I have heard that after session.close() , the objects are in this session will be removed from cache.
If 'yes'
then why 'session2' object retrieving object from cache?
I am closing 'session1' when I'm fetching data first time(query executing) and data will be stored in cache(default first level cache), RIGHT !
But, after creating 'session2' object, I'm still able to retrieve particular object(no query is executing), means it is taking object from cache !
....Why ?
In image, I have paste my code
Image : my java files, table, persistent class, output
There are cache leves in Hibernate that you need to understand.
First-level cache:
The first-level cache is the Session cache and is a mandatory cache
through which all requests must pass. The Session object keeps an
object under its own power before committing it to the database.
Second-level cache:
Second level cache is an optional cache and first-level cache will
always be consulted before any attempt is made to locate an object in
the second-level cache. The second-level cache can be configured on a
per-class and per-collection basis and mainly responsible for caching
objects across sessions.
Query-level cache:
Hibernate also implements a cache for query resultsets that integrates
closely with the second-level cache.
All is quoted from here Cache Hibernate Tutorial. So in short the First Level of cache of that Session is removed. But that doesn't mean that the other Session Cache or the Level 2 cache need to be removed, because has his own Cache. Hope it helped you.
Related
I have read that using hibernate's second level cache, it can improve applications performance by having less database hits for data / object retrieval.
However, how does hibernate ensure that second level cache is up to date with the data in database.
For, example:
Suppose the below class is entity and persisted into the DB.
#Entity
class User {
Id
private int id;
private String str;
}
Now, if we have enabled second level cache, I understand that if we open different sessions then each session will hit the second level cache for retrieving object value.
Now, if data in database gets changes (for e.g. for row with id=1) say by some independent process/ manually changing the values, and we try to access the value, how does hibernate detect that the cache is having latest value (for id = 1).
In general, how does hibernate ensure that data in second level cache is consistent with the db values.
Thanks for your help.
Hibernate manages the cache himself, so when you update some entity thru hibernate Session it will invalidate cache entry assocciated with this entity - so cache is always fresh.
If another process (or even second JVM running the same hibernate application) updates record in database, Hibernate on first JVM is unaware of this fact and has stale object in his cache.
However you can use any cache implementation (cache provider) you want. There are many production-ready cache providers that allow you to configure how long given entity will be stored in cache. For example you can configure your cache to invalide all entities after 30 seconds and so on.
If you use EhCache cache provider you can provide such configuration:
<cache name="com.my.company.Entity"
maxElementsInMemory="1000"
eternal="false"
timeToIdleSeconds="7200"
timeToLiveSeconds="7200"
overflowToDisk="false"
memoryStoreEvictionPolicy="LRU"/>
You can find more information abount L2 cache here:
http://www.tutorialspoint.com/hibernate/hibernate_caching.htm
however there is a lot of useful tutorials about this.
It doesn't.
If you change data in the database without using hibernate it won't know about it and your cache and the database get out of sync.
How to force reread db data (without evicting second level cache before reading) and then put it to cache.
The use case is as following:
There is a service, which uses dictionary data. The service experiences high load, so performance is the must.
I would like dictionary data to be always available in the second level cache for the service.
The dictionary data can be externally modified, so the cache needs to be refreshed periodically.
Is there a possibility to 'refresh' second level cache in such a way that other clients of SessionFactory will not cause a hit to db (will get old items while updated data is being read).
ehcache is used as cache provider, but it can be reasonably changed.
Every now and again, to cause a read through to the db, you can set the CacheMode on the session to REFRESH - the data will be fetched from the db for that one client, and any updates added to the second-level cache. Other clients querying using the normal cache mode will read from the second-level cache without hitting the db.
See CacheMode
I just want some general info about standard purpose of using L1 cache and L2 cache.
I'm curious because I'm investigating the system with terracotta as 2nd level cache and I've found that it also has 1st-level cache.
L1 Cache is the cache that exists per Hibernate session, and this cache is not shared among threads. This cache makes use of Hibernate's own caching.
L2 Cache is a cache that survives beyond a Hibernate session, and can be shared among threads. For this cache you can use either a caching implementation that comes with Hibernate like EHCache or something else like JBossCache2
In JPA/Hibernate (and other similar ORM tools), the L1 cache is the transactional cache i.e. the entities stored from when you open a transaction to when you close it. This is almost never a shared cache (other threads can't make use of it). In JPA, this would usually be held by the EntityManager.
The L2 cache is a full (typically) shared cache. If you have multiple threads/queries pulling in data, then they can make use of entities that have already been retrieved by other threads that are still live in the cache. In JPA, this would usually be held by the EntityManagerFactory.
GaryF is not wrong, but is not technically right :-) Anton is more correct on this, but to complement his answer:
First Level Cache: this is a "cache" which stores all the entities known by a specific session. So, if you have 3 transactions inside this session, it'll hold all entities touched by all three transactions. It gets cleared when you close the session or when you perform the "clear" method.
Second Level Cache: this is a "real" cache and is delegated to an external provider, such as Infinispan. In this cache, you have full control over the contents of the cache, meaning that you are able to specify which entries should be evicted, which ones should be retained longer and so on.
If Hibernate is anything similar to NHibernate (which it is, except the other way round), the Session is the first-level cache. Except that it is not cache in a general sense, but rather an identity map.
L1 By default enabled, you have to add some third party library like EH cache, Redis for L2.
You can't disable L1 in hibernate.
L1:The first-level cache is the cache per Hibernate Session cache and is a mandatory cache through which all requests must pass and this cache is not shared among threads.
L2:The second-level cache can be configured on a per-class and per-collection basis and mainly responsible for caching objects across sessions.L2 Cache is a cache that survives beyond a Hibernate session and can be shared among threads.
As mentioned in this article these are important differences :
First level Cache vs. Second-level Cache in Hibernate Now that we have
got some basic understanding of the first level and second level
cache, here are some differences between them:
The primary difference is that the first level cache is maintained
at the Session level while the second level cache is maintained at the
SessionFactory level.
The data stored in the first level cache is accessible to the only
Session that maintains it, while the second level cache is accessible
to all.
The first level cache is by default enabled while the second level
cache is by default disabled.
A couple of things to know about Hibernate's first level cache:
You can use the Session.evict() to remove the loaded entity from
the first level cache, can use the refresh() method to refresh the
cache, and can use the clear() method to remove all entities in cache.
You cannot disable the first level cache, it is always enabled.
Hibernate entities or database rows remain in cache only until
Session is open, once Session is closed, all associated cached data is
lost.
Read more: https://www.java67.com/2017/10/difference-between-first-level-and-second-level-cache-in-Hibernate.html#ixzz7B8pzzLzL
Does Hibernate use cache (second level or otherwise) if all I am doing is batch inserts?
No entities are being requested from the database, and no generators are used.
Also, would StatelessSession vs Session change the answer? What if I was using a Session with a JDBC batch size of 50? The cache I will be using is Ehcache
Doe Hibernate use cache (second level or otherwise) if all I am doing is batch inserts?
Newly inserted Entities instances are cached in the L1 cache (the session-level cache) before they are flushed to the database (see the section 13. Batch processing), hence the need to flush and clear your session regularly to prevent OOM.
Also, would StatelessSession vs Session change the answer?
Yes. As written in section 13.3. The StatelessSession interface : A StatelessSession has no persistence context associated with it and does not provide many of the higher-level life cycle semantics. In particular, a stateless session does not implement a first-level cache nor interact with any second-level or query cache.
What if I was using a Session with a JDBC batch size of 50?
This just means that you should flush/clear the session every 50 insert.
As far as I know Hibernate let's you configure entities and collections to be stored in a second-level cache.
When does Hibernate try to read these cached entities from the second-level cache and when does it hit the DB? Does Hibernate only read from the second-level cache when loading entities by calling Sesssion.get() and when initializing proxies (including collections)? Does Hibernate ever hit the second-level cache when executing HQL- or Criteria-Queries?
Examples?
2nd level cache contains only entities by their ids, so when retrieving an entity by id (i.e. get, load or resolving a proxy implicitly) a 2nd level cache may be accessed. Any other queries (hal, criteria) will bypass the cache and hit the DB - at least as long as no query cache is used as well.
(Note: the easiest way to answer that type of questions is to turn show_sql on and see what queries Hib generates.)
Sometimes query only return PKs of the records (e.g. for iteration queries) and then Hib can use the cache.
When retrieving linked objects cache can be used too.
I cannot though give you the precise rule here. I also suspect the answer depends on capabilities of dialect used.