获取EntityManager的不同方法 [英] Different ways of getting the EntityManager

问题描述

我看到的用于创建EntityManager的惯用法是这样的:

The usual idiom I see for creating the EntityManager is something like this:

public class BaseDao {
    private static final String PERSISTENCE_UNIT_NAME = "Employee";

    EntityManagerFactory factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);

    public EntityManager getEntityManager() {
      return factory.createEntityManager();
    } 
}

然后按如下所示使用它:

Employee emp = new Employee();
emp.setName("Joe M");
getEntityManager().persist(emp);

问题是为什么不这样做:

public class BaseDao{
    private static final String PERSISTENCE_UNIT_NAME = "Employee";
    EntityManagerFactory factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
    private EntityManager entityManager = null;


public void setEntityManger() {
    EntityManagerFactory factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
    this.entityManager = factory.createEntityManager();

    }

    public EntityManager getEntityManager() {
        return this.entityManager;
    }
}

换句话说，是否需要始终通过factory.createEntityManager()获取实体管理器?还是可以将其创建为实例(甚至是静态)变量并像这样进行检索?

In other words is there a need to always get the entity manager through factory.createEntityManager()? or can it be created as an instance (or even static) variable and retrieved like that?

为澄清起见，我正在谈论的是不使用EJB或Spring容器的环境.

To clarify, I am talking about an environment that doesn't use EJB or Spring containers.

谢谢.

推荐答案

有两种创建EntityManager 实例的方法.

There are two ways to create EntityManager instances.

一种方法是用于SDK应用程序，在单元测试中，我经常使用这种方法.这就是您的示例中的内容:

One way is for SDK applications, and I use this way a lot in unit testing. This is what you have in your example:

EntityManagerFactory factory = 
  Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);

在企业应用程序中，您可以让容器为您创建它们，并在需要时注入它们.

In Enterprise applications you let the container create them for you and inject them when needed.

EntityManager只是JDBC连接的包装.它的重量很轻，可以创建和销毁而不会降低性能.

EntityManager is just a wrapper around a JDBC connection. It's very light weight and can be created and destroyed without performance penalty.

请记住，EntityManager并非线程安全的，因此，如果您有一个实例，则可能需要同步对其的访问.参见交易基础了解详情.

Keep in mind that the EntityManager is not thread safe, so if you have one instance, you may need to synchronize access to it. See transaction basics for details.

这是我大致的处理方式:

Here's how I would do it (roughly):

public class BaseDao{
  private static final String PERSISTENCE_UNIT_NAME = "Employee";
  private static EntityManagerFactory factory = 
    Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);

  public void create(MyEntiy person){
    EntityManager em = factory.createEntityManager();
    em.getTransaction().begin();
    // do what ever you need 
    em.getTransaction().commit();
    em.close();
  }

  // add more methods to the dao.
}

一旦您将此原型制作好并准备就绪，就可以使用通用DAO.

Once you get this protoyped and ready, you can use a generic DAO.

这篇关于获取EntityManager的不同方法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！