What is a good way to pass useful state information to an exception in Java?

Question

I noticed some confusion initially with my question. I\'m not asking about how to configure a logger nor how to use a logger properly, but rather how to capture all of the infor

Answer 1

Why not keep a local copy/list of all messages that would have gone to the debug log if it was enabled, and pass that to the custom exception when you throw it? Something like:

static void logDebug(String message, List msgs) {
    msgs.add(message);
    log.debug(message);
}

//...

try {

    List debugMsgs = new ArrayList();

    String myValue = someObject.getValue();
    logDebug("Value: " + myValue, debugMsgs);
    doSomething(myValue);

    int x = doSomething2();
    logDebug("doSomething2() returned " + x, debugMsgs);

}
catch (BadThingsHappenException bthe) {
    // at the point when the exception is caught, 
    // debugMsgs contains some or all of the messages 
    // which should have gone to the debug log
    throw new UnhandledException(bthe, debugMsgs);
}

Your exception class can make use of this List parameter when forming getMessage():

public class UnhandledException extends Exception {
    private List debugMessages;

    public UnhandledException(String message, List debugMessages) {
        super(message);
        this.debugMessages = debugMessages;
    }

    @Override
    public String getMessage() {
        //return concatentation of super.getMessage() and debugMessages
    }
}

The usage of this would be tedious - as you'd have to declare the local variable in every single try/catch where you wanted this type of information - but it might be worth it if you have just a few critical sections of code in which you'd like to maintain this state information on an exception.