Too many connections using Hibernate and mysql - mysql

Question

I am using Hibernate 3 and mysql sever 5.5 for myweb application with spring 3.0
I am getting exception as Too many connections......
My java file where I create session is as follows:
public class DBConnection {
static{
}
public Session getSession(){
Session session = null;
SessionFactory sessionFactory= null;
sessionFactory = new Configuration().configure().buildSessionFactory();
session = sessionFactory.openSession();
return session;
}
}
and I call this method where I need session
as
Session session=new DBConnection().getSession();
and after
transaction.commit();
I close session by using
session.close();
please help me in solving problem.......
my hibernate.cfg.xml is :
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/dbname</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">lax</property>
<property name="hibernate.connection.pool_size">100</property>
<property name="show_sql">true</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="hibernate.hbm2ddl.auto">update</property>
<property name=""></property>
<property name="hibernate.connection.release_mode">on_close</property>
</session-factory>
</hibernate-configuration>

Answer 1

This is because you are using a connection pool which got created as soon as you build SessionFactory, but the connections are acquired only when you open a Session. Now, you are closing the session, due to which connections are released, but are not closed and are held up by the pool. Now, you are again creating a SessionFactory, hence creating a new pool, then getting a session, hence creating a new connection and so on.. which will eventually reach the maximum number of connections allowed.
What you have to do is using one Connection Pool (using one SessionFactory) and getting and releasing the connections from the same pool.
public class DBConnection {
private static SessionFactory factory;
static {
factory = new Configuration().configure().buildSessionFactory();
}
public Session getSession() {
return factory.openSession();
}
public void doWork() {
Session session = getSession();
// do work.
session.close();
}
// Call this during shutdown
public static void close() {
factory.close();
}
}

Answer 2

You create new SessionFactorys every time you need a Session and don't close them.
Usually you need to create session factory only once during startup of your application, and close it during shutdown. For example, as follows:
public class DBConnection {
private static SessionFactory factory;
static {
factory = new Configuration().configure().buildSessionFactory();
}
public Session getSession() {
return factory.openSession();
}
// Call this during shutdown
public static void close() {
factory.close();
}
}
Also take a look at contextual sessions pattern.

Answer 3

I have used
Session session = SessionFactoryUtils.getSession(this.sessionFactory,
true);
to get session. Above the DAO layer, transaction with annotations (#Transactional) is used in add, edit or delete. Our application uses shiro for authentication and authorization. In each page access, it creates at least 5 connections. So the subsequent request keeps on increasing mysql connection. Thanks to #Venu I found out that
show variables like "max_connections";
shows default maximum connection available. Initially (I guess) it was set to 151. This number would soon exhaust because another variable "innodb_open_files" was set to 300. By intuition I can say if one table reaches maximum connection of 300 it would only go for clean up (just like garbage collection in java).
One would think, if we would increase the value of "max_connections" to 300 it solve this error, but as I said earlier "innodb_open_files" does not go for cleanup until one table reaches that limit. In our case user table which would be called frequently would reach that limit for cleanup to happen. This means, you cannot know before hand what is the perfect number for "max_connections".
In mysql configuration file "/etc/mysql/my.cnf" max_connections is set as
max_connections = 1000
I did not change
value of "innodb_open_files", which is default to "300"
mysql> show variables like "innodb_open_files";
During execution of your program you can use
mysql> show processlist;
to find out how many connections are open. I guess all the processes in Sleep state can be deleted in next cycle of cleanup (but its only my guess).
To simulate more than 1000 users, executing some url in your web project, you can use software like jmeter and set number of thread (users) to 1000. Use of more user also depends upon available RAM in your computer. I was able to simulate more than 3000 connections sending requests at 1 second interval. A good and sufficient tutorial on how to do this is listed on roseindia
Now my query. I have not closed session on any place. I guess #Transaction was created for that purpose (mysql proves that, because all those connections get cleaned once the limit reaches). Due to merge problem of detached objects, I had to used OpenSessionInViewFilter. OpenSessionInViewFilter (can be OpenSessionInViewInterceptor) opens the session a little longer for the merge to happen, which might be the reason so many connections remain open in mysql (Don't know clearly). Since it works for so many users it seems a good solution, but is there a better solution to make it work under default max connection size of mysql. i.e 151 connections.

Answer 4

I was facing the same issues with Hibernate mysql conenctions
The following two things helped fix my connection problems
import javax.inject.Singleton;
#Singleton
public class DBConnection {
private static SessionFactory factory;
static public Session getSession() {
if (factory == null) {
System.out.println("--------New connection----");
factory = new Configuration().configure("hibernate.cfg.xml")
.addAnnotatedClass(SomeClass.class)
.buildSessionFactory();
}
//System.out.println("--------old connection----");
return factory.openSession();
}
// Call this during shutdown
public static void close() {
factory.close();
}
}
Create a singleton class for getting a DBConnection
Do not forget to perform session.close() when your query is complete
Also in "hibernate.cfg.xml"
add this line
<hibernate-configuration>
<session-factory>
....
<property name="connection.pool_size">100</property>
....
</session-factory>
</hibernate-configuration>
under

Answer 5

JAVA ISSUE
Please check this answer : https://stackoverflow.com/a/10785770/598424
MYSQL ISSUE
YOU NEED TO LOGIN AS SUPERUSER. THEN EXECUTE THIS CODE:
SET GLOBAL MAX_CONNECTIONS = 200;
HOWEVER, THIS ONLY LASTS UNTIL THE MYSQL SERVER RESTARTS. IF YOU WANT TO CHANGE IT PERMANENTLY, YOU CAN ADD THE FOLLOWING LIKE UNDER MYSQLD SECTION.
MAX_CONNECTIONS = 500
OR
IF YOU HAVE TOO MANY PROCESSES LINGERING AROUND ON THE DATABASE SERVER, THEN YOU MAY REACH A LIMIT AND NO NEW ONES WILL BE ALLOWED TO BE STARTED.
To see if this is the case, take a look at your processes from the MySQL Monitor prompt by typing:
show processlist;
Then you will see the lingering processes. Each has an ID, and you can kill them by process ID right in the MySQL Monitor by typing:
kill 2309344;