java - java.sql.SQLException: The statement is closed

Question

I am using sql server 2008 R2. In that I am using a Ejb stateless. In that i am declaring a instance variable of my Database class and instantiated on either on the create method or at the instance variable also.

and when i call a execute a executeQuery method that throw an Exception:

java.sql.SQLException: The statement is closed.

But it works fine if i define local variable of my database class object in the Ejb bussiness method .

I don't understand why that happens.