JPA2.1更新数据库实体的正确方法（使用eclipseLink）？

我想知道我正在做什么来读取和更新我的数据库，这是一种方法，也是一种良好的做法

运行程序时，我会将数据库加载到内存中：

 public static void open() {
  EntityManagerFactory emf = Persistence.createEntityManagerFactory("htTeamManagerPU");
  EntityManager em = emf.createEntityManager();
  TypedQuery<Season> query = em.createQuery("select s from Season s",Season.class);
  seasons = query.getResultList(); }

---

这是工作良好，并更新我的数据库。我想知道的是，这是否是正确和最好的方法。谢谢

首先，您没有做两次相同的更改

在EntityManager生命周期内（这是一个简化过程，因为存在事务的超时概念和更多内容），它监视检索的实体发生了什么，因此，如果修改，它们的更改将转换为数据库，但一旦EntityManager生命周期结束，所有监视也将结束。问题是，您习惯于隐式操作（在代码中没有EntityManager的合并、删除或删除函数，所以只有自己的EntityManager才关心如何处理更改），而不是显式操作（使用EntityManager的合并、删除或创建由程序员声明的方法）所以您可能认为，一旦实体被修改，它就会自动提交，但不会：更改由EntityManager处理

由于playerList可能是在EntityManager生命周期之外修改的，所以没有人关心它的实体，所以更改不会在数据库中刷新，这会导致您的问题

在我描述的情况下，这种缺乏刷新是完全正常的。对于未处理的实体，获取实体管理器、开始事务等过程是完全正常的

关于您的问题“我想知道这是否是正确和最好的方法”，我已经解释了这个概念，但我可以提供一些代码改进建议。 例如，我将避免隐式EntityManager操作以避免混淆；使用显式方法，我会这样更改您的方法：

public void modifiedFinishTraining(int id){

/*This three lines are mandatory. You have to get an EntityManager to get access to     database and start a transaction to store your changes*/
EntityManagerFactory emf = Persistence.createEntityManagerFactory("htTeamManagerPU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();

/*case 1: use merge method. If you want to update your player or create if it is not at database, use it and avoid the em.find(Player.class, id) step*/
for (Player player: playerList){

if (player.getPlayerId()==id) {
//This way you avoid the step of find by be careful: merge updates existing or creates a new one.
em.merge(player);
}

}
/*case 2: If you want to check if exists and you dont want to create it in other case  do it*/
for (Player player: playerList){
  if (player.getPlayerId()==id) {
      //you find the player in database
       Player p1 = em.find(Player.class, player.getPlayerId());
       //check if player exists and if true, update           
       if(p1  != null){
         p1.setIsTeam((short)0);
         p1.setIsToSell((short)1);

         em.merge(p1);
       }
   }
}

//this would be done at the end of the operation to commit all changes at the same time
em.getTransaction().commit();   
em.close();
emf.close();
/*the upper three lines are mandatory. You must commit your changes because transaction is manually handled and once commited you have to close all to free resources*/

}   

---

您应该只在应用程序中创建一个

EntityManagerFactory

（可能在启动或缓存时创建）。您的问题的答案如下：

public void modifiedFinishTraining(int id){

/*This three lines are mandatory. You have to get an EntityManager to get access to     database and start a transaction to store your changes*/
EntityManagerFactory emf = Persistence.createEntityManagerFactory("htTeamManagerPU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();

/*case 1: use merge method. If you want to update your player or create if it is not at database, use it and avoid the em.find(Player.class, id) step*/
for (Player player: playerList){

if (player.getPlayerId()==id) {
//This way you avoid the step of find by be careful: merge updates existing or creates a new one.
em.merge(player);
}

}
/*case 2: If you want to check if exists and you dont want to create it in other case  do it*/
for (Player player: playerList){
  if (player.getPlayerId()==id) {
      //you find the player in database
       Player p1 = em.find(Player.class, player.getPlayerId());
       //check if player exists and if true, update           
       if(p1  != null){
         p1.setIsTeam((short)0);
         p1.setIsToSell((short)1);

         em.merge(p1);
       }
   }
}

//this would be done at the end of the operation to commit all changes at the same time
em.getTransaction().commit();   
em.close();
emf.close();
/*the upper three lines are mandatory. You must commit your changes because transaction is manually handled and once commited you have to close all to free resources*/

}