student0_.sid as sid1_0_0_,
            student0_.sage as sage2_0_0_,
            student0_.sname as sname3_0_0_
        from
            Student student0_
        where
            student0_.sid=?

     */
    em.remove(s);

    em.persist(s);

    tx.commit();
    em.close();
    factory.close();
}

find 找不到返回 null

@Test
public void test2() {
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
    EntityManager em = factory.createEntityManager();
    EntityTransaction tx = em.getTransaction();
    tx.begin();

    Student s = em.find(Student.class, 1027500L); // 找不到返回 null
    /*

    Hibernate:
        select
            student0_.sid as sid1_0_0_,
            student0_.sage as sage2_0_0_,
            student0_.sname as sname3_0_0_
        from
            Student student0_
        where
            student0_.sid=?

     */
    System.out.println(s); // null

    tx.commit();
    em.close();
    factory.close();
}