could not get database metadata java.sql.SQLException:

I am using hibernate sharedFactory for database updation. When am trying to do one updation am getting the following errors on log:

2011-09-01 16:17:50,406 ERROR (org.hibernate.tool.hbm2ddl.SchemaUpdate:134:execute) - could not get database metadata
java.sql.SQLException: Connections could not be acquired from the underlying database!

2011-09-01 16:32:05,734 ERROR (org.hibernate.tool.hbm2ddl.SchemaUpdate:165:execute) - could not complete schema update
java.sql.SQLException: Connections could not be acquired from the underlying database!

And the exception is exactly t开发者_如何学Pythonhrowing on this line of code..

sessionFactoryShard = shardedConfig.buildShardedSessionFactory();

Can anyone tell why this error occurs?