blocks|key|3216336|text|JPA+specification包含了对这些操作的语义的非常精确的描述，比javadoc更好：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3216337|3216338|应用于实体X的持久化操作的语义如下：|blockquote|style|BOLD|3216339|3216340|3216341|3216342|如果X是一个新实体，它将成为托管实体。实体X将在事务提交时或之前或作为刷新操作的结果输入到数据库中。|unordered-list-item|3216343|如果X是预先存在的托管实体，则持久化操作将忽略它。但是，如果从X到这些其他实体的关系是用cascade=PERSIST或cascade=ALL注释元素值注释的，或者是用等效的XML描述符元素指定的，则持久化操作将级联到X引用的实体。|CODE|3216344|如果X是已移除的实体，则它将成为托管实体。|3216345|如果X是分离的对象，则在调用持久化操作时可能会抛出EntityExistsException。对于X中的关系引用的所有实体Y，如果与Y的关系已用级联元素值cascade=PERSIST或cascade=ALL进行了批注，则|3216346|操作将应用于Y。|3216347|3216348|3216349|3216350|应用于实体X的merge操作的语义如下：|3216351|3216352|3216353|3216354|如果X是分离的实体，则将X的状态复制到具有相同身份的预先存在的受管实体实例X‘上，或者创建X的新的受管副本X’。|3216355|如果X是新的实体实例，则创建新的受管实体实例X‘，并且将X的状态复制到新的受管实体实例X’中。|3216356|如果X是移除的实体实例，合并操作将引发IllegalArgumentException+(否则事务提交将失败)。|3216357|如果X是托管实体，则合并操作将忽略它，但是，如果X中的关系已使用级联元素值cascade=MERGE或cascade=ALL对这些关系进行了批注，则合并操作将级联到这些关系引用的实体。如果X中的关系引用的所有实体Y都具有级联元素值cascade=MERGE或cascade=ALL，则Y将递归合并为Y‘。对于X引用的所有这样的Y，X‘被设置为引用Y’。(请注意，如果X是托管的，则X与X‘是相同的对象。)|3216358|如果X是合并到X’的实体，则引用另一个实体Y，其中未指定cascade=MERGE或cascade=ALL，则从X‘导航相同的关联将产生对具有与Y相同的持久标识的托管对象Y’的引用。|3216359|3216360|entityMap|0|LINK|mutability|MUTABLE|url|http://www.jcp.org/en/jsr/detail?id=317^0|0|H|0|0|0|7|3|0|0|0|0|0|18|F|1O|B|0|0|P|L|25|F|2L|B|0|0|0|0|0|7|5|0|0|0|0|0|0|J|O|0|11|D|1F|B|37|D|3L|B|0|S|D|16|B|0|0^^$0|@$1|2|3|4|5|6|7|1P|8|@]|9|@$A|1Q|B|1R|1|1S]]|C|$]]|$1|D|3|-4|5|6|7|1T|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|1U|8|@$A|1V|B|1W|H|I]]|9|@]|C|$]]|$1|J|3|-4|5|6|7|1X|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|1Y|8|@]|9|@]|C|$]]|$1|L|3|-4|5|6|7|1Z|8|@]|9|@]|C|$]]|$1|M|3|N|5|O|7|20|8|@]|9|@]|C|$]]|$1|P|3|Q|5|O|7|21|8|@$A|22|B|23|H|R]|$A|24|B|25|H|R]]|9|@]|C|$]]|$1|S|3|T|5|O|7|26|8|@]|9|@]|C|$]]|$1|U|3|V|5|O|7|27|8|@$A|28|B|29|H|R]|$A|2A|B|2B|H|R]|$A|2C|B|2D|H|R]]|9|@]|C|$]]|$1|W|3|X|5|O|7|2E|8|@]|9|@]|C|$]]|$1|Y|3|-4|5|6|7|2F|8|@]|9|@]|C|$]]|$1|Z|3|-4|5|6|7|2G|8|@]|9|@]|C|$]]|$1|10|3|-4|5|6|7|2H|8|@]|9|@]|C|$]]|$1|11|3|12|5|G|7|2I|8|@$A|2J|B|2K|H|I]]|9|@]|C|$]]|$1|13|3|-4|5|6|7|2L|8|@]|9|@]|C|$]]|$1|14|3|-4|5|6|7|2M|8|@]|9|@]|C|$]]|$1|15|3|-4|5|6|7|2N|8|@]|9|@]|C|$]]|$1|16|3|17|5|O|7|2O|8|@]|9|@]|C|$]]|$1|18|3|19|5|O|7|2P|8|@]|9|@]|C|$]]|$1|1A|3|1B|5|O|7|2Q|8|@$A|2R|B|2S|H|R]]|9|@]|C|$]]|$1|1C|3|1D|5|O|7|2T|8|@$A|2U|B|2V|H|R]|$A|2W|B|2X|H|R]|$A|2Y|B|2Z|H|R]|$A|30|B|31|H|R]]|9|@]|C|$]]|$1|1E|3|1F|5|O|7|32|8|@$A|33|B|34|H|R]|$A|35|B|36|H|R]]|9|@]|C|$]]|$1|1G|3|-4|5|6|7|37|8|@]|9|@]|C|$]]|$1|1H|3|-4|5|6|7|38|8|@]|9|@]|C|$]]]|1I|$1J|$5|1K|1L|1M|C|$1N|1O]]]]

<a href="http://www.jcp.org/en/jsr/detail?id=317" rel="noreferrer">JPA specification</a> contains a very precise description of semantics of these operations, better than in javadoc:

<blockquote>
 The semantics of the persist
 operation, applied to an entity X are
 as follows: 
 
 <ul>
 <li>If X is a new entity, it
 becomes managed. The entity X will be
 entered into the database at or before
 transaction commit or as a result of
 the flush operation.</li>
 <li>If X is a
 preexisting managed entity, it is
 ignored by the persist operation.
 However, the persist operation is
 cascaded to entities referenced by X,
 if the relationships from X to these
 other entities are annotated with the
 <code>cascade=PERSIST</code> or <code>cascade=ALL</code>
 annotation element value or specified
 with the equivalent XML descriptor
 element.</li>
 <li>If X is a removed entity,
 it becomes managed.</li>
 <li>If X is a
 detached object, the
 <code>EntityExistsException</code> may be thrown
 when the persist operation is invoked,
 or the <code>EntityExistsException</code> or
 another <code>PersistenceException</code> may be
 thrown at flush or commit time.</li>
 <li>For
 all entities Y referenced by a
 relationship from X, if the
 relationship to Y has been annotated
 with the cascade element value
 <code>cascade=PERSIST</code> or <code>cascade=ALL</code>, the
 persist operation is applied to Y.</li>
 </ul>
</blockquote>

<hr>

<blockquote>
 The semantics of the merge operation
 applied to an entity X are as follows:
 
 <ul>
 <li>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.</li>
 <li>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'.</li>
 <li>If X is a
 removed entity instance, an
 <code>IllegalArgumentException</code> will be
 thrown by the merge operation (or the
 transaction commit will fail).</li>
 <li>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 <code>cascade=MERGE</code> or
 <code>cascade=ALL</code> annotation.</li>
 <li>For all
 entities Y referenced by relationships
 from X having the cascade element
 value <code>cascade=MERGE</code> or <code>cascade=ALL</code>, Y
 is merged recursively as Y'. For all
 such Y referenced by X, X' is set to
 reference Y'. (Note that if X is
 managed then X is the same object as
 X'.)</li>
 <li>If X is an entity merged to X',
 with a reference to another entity Y,
 where <code>cascade=MERGE</code> or <code>cascade=ALL</code> is
 not specified, then navigation of the
 same association from X' yields a
 reference to a managed object Y' with
 the same persistent identity as Y.</li>
 </ul>
</blockquote>

blocks|key|290349|text|这是来自JPA的。以一种非常简单的方式：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|290350|290351|persist(entity)应该与全新的实体一起使用，以将它们添加到数据库中(如果实体已经存在于数据库中，则会抛出EntityExistsException+)。如果实体被分离并更改，则应使用|unordered-list-item|290352|merge(entity)将实体放回持久性上下文。|290353|290354|entityMap^0|4|3|0|0|0|F|0|0|D|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|R|8|@]|D|@]|E|$]]|$1|G|3|H|5|I|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|J|3|K|5|I|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|N|$]]

This is coming from <code>JPA</code>. In a very simple way:

<ul>
<li><code>persist(entity)</code> should be used with totally new entities, to add them to DB (if entity already exists in DB there will be EntityExistsException throw). </li>
<li><code>merge(entity)</code> should be used, to put entity back to persistence context if the entity was detached and was changed.</li>
</ul>

blocks|key|284808|text|Persist应该只在新实体上调用，而merge则用于重新附加分离的实体。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|284809|如果您使用的是指定的生成器，那么使用merge而不是persist可能会导致冗余的SQL语句。|284810|此外，为托管实体调用merge也是错误的，因为托管实体是由Hibernate自动管理的，并且在刷新持久性上下文时，脏检查机制会将它们的状态与数据库记录同步。|284811|entityMap^0|J|5|0|I|5|Q|7|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|T|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|K|$]]

Persist should be called only on new entities, while <code>merge</code> is meant to reattach detached entities.
If you're using the assigned generator, using <code>merge</code> instead of <code>persist</code> can cause a redundant SQL statement.
Also, calling merge for managed entities is also a mistake since managed entities are automatically managed by Hibernate, and their state is synchronized with the database record by the dirty checking mechanism upon flushing the Persistence Context.

blocks|key|287011|text|最重要的区别是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|287012|287013|在persist方法的情况下，如果要在持久性上下文中管理的实体已经存在于持久性上下文中，则忽略新实体。(什么也没发生)|unordered-list-item|offset|length|style|CODE|287014|，但在使用merge方法的情况下，已经在持久性上下文中管理的实体将被新实体替换(已更新)，并且此更新的实体的副本将返回。(从现在开始，如果您希望在持久性上下文中反映您的更改，则应对此返回的实体进行任何更改)|287015|287016|entityMap^0|0|0|1|7|0|5|5|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|P|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|Q|8|@$F|R|G|S|H|I]]|9|@]|A|$]]|$1|J|3|K|5|E|7|T|8|@$F|U|G|V|H|I]]|9|@]|A|$]]|$1|L|3|-4|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|N|$]]

The most important difference is this:

<ul>
<li>In case of <code>persist</code> method, if the entity that is to be managed in the persistence context, already exists in persistence context, the new one is ignored. (NOTHING happened)</li>
<li>But in case of <code>merge</code> method, the entity that is already managed in persistence context will be replaced by the new entity (updated) and a copy of this updated entity will return back. (from now on any changes should be made on this returned entity if you want to reflect your changes in persistence context)</li>
</ul>

What is the difference between persist() and merge() in Hibernate?

<code>persist()</code> can create a UPDATE &amp; INSERT query, eg:

<pre><code>SessionFactory sef = cfg.buildSessionFactory();
Session session = sef.openSession();
A a=new A();
session.persist(a);
a.setName("Mario");
session.flush();
</code></pre>

in this case query will be generated like this:

<pre><code>Hibernate: insert into A (NAME, ID) values (?, ?)
Hibernate: update A set NAME=? where ID=?
</code></pre>

so <code>persist()</code> method can generate an Insert and an Update.

Now with <code>merge()</code>:

<pre><code>SessionFactory sef = cfg.buildSessionFactory();
Session session = sef.openSession();
Singer singer = new Singer();
singer.setName("Luciano Pavarotti");
session.merge(singer);
session.flush();
</code></pre>

This is what I see in the database:

<pre><code>SINGER_ID SINGER_NAME
1 Ricky Martin
2 Madonna
3 Elvis Presley
4 Luciano Pavarotti
</code></pre>

Now update a record using <code>merge()</code>

<pre><code>SessionFactory sef = cfg.buildSessionFactory();
Session session = sef.openSession();
Singer singer = new Singer();
singer.setId(2);
singer.setName("Luciano Pavarotti");
session.merge(singer);
session.flush();
</code></pre>

This is what I see in the database:

<pre><code>SINGER_ID SINGER_NAME
1 Ricky Martin
2 Luciano Pavarotti
3 Elvis Presley
</code></pre>

What is the difference between persist() and merge() in JPA and Hibernate?

数据库

Hibernate中persist()和merge()有什么不同？persist()可以创建UPDATE & INSERT查询，例如：SessionFactory sef = cfg.buildSessionFactory();Session session = sef.openSession();A a=new A(...

问在JPA和Hibernate中persist()和merge()有什么区别？
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