blocks|key|1070428|text|如果您只想知道对象是否存在，请向数据库发送一个SELECT+COUNT。这将返回0或1。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1070429|异常处理的开销不是很大(除非您在正常操作期间执行了数百万次)，所以我不会费心。|1070430|但是代码并不能真正反映你的意图。因为getSingleResult()在内部调用getResultList()，所以更清晰，如下所示：|1070431|public+boolean+objExists(...)+{
++++return+getResultList(...).size()+==+1;
}|code-block|syntax|javascript|1070432|如果您按对象id查询，并且启用了缓存，那么在对象已经加载的情况下，这将变成一个在缓存中的简单查找。|1070433|entityMap^0|N|C|0|0|I|H|14|F|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|J|3|K|5|L|7|11|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|12|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|R|$]]

If you just want to know whether the object exists, send a <code>SELECT COUNT</code> to your database. That will return 0 or 1.

The cost of the exception handling isn't that big (unless you do that millions of times during a normal operation), so I wouldn't bother.

But the code doesn't really reflect your intention. Since <code>getSingleResult()</code> calls <code>getResultList()</code> internally, it's clearer like so:

<pre><code>public boolean objExists(...) {
 return getResultList(...).size() == 1;
}
</code></pre>

If you query by object id and you have caching enabled, that will become a simple lookup in the cache if the object has already been loaded.

blocks|key|1070481|text|如果按主键进行搜索，还可以使用Entitymanger.find(entityClass,+primaryKey)，它在实体不存在时返回null。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1070482|entityMap^0|F|16|1W|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

If you are searching by primary key you can also use <code>Entitymanger.find(entityClass, primaryKey)</code> which returns <code>null</code> when the entity does not exist.

blocks|key|1070548|text|尽量避免仅仅为了检查实体是否存在而将实体加载到会话(getSingleResult())中。在这里，计数更好。使用Criteria+Query+API，它将如下所示：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1070549|public+<E+extends+AbstractEntity>+boolean+exists(final+Class<E>+entityClass,+final+int+id)+{
++++final+EntityManager+em+=+getEntityManager();
++++final+CriteriaBuilder+cb+=+em.getCriteriaBuilder();

++++final+CriteriaQuery<Long>+cq+=+cb.createQuery(Long.class);
++++final+Root<E>+from+=+cq.from(entityClass);

++++cq.select(cb.count(from));
++++cq.where(cb.equal(from.get(AbstractEntity_.id),+id));

++++final+TypedQuery<Long>+tq+=+em.createQuery(cq);
++++return+tq.getSingleResult()+>+0;
}|code-block|syntax|javascript|1070550|entityMap^0|Q|H|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Try to avoid loading the entity into the session (<code>getSingleResult()</code>) just to check for it's existence. A count is better here. With the Criteria Query API it would look something like this:

<pre><code>public &lt;E extends AbstractEntity&gt; boolean exists(final Class&lt;E&gt; entityClass, final int id) {
 final EntityManager em = getEntityManager();
 final CriteriaBuilder cb = em.getCriteriaBuilder();

 final CriteriaQuery&lt;Long&gt; cq = cb.createQuery(Long.class);
 final Root&lt;E&gt; from = cq.from(entityClass);

 cq.select(cb.count(from));
 cq.where(cb.equal(from.get(AbstractEntity_.id), id));

 final TypedQuery&lt;Long&gt; tq = em.createQuery(cq);
 return tq.getSingleResult() &gt; 0;
}
</code></pre>

blocks|key|856182|text|只需在查询中使用count(e)，就不会抛出NoResultException，从而避免加载entity对象|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|856183|因此Java代码可以如下所示：|856184|public+boolean+isRecordExist()+{
++++String+query+=+"select+count(e)+from+YOUR_ENTITY+e+where+....";
++++//+you+will+always+get+a+single+result
++++Long+count+=+(Long)+entityManager.createQuery(+query+).getSingleResult();
++++return+(+(+count.equals(+0L+)+)+?+false+:+true+);
}|code-block|syntax|javascript|856185|希望这对某人有帮助:)|856186|entityMap^0|8|8|M|H|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

Simply use <code>count(e)</code> in your query, so no <code>NoResultException</code> will be thrown and you will avoid loading the entity object 
So the Java code can be as follow:

<pre><code>public boolean isRecordExist() {
 String query = "select count(e) from YOUR_ENTITY e where ....";
 // you will always get a single result
 Long count = (Long) entityManager.createQuery( query ).getSingleResult();
 return ( ( count.equals( 0L ) ) ? false : true );
}
</code></pre>

Hope that helps someone :)

blocks|key|1070657|text|尝尝这个|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1070658|public+boolean+exists(Object+id){
++++return+getEntityManager().find(entityClass,+id)!=null;
}|code-block|syntax|javascript|1070659|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Try this 

<pre><code>public boolean exists(Object id){
 return getEntityManager().find(entityClass, id)!=null;
}
</code></pre>

I want to know whether a given record is present in a database or not. so far I have achieved this by writing a JPA query and the running it by <code>getSingleResult()</code> method. this would throw a <code>NoResultException</code> if the record with the given parameter does not exist. Of course, it's not a must for the record to exist, so it's the normal behaviour sometimes, that's why I asked to myself, is it neccessary to throw an Exception which I have to handle by a catch block? As far as I know the cost of Exception handling is quite big, so I'm not very satisfied with this solution, also, I don't even need the object, I only need to know it's existence in the DB.

Is there a better way to check whether an object exist or not? eg. using <code>getResultList()</code> and checking it's size maybe?

How to check if a record exists using JPA

数据库

Java

我想知道给定的记录是否存在于数据库中。到目前为止，我已经通过编写一个JPA查询并通过getSingleResult()方法运行它来实现这一点。如果带有给定参数的记录不存在，则会抛出NoResultException。当然，这不是记录存在的必要条件，所以有时这是正常的行为，这就是为什么我问自己，有必要抛出一个异常，我必须...

问如何使用JPA检查记录是否存在
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