我的问题涉及EntityManager.getReference.鉴于我在一个JPA会话中,我可以确定对于同一个实体和相同的主键两次调用EntityManager.getReference我总是得到相同的
java对象实例吗？
对于两个不同的会话,我怀疑会得到两个不同的java对象实例 – 是不是真的如此？
我有兴趣了解一般规则,而不知道具体实施的工作原理.是否由规格定义？ (我自己无法找到它).
Person p1 = EntityManager.getReference(Person.class, 1L);
Person p2 = EntityManager.getReference(Person.class, 1L);
if (p1 == p2) {
System.out.println("SAME");
} else {
System.out.println("DIFF");
}
最佳答案 是的,它是JPA的基本保证 – 在持久化上下文(即session,EntityManager)的范围内,托管实体的对象标识与其数据库标识匹配：
7.1 Persistence Contexts
A persistence context is a set of managed entity instances in which for any persistent entity identity there is a unique entity instance.
和getReference()返回一个托管实例：
3.2.8 Managed Instances
…
The contains() method can be used to determine whether an entity instance is managed in the current persistence context.
The contains method returns true:
- If the entity has been retrieved from the database or has been returned by getReference, and has not been removed or detached.
- If the entity instance is new, and the persist method has been called on the entity or the persist operation has been cascaded to it.
而且,这个保证意味着在持久化上下文的范围内,无论你如何获得它,你都会得到相同id的实体实例(通过find(),getReference(),merge(),query或relationship遍历) .
例如,如果您从getReference()获取代理,则该实体的所有进一步工作都将通过该代理进行：
Person p1 = EntityManager.getReference(Person.class, 1L);
Person p2 = EntityManager.find(Person.class, 1L);
assertSame(p1, p2);
也可以看看：
