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My database uses MySQL and I'm using hibernate as an ORM framework. I have one instance where I need to access the MySQL Connection object. I use the following code to get a java.sql.Connection object:

getHibernateTemplate().getSessionFactory().getCurrentSession().connection();

However, when I try to cast it to a (com.mysql.jdbc.Connection) object, I get the following exception:

java.lang.ClassCastException: $Proxy50 cannot be cast to com.mysql.jdbc.Connection

Strangely, if I do conn.getClass().getName(), the class type returned is '$Proxy50' and not 'java.sql.Connection' or some other meaningful type.

What is the correct method for obtaining a vendor specific Connection object from Hibernate? (I'm trying to read a MySQL system property). Why does my above example not work?

Bhesh Gurung
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David
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    Not sure how Spring's HibernateTemplate plays a role, but try `Connection#unwrap()`. – BalusC Dec 20 '11 at 22:03
  • Be very careful doing this. The "raw" JDBC connection is usually under a few different layers of classes that all implement connection. The most common case is the layer a connection pool will add so that when you "close" the connection, what you really do is return it to the pool. If you do something with the raw connection that one of these layers is tracking, you risk problem. Be absolutely sure you need to do this, as opposed to achieving your goal other ways. – rfeak Dec 20 '11 at 22:14
  • @BalusC - Good idea. I tried it out, but my version of the Apache DBCP connection pooler did not implement the unwrap method. When I tried it, I got the following exception: java.lang.AbstractMethodError: org.apache.commons.dbcp.PoolingDataSource$PoolGuardConnectionWrapper.unwrap(Ljava/lang/ClassLjava/lang/Object; – David Dec 20 '11 at 22:39
  • @rfeak - I'm just trying to read the value of the MySQL allowNanAndInf property (which determines whether or not NaN values are permitted in double fields). I'm not planning to perform any queries or inserts/updates against the vendor MySQL Connection object. – David Dec 20 '11 at 22:40
  • DBCP is indeed not JDBC4 compliant. Well, if you can, replace it. It's a terrible pool anyway. See also http://stackoverflow.com/questions/520585/connection-pooling-options-with-jdbc-dbcp-vs-c3p0/3481821#3481821 – BalusC Dec 21 '11 at 00:44

3 Answers3

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As a rule, you should not need to access the concrete Connection implementation.

But if you really, really need to, here's an explanation: $Proxy40 means it is a JDK dynamic proxy. Spring tends to create these. If yours is created by spring, then, you can get the real object by:

Advised advised = (Advised) connection;
Connection conn = (Connection) advised.getTargetSource().getTarget();

If spring is not involved in the proxying, you should be able to call Connection.unwrap(..) as BalusC suggested (you should be able to to that either way)

Bozho
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  • I am using Spring so I tried your code sample and got the following exception: java.lang.ClassCastException: $Proxy50 cannot be cast to org.springframework.aop.framework.Advised – David Dec 20 '11 at 22:37
  • then perhaps the proxy hasn't been created by spring. Did you try the .unwrap(..) method? – Bozho Dec 20 '11 at 23:04
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What is the correct method for obtaining a vendor specific Connection object from Hibernate?

It depends.

I've unwrapped Proxy's successfully like this. How to unwrap the original object from a dynamic proxy

Kinda scary stuff though, avoid it if you can.

Community
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Andy
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com.mysql.jdbc.Connection implements java.sql.Connection interface. Therefore when you create com.mysql.jdbc.Connection object is also java.sql.Connection object. So you cannot cast operation there because returning object is java.sql.Connection not com.mysql.jdbc.Connection.

olyanren
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    You need to read up on [downcasting](http://en.wikipedia.org/wiki/Downcasting), which is precisely this sort of operation. It is dangerous since, as is the original posters issue, if you do not know the precise type of Connection, you can end up with a `ClassCastException`. In this case, the object is a type of `java.sql.Connection` it just happens to not directly be a `com.mysql.jdbc.Connection`. – ig0774 Dec 20 '11 at 23:15