关于hibernate的merge、attachDirty、attachClean方法的应用

今天自动生成了hibernate的方法之后发现了这几个方法,从网上查找了一下资料，还是不明白，有哪位大侠可以详细解释一下吗？究竟什么时候用的，好像没怎么用到？
// 关于merge() attachDirty() attachClean()三种方法下面做一个简单的介绍

public CodeDepts merge(Usertable detachedInstance) {
log.debug("merging Usertable instance");
try {
Usertable result = (Usertable) getSession()
.merge(detachedInstance);
log.debug("merge successful");
return result;
} catch (RuntimeException re) {
log.error("merge failed", re);
throw re;
}
}


public void attachDirty(Usertable instance) {
log.debug("attaching dirty Usertable instance");
try {
getSession().saveOrUpdate(instance);
log.debug("attach successful");
} catch (RuntimeException re) {
log.error("attach failed", re);
throw re;
}
}


public void attachClean(Usertable instance) {
log.debug("attaching clean Usertable instance");
try {
getSession().lock(instance, LockMode.NONE);
log.debug("attach successful");
} catch (RuntimeException re) {
log.error("attach failed", re);
throw re;
}
｝

老大 你给出的例子中讲得很清楚的

** * 将传入的detached状态的对象的属性复制到持久化对象中，并返回该持久化对象 * 如果该session中没有关联的持久化对象，加载一个，如果传入对象未保存，保存一个副本并作为持久对象返回，传入对象依然保持detached状态。 * @see com.codedepts */ public codedepts merge(codedepts detachedinstance) { log.debug("merging codedepts instance"); try { codedepts result = (codedepts) getsession() .merge(detachedinstance); log.debug("merge successful"); return result; } catch (runtimeexception re) { log.error("merge failed", re); throw re; } } public void attachdirty(codedepts instance) { log.debug("attaching dirty codedepts instance"); try { getsession().saveorupdate(instance); log.debug("attach successful"); } catch (runtimeexception re) { log.error("attach failed", re); throw re; } } public void attachclean(codedepts instance) { log.debug("attaching clean codedepts instance"); try { getsession().lock(instance, lockmode.none); log.debug("attach successful"); } catch (runtimeexception re) { log.error("attach failed", re); throw re; } }