java - Spring-data-jpa - .save() 什么时候不返回相同的实体？

Question

在这段代码中，我保存了主对象和该对象的外键。 我打印出“实体”和“事物”，它们应该是完全相同的对象。但他们不是。为什么？

thingList.forEach(entity -> {
            System.out.println(entity);

            // Save if the foreign key exists and isn't already saved in the database
            if(entity.ForeignKey() != null && ForeignKeyRepository.findOne(entity.ForeignKey().getId()) == null)
            {
                ForeignKeyRepository.save(entity.getForeignKey());
            }
            Thing thing = thingRepository.save(entity);
            System.out.println(thing);
        });

Answer 1

如果实体不是新的，并且在 EntityManager 的 session 中已经有一个代表数据库行的不同实例，您将获得该实例，并进行修改以匹配作为参数传递的实例，如返回值。

您可以通过检查实现和相关 JPA 文档来确认这一点。

save 方法在 SimpleJpaRepository 中实现

public <S extends T> S save(S entity) {

    if (entityInformation.isNew(entity)) {
        em.persist(entity);
        return entity;
    } else {
        return em.merge(entity);
    }
}

em 是 EntityManager。从它的 merge 方法文档中:

Returns: the managed instance that the state was merged to

JPA specification第 3.2.7.1 节更明确一点:

• 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.