How does Hibernate determine whether an entity needs to be updated?

Hibernate ORM
1

User user = userService.findById(1L);
// do nothing
user = userService.save(user);

no update sql to run because (user) have nothing change.
I want to know where is the source code that hibernate5 to decide whether need update.

23333…my english is so bad.

2

When you call save, Spring calls merge so the detached entity state is copied onto a newly fetched one. Then, the dirty checking mechanism detects all the modifications that were done due to copying the detached entity state