There is no "Default" when it comes to database drivers. It uses whichever driver you have told it to use ( The whole Class.forName or DriverManager.registerDriver() part in your code ). So, if you want to be sure, download the latest jdbc drivers available for your version of DB2, then you'll know that you are not using a legacy/out-dated driver.
**NOTE**- If your java code is running on the AS/400 ( native OS for DB2 ), there are property settings telling the AS/400 what jdk it is supposed to use. That is the only restriction that I can see related to your question.
If you are having a specific problem, post the exception and a code snippet along with an explanation of the program behaviour.