Q. Why do I get a NoClassDefFoundError: org/aspectj/lang/Signature when I try to connect to MySQL
This issue relates to some confusion on the MySQL site between the normal MySQL driver and a special debugging/tracing mode driver. This error indicates that you are attempting to use the tracing driver instead of the normal driver.
Download and use the normal driver instead. It is the one without the g in the name of the jar file.
Q. Why do I get a connection refused exception
There are several things you need to check to isolate the source of this type of exception.
your database server is accepting TCP connections.
Check that your database server is running on the server address you think it is
Check for firewalls. Is there a port or ports being blocked between your application and the database?
Check that you are not having a security problem. Is your application an Applet? If you are connecting to a database that is not on the same physical box as your web server then you need to sign your applet.
Check that you are using the right username and password
Check that the database user has the correct permissions and is able to log in from the location you are attempting to log in from.
Check that you are using the correct driver for your database. There often different versions of the same driver for the same database but different versions of the database since the underlying proctocol had some changes between versions.
If all that fails then consult with your DBA for assistance.
Q. I'm getting exceptions when my applet tries to make a connection to the DB
Applets are restricted in what they are allowed to do; that's a security precaution. The applet needs to be digitally signed, or the local security policy adjusted to get around this. Read this for solutions: HowCanAnAppletReadFilesOnTheLocalFileSystem
You should note that you can connect unsigned Applets to a database server provided that database server is the same server that the Applet came from.
However signed or unsigned it is not considered good practice to connect your Applet directly with a database of any kind. Doing so means opening your database to connections from the internet in general and is a highly unsafe practice.