find 方法

• 立即載入：在呼叫find方法的時候，就會發送sql語句查詢資料庫。
• 找到返回實體類物件，可以用於remove，setter，remove之後還可以呼叫persist儲存。
• 找不到返回null。

find 方法測試

find之後setter實現更新

@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狀態的實體類物件就相當於操作表中的記錄

    tx.commit();
    /*

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

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

find之後remove實現刪除

@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狀態的實體類物件就相當於操作表中的記錄

    tx.commit();
    /*

    Hibernate:
        delete
        from
            Student
        where
            sid=?

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

find之後remove再persist相當於刪除後又儲存

@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();
}