I have assigned hibernate session to instance variable of an immutable class. The class uses this session object (instance variable) to create hibernate query. The code works fine.
Due to this is it possible that the connection pool does not have free connections to database.
Session will take a single connection from the connection pool. Other connections will be free. Please refer the Hibernate manual,
*It is advisable to have the org.hibernate.SessionFactory create and pool JDBC connections for you. If you take this approach, opening a org.hibernate.
Session is as simple as:
Session session = sessions.openSession(); // open a new Session
Once you start a task that requires access to the database, a JDBC connection will be obtained from the pool.*
Related
I have the below doubts on the Hikari or any database pooling concept.
When does the connection pool gets created?
Suppose if I have mentioned spring.datasource.hikari.maximum-pool-size=50
Will it create 50 database instances?
As by default, Spring scopes on classes in singleton, how 50 instances are created?
1:
A connection pool will be created when the first connection is creating. For example when the first SQL is executed. {#link com.zaxxer.hikari.HikariDataSource#getConnection()}.
This is my case and maybe it's different according the ORM you are using.
Connection instances will be created by the connection pool. {#link com.zaxxer.hikari.pool.HikariPool#poolEntryCreator} {#link com.zaxxer.hikari.pool.HikariPool#postFillPoolEntryCreator}
"spring.datasource.hikari.maximum-pool-size=50" means that the connection pool would not create connection instance more than 50. So it is the limit for connection instance count in connection pool.
2:
Spring Bean is singleton by default. But connection instances in the conneciton pool are not "spring bean" and they are created from "new PoolEntry". {#link com.zaxxer.hikari.pool.HikariPool#createPoolEntry}
Connection pooling helps reducing in creating database connections everytime a database call is encountered.
Pool would be nothing but set of connections (ideally active) (Like we have thread pool ), when requested will return one active connection ( or create one for first database request) , instance would be a misfit word here !
To answer first part of your question, it is initialized during application startup when that spring.datasource.hikari... property is encountered.
Below link explains this concept well
https://coderstea.in/post/best-practices/jdbc-connection-pooling-explained-with-hikaricp/
I was understanding about spring + hibernate + java integration. So i got clear picture on below given points:
Spring uses LocalSessionFactoryBean class to create SessionFactory which is hibernate class.
Application context load the definition for datasource, hibernate properties etc.
we can inject SessionFactory class in spring dao classes.
My question is on crud operation invocation, like
sessionfactory.getCurrentSession().get(--, -)
How it works internally in terms of using datasource or any other spring/hibernate related resources ?
I try to explain at hight level what happen, for more detail you can check the source code.
First of all in your line of code 2 methods are called:
getCurrrentSession() on session factory
get(--,--) in the returned session
In the first method hibernate use the CurrentSessionContext to retreive the acual session. The CurrentSessionContext implementation substantially looks if there is an open session with an open transaction related to your thread and your session facotry and return it; to make it simpler it loooks if you are doing something on this DB in this thread and allow You to continue. If the CurrentSessionContext doesn't find a session a new one is create. (Note there are many implementation of CurrentSessionContext by default JPA one is used)
After retreive the session the second method is executed. In the second method referring to the dialect and other objects a native sql query is generated. In your specific case the sql is sent over the session to the database, the step looks like:
if the session is not binded to a connection, ask for a db connection from the pool
send the sql over the connection and retreive a resultset
get the resultset and tranlaste it to the entity object wich will be returned
if you are doing a dml operation (update, insert ...) the sql is tored on the session and it will be sent after flushing the session(you can use the flush() method it, otherwise just commit and wait hibernate do ti for you). Important note, committing the Transaction means that the code will be execute on the DB. After commit hibernate, with his own timing, will open a transaction over the db connection (important hibernate transacion is not a db transaction) will execute all the generate sql statement and will commit the DB transaction. This is the flush operation, remember yo don't exactally know when the flush happen since you don't force it manually with the flush() method.
hope this hepl
r.
Hibernate use sessions to interact with the database, then what is the use of connection pool?
As per the configuration, hibernate default connection pool size is 1. If we open multiple sessions(as per the default configuration each session is a thread - current_session_context_class) in the hibernate, all those sessions will use the same connection object? If not what is the use of taking new connection in hibernate and how to get that new connection object?
I'm confused about the difference between DataSource and SessionFactory.
I think SessionFactory is a manager for retrieve Sessions (which are in fact connections to a database I guess).
DataSource has the method getConnection()
"Attempts to establish a connection with the data source that this DataSource object represents."
It means a DataSource object always works directly with the database?
If I have a pool of connections, DataSource will ask to the pool for a connection? Or will it try to get another connection to the database.
Are in fact DataSource and SessionFactory the same? Both try to give sessions/connections?
Datasource maintains the pool of connections, establishes the connection, knows how to connect etc.,
User configures data source either in the server(like websphere/weblogic) or in the spring/hibernate configuration file. Either way you give the handle of this data source to the sessionfactory during the app startup.
SessionFActory is an interface which hides all the details like opening the connection / closing the connections. Through out your development time you dont worry about these small things like loading driver/closing connection/opening etc., so you can concentrate on business logic/other important stuff.
How properly "lifecycle" of a Hibernate session under Spring should be done?
The SessionFactory is created automatically by Spring and is taking its DB connections from Glassfish connection pool. At the moment I am getting a Hibernate session via SessionFactory.getCurrentSession(). Then I start transaction, do the work and then commit() or rollback() at the end. Do I need to do any other actions like disconnect(), close(), flush() or any others at any time so connections would be properly returned back to the pool or is everything already automatically done by Spring?
With plenty of these methods it is a little confusing for me at the moment to understand when what should be done, maybe someone can point to right direction?
As SessionFactory is created automatically by Spring, Spring framework will take care of closing the connection.
Check out Spring Resource Management
If you want to check. You can check the log, if you are using logging for your app. It'll be like :
(main) INFO [AnnotationSessionFactoryBean] Closing Hibernate SessionFactory
I get following lines from this link
The main contract here is the creation of Session instances. Usually
an application has a single SessionFactory instance and threads
servicing client requests obtain Session instances from this factory.
The internal state of a SessionFactory is immutable. Once it is
created this internal state is set. This internal state includes all
of the metadata about Object/Relational Mapping.
Implementors must be threadsafe.
The policies about how the connection releases back to the connection pool have nothing to do with Spring .It is configured by Hibernate itself through the configuration parameter hibernate.connection.release_mode , which is identified by the enum in the org.hibernate.ConnectionReleaseMode
Start from version 3.1+ , the default value of the hibernate.connection.release_mode is auto which the corresponding ConnectionReleaseMode value depends on whether JTA or JDBC transaction is used. In case of JDBC transaction is used , it is set to ConnectionReleaseMode.AFTER_TRANSACTION (i.e after_transaction ).
The behaviour of ConnectionReleaseMode.AFTER_TRANSACTION is that : The connection will be returned to the connection pool after each transaction , that is by calling either transaction.commit() or transaction.rollback() , as well as calling session.close() and session.disconnect()
You can verify this behaviour in hibernate documentation Section 11.5.
Hope this link will guide you about session and transactions.
Then I start transaction, do the work and then commit() or rollback()
at the end. Do I need to do any other actions like disconnect(),
close(), flush() or any others at any time so connections would be
properly returned back to the pool or is everything already
automatically done by Spring?
As you call commit() on Transaction it will automatically closes the session, which ultimately calls close method on connection to return to it's pool.
When you are executing a hibernate query through SessionFactory.getCurrentSession() , Spring performs the necessary task of opening and closing the connection . The SessionFactory you are using in the spring config also calls the config.buildSessionFactory method internally .
Most of this happens in the implementations of the AbstractSessionFactoryBean. The closing of connecting is done by hibernate in the SessionFactoryImpl class using the statement settings.getConnectionProvider().close(); . In short , hibernate does everything for you . Spring just calls it's help when necessary.