Question

I have the following code

    Configuration config = new Configuration().configure();
    config.buildMappings();
    serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry(); 
    SessionFactory factory = config.buildSessionFactory(serviceRegistry);
    Session hibernateSession = factory.openSession();
    Transaction tx = hibernateSession.beginTransaction();
    ObjectType ot = (ObjectType)hibernateSession.merge(someObj);
    tx.commit();
    return ot;

hibernate.cfg.xml contains:

<session-factory>

    <property name="connection.url">jdbc:postgresql://127.0.0.1:5432/dbase</property>
    <property name="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</property>
    <property name="connection.driver_class">org.postgresql.Driver</property>

    <property name="connection.username">username</property>
    <property name="connection.password">password</property>

    <property name="connection.provider_class">org.hibernate.connection.C3P0ConnectionProvider</property>        
    <property name="hibernate.c3p0.acquire_increment">1</property>
    <property name="hibernate.c3p0.min_size">5</property>
    <property name="hibernate.c3p0.max_size">20</property>
    <property name="hibernate.c3p0.max_statements">50</property>
    <property name="hibernate.c3p0.timeout">300</property>
    <property name="hibernate.c3p0.idle_test_period">3000</property>
    <property name="hibernate.c3p0.acquireRetryAttempts">1</property>
    <property name="hibernate.c3p0.acquireRetryDelay">250</property>

    <property name="hibernate.show_sql">true</property>
    <property name="hibernate.use_sql_comments">true</property>

    <property name="hibernate.transaction.factory_class">org.hibernate.transaction.JDBCTransactionFactory</property>
    <property name="hibernate.current_session_context_class">thread</property>

    <mapping class="...." />

</session-factory>

After a few seconds and some successful inserts, the following exception appears:

org.postgresql.util.PSQLException: FATAL: sorry, too many clients already
    at org.postgresql.core.v3.ConnectionFactoryImpl.doAuthentication(ConnectionFactoryImpl.java:291)
    at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:108)
    at org.postgresql.core.ConnectionFactory.openConnection(ConnectionFactory.java:66)
    at org.postgresql.jdbc2.AbstractJdbc2Connection.<init>(AbstractJdbc2Connection.java:125)
    at org.postgresql.jdbc3.AbstractJdbc3Connection.<init>(AbstractJdbc3Connection.java:30)
    at org.postgresql.jdbc3g.AbstractJdbc3gConnection.<init>(AbstractJdbc3gConnection.java:22)
    at org.postgresql.jdbc4.AbstractJdbc4Connection.<init>(AbstractJdbc4Connection.java:30)
    at org.postgresql.jdbc4.Jdbc4Connection.<init>(Jdbc4Connection.java:24)
    at org.postgresql.Driver.makeConnection(Driver.java:393)
    at org.postgresql.Driver.connect(Driver.java:267)
    at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:135)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
    at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
    at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
    at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
12:24:19.151 [          Thread-160] WARN                   internal.JdbcServicesImpl - HHH000342: Could not obtain connection to query metadata : Connections could not be acquired from the underlying database!
12:24:19.151 [          Thread-160] INFO                             dialect.Dialect - HHH000400: Using dialect: org.hibernate.dialect.PostgreSQLDialect
12:24:19.151 [          Thread-160] INFO                  internal.LobCreatorBuilder - HHH000422: Disabling contextual LOB creation as connection was null
12:24:19.151 [          Thread-160] INFO        internal.TransactionFactoryInitiator - HHH000268: Transaction strategy: org.hibernate.engine.transaction.internal.jdbc.JdbcTransactionFactory
12:24:19.151 [          Thread-160] INFO               ast.ASTQueryTranslatorFactory - HHH000397: Using ASTQueryTranslatorFactory
12:24:19.151 [          Thread-160] INFO                        hbm2ddl.SchemaUpdate - HHH000228: Running hbm2ddl schema update
12:24:19.151 [          Thread-160] INFO                        hbm2ddl.SchemaUpdate - HHH000102: Fetching database metadata
12:24:19.211 [Runner$PoolThread-#0] WARN              resourcepool.BasicResourcePool - com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@ee4084 -- Acquisition Attempt Failed!!! Clearing pending acquires. While trying to acquire a needed new resource, we failed to succeed more than the maximum number of allowed acquisition attempts (1). Last acquisition attempt exception: 
org.postgresql.util.PSQLException: FATAL: sorry, too many clients already
    at org.postgresql.core.v3.ConnectionFactoryImpl.doAuthentication(ConnectionFactoryImpl.java:291)

It seems that the hibernate doesn’t realse the connection. But hibernateSession.close() causes exception Session is closed because tx.commit() is called.

Answer 1

I found the cause why c3p0 behaved in this way.The issue was quite trivial…
This part of code:

Configuration config = new Configuration().configure();
config.buildMappings();
serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry(); 
SessionFactory factory = config.buildSessionFactory(serviceRegistry);

was executed multiple times. Thank you Steve for the tip.