JPA 增删改查 2

1.     <!--[endif]-->查找对象
   1）  <!--[endif]-->用find()方法查找对象

public void getPerson(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa");
        EntityManager em = factory.createEntityManager();
        //相当于Hibernate中的get方法
        Person person = em.find(Person.class, 1);
        System.out.println(person.getUserName());
        em.close();
        factory.close();
    }

 

  2）  <!--[endif]-->用getReference()方法查找对象

public void getPerson2(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa");
        EntityManager em = factory.createEntityManager();
        //相当于Hibername中的load方法，延迟加载
        Person person = em.getReference(Person.class, 1);
        System.out.println(person.getUserName());
        em.close();
        factory.close();
    }

 

 

find方法相当于Hibernate中的get方法，getReference方法相当于Hibernate中的load方法，具有延迟加载的作用；当查找的对象不存在时，find的方法返回null值，而getReference方法则抛出异常javax.persistence.EntityNotFoundException
  2.     <!--[endif]-->更新对象 

public void update(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();
        Person person = em.find(Person.class, 1);
        person.setUserName("kate");
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
将游离状态下的数据更新到数据库中 
public void updatePerson(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();
        Person person = em.find(Person.class, 1);
        //把实体管理器中的实体变成游离状态
        em.clear();
        person.setUserName("jerry");
        //把游离状态下的更新同步到数据库中
        em.merge(person);
        em.getTransaction().commit();
        em.close();
        factory.close();
    }

 

 

 

 

  3.     <!--[endif]-->删除

public void delete(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();
        Person person = em.find(Person.class, 1);
        em.remove(person);
        em.getTransaction().commit();
        em.close();
        factory.close();
    }