JPA：合并和持久化上下文

Question

我有以下代码在使用 class:

public void updateEntity(Entity e){
   EntityManager em=entityManagerFactory.createEntityManager();
   em.getTransaction().begin();
   em.merge(e);
   em.getTransaction().commit();
   em.close();
}

public Entity readEntity(int id){
   EntityManager em=entityManagerFactory.createEntityManager();
   Entity result=em.find(Entity.class, id);
   em.close();
   return result;
}

请注意，在这两个函数中，实体管理器（据我所知是持久性上下文）已关闭。

所以当我这样做时：

Entity entity=service.readEntity(100);
entity.setXxx("lalala");
service.updateEntity(entity);

一切正常，我在日志中看到生成的 sql 仅更改了一个字段。但是当我这样做时：

Entity entity=new Entity(100);
entity.setXxx("lalala");
service.updateEntity(entity);

更改未写入数据库。请解释为什么。我使用 EclipseLink。

Answer 1

JPA 2.0 规范的以下摘录应该给出解释：

The merge operation allows for the propagation of state from detached entities onto persistent entities managed by the entity manager.

The semantics of the merge operation applied to an entity X are as follows:

• If X is a detached entity, the state of X is copied onto a pre-existing managed entity instance X' of the same identity or a new managed copy X' of X is created.

• If X is a new entity instance, a new managed entity instance X' is created and the state of X is copied into the new managed entity instance X'.

• If X is a removed entity instance, an IllegalArgumentException will be thrown by the merge operation (or the transaction commit will fail).

• If X is a managed entity, it is ignored by the merge operation, however, the merge operation is cascaded to entities referenced by relationships from X if these relationships have been annotated with the cascade element value cascade=MERGE or cascade=ALL annotation.

• For all entities Y referenced by relationships from X having the cascade element value cascade=MERGE or cascade=ALL, Y is merged recursively as Y'. For all such Y referenced by X, X' is set to reference Y'. (Note that if X is managed then X is the same object as X'.)

• If X is an entity merged to X', with a reference to another entity Y, where cascade=MERGE or cascade=ALL is not specified, then navigation of the same association from X' yields a reference to a managed object Y' with the same persistent identity as Y.

根据规范，您的 Entity 应该保存到数据库中。但是因为您的更改在两种情况下都是相同的（lalala 和 100）（find() 然后 merge() 然后创建 merge()），可能您没有注意到数据库中的更改。我已经测试了以下情况，一切都按预期工作：

我创建了一个新实体并合并如下（这在数据库中创建了一个 ID = 101 的新条目）：

@Test
public void createEmployee() {
    Employee emp = new Employee();
    emp.setName("Test");
    emp.setSalary(1000);
    tx.begin();
    em.merge(emp);
    tx.commit();
}

然后我做了以下并调用了merge()，这次只为ID=101的实体修改了name属性：

@Test
public void createEmployee() {
    Employee emp = new Employee();
    emp.setName("Test2");
    emp.setSalary(1000);
    emp.setId(101);
    tx.begin();
    em.merge(emp);
    tx.commit();
}

JPA：合并和持久化上下文

JPA: merge and persistence context

java

jpa

eclipselink