A third built-in extension point for MySQL Connector/J is the ExceptionInterceptor interface. This is the third extension point covered in my recent JavaOne and Silicon Valley Code Camp presentations, and is very useful for diagnosing specific Exceptions encountered without modifying application-side code. This corresponds to slide #60 in my slide deck, and there are two Java files we’ll reference from my demo code:
To implement an exception interceptor, you need to do the following:
- Create a Java class which implements com.mysql.jdbc.ExceptionInterceptor
- Configure Connector/J to use your exception interceptor by passing the fully-qualified class name as the value for the “exceptionInterceptors” property.
Like statement interceptors, this extension point is stackable – you can create multiple exception interceptors, passing them in as a comma-delimited list of fully-qualified class names. The exception interceptors are executed in order in which they are defined in the connection property.
The demo here simply demonstrates how this work, but doesn’t give much of an idea of the power behind this interface. The demo code executes a command that’s not valid SQL syntax – which will trigger a server-side error – catches the normal exception, wraps it with some additional text in the message before returning it to the application (where it is hopefully logged). So what can you do with this?
There are certain errors where diagnosis requires additional information about either connection or server state – at the time the exception is raised. This gives you a hook to enable collection of this data. For example, certain NDB (Cluster) errors map to the same MySQL Server error code and message, and you can get more information from the NDB problems by issuing SHOW WARNINGS immediately after the error. Unless you want to recode your application, there’s a lot of value in the ability to add an ExceptionInterceptor which looks for such errors, executes the SHOW WARNINGS, takes the details returned and shoves them into the Exception message text.
Another example might be when you get a server error that indicates the connection character set is set to something other what it should be set to, based on the configuration settings, and you want to know what session character set or collation is in use. Unless you change your application code, you can only assume what it should be – unless you implement an ExceptionInterceptor that collects that data when the problem is encountered and logs that information for you.
Generally speaking, this is an extension point that you probably won’t leverage for normal operations, but can be very useful for diagnosing Exceptions that are difficult to reproduce outside the context of your Java application deployment. Because ExceptionInterceptors are only called when handling a SQLException thrown from Connector/J code, you don’t have to worry too much about performance penalties in production deployments.