主键一定要有么是什么意思？是问可不可以修改一个没有主键的表，还是问obj这个对象中表示主键的属性必须有值？
关于主键生成策略，啥是逻辑上的，啥是物理上的？

建议你看看官网文档，
http://docs.jboss.org/hibernate/orm/4.1/devguide/en-US/html/
不知道你用的是哪个版本的，这个链接是4.1最新版的。
Entity states
new, or transient - the entity has just been instantiated and is not associated with a persistence context. It has no persistent representation in the database and no identifier value has been assigned.
managed, or persistent - the entity has an associated identifier and is associated with a persistence context.
detached - the entity has an associated identifier, but is no longer associated with a persistence context (usually because the persistence context was closed or the instance was evicted from the context)
removed - the entity has an associated identifier and is associated with a persistence context, however it is scheduled for removal from the database.

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