What are the required C3P0 settings for hibernate in order to avoid Deadlocks

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I use Hibernate together with MySQL 5.1.30.

I have the next libraries:

  • c3p0-0.0.1.2.jar
  • mysql-connector-java-5.0.3-bin.jar
  • hibernate3.jar

I use a hibernate.cfg.xml for configuration: 

<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
    <session-factory>
        <!-- Database connection settings -->
        <property name="connection.driver_class">org.gjt.mm.mysql.Driver</property> 

        <property name="connection.url">jdbc:mysql://localhost/fooDatatbase</property>
    <property name="connection.username">foo</property>
    <property name="connection.password">foo123</property>

        <!-- Use the C3P0 connection pool provider -->
    <property name="hibernate.c3p0.min_size">5</property>
    <property name="hibernate.c3p0.max_size">20</property>
    <property name="hibernate.c3p0.timeout">300</property>
    <property name="hibernate.c3p0.max_statements">50</property>
    <property name="hibernate.c3p0.idle_test_periods">3000</property>       

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

        <mapping resource="databaselayer/mail/Mail.hbm.xml"/>
        <mapping resource="databaselayer/courses/Course.hbm.xml"/>
        <mapping resource="databaselayer/price/Price.hbm.xml"/>        
        <mapping resource="databaselayer/contact/Contact.hbm.xml"/>
        <mapping resource="databaselayer/artists/Musician.hbm.xml"/>
        <mapping resource="databaselayer/concerts/Concert.hbm.xml"/>     
        <mapping resource="databaselayer/welcome/Welcome.hbm.xml"/>
        <mapping resource="databaselayer/information/Information.hbm.xml"/>                             
    </session-factory>
</hibernate-configuration>

In the JAVA persistance with hibernate book, c3p0 configuration options are explained:

  • hibernate.c3p0.min_size This is the minimum number of JDBC connections that C3P0 keeps ready at all times
  • hibernate.c3p0.max_size This is the maximum number of connections in the pool. An exception is thrown at runtime if this number is exhausted.
  • hibernate.c3p0.timeout You specify the timeout period (in this case, 300 seconds) after which an idle connection is removed from the pool).
  • hibernate.c3p0.max_statements Maximum Number of statements that will be cached. Caching of prepared statements is essential for best performance with Hibernate.
  • hibernate.c3p0.idle_test_periods This is the iddle time in seconds before a connection is automatically validated.

I use Java 1.5.0_09 and tomcat 6.0. I have three applications deployed in tomcat. Each of them uses hibernate with a configuration file almost equivalent the shown above (only username, databasename, password and the mapping resoruces change).

Unfortunately with the above settings, after some hours running i get some nasty Deadlock errors which end killing tomcat. 

Jan 22, 2009 3:29:07 PM com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector run
WARNING: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@2437d -- APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks!
Jan 22, 2009 3:29:07 PM com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector run
WARNING: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@1dc5cb7 -- APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks!
Jan 22, 2009 3:29:07 PM com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector run
WARNING: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@9cd2ef -- APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks!
Jan 22, 2009 3:29:07 PM com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector run
WARNING: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@4af355 -- APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks!
Jan 22, 2009 3:29:07 PM com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector run
WARNING: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@1275fcb -- APPARENT DEADLOCK!!! Creating emergency threads for unassigned pending tasks!
Jan 22, 2009 3:29:35 PM com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector run

This seems to be an error several people already got. I changed my settings trying to follow the workaround described herehttp://forum.hibernate.org/viewtopic.php?p=2386237 to:

<property name="hibernate.c3p0.acquire_increment">1</property>
<property name="hibernate.c3p0.min_size">0</property>
<property name="hibernate.c3p0.max_size">48</property>
<property name="hibernate.c3p0.timeout">0</property>
<property name="hibernate.c3p0.max_statements">0</property>

With the new settings, I do not get Deadlocks, but I get: 

WARNING: SQL Error: 0, SQLState: 08S01
Jan 24, 2009 5:53:37 AM org.hibernate.util.JDBCExceptionReporter logExceptions
SEVERE: Communications link failure due to underlying exception: 

** BEGIN NESTED EXCEPTION ** 

java.io.EOFException

STACKTRACE:

java.io.EOFException
    at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1913)

Does anyone knows what I am doing wrong, and how I can setup c3p0 correctly?

c3p0 hibernate
6 Answers

Do the three applications share the same connection pool, or does each one get its own? I'd recommend the latter.

user_372130
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