Capture and log / notify unhandled exceptions in a JSF based application

Question

I would like to check and log all unhandled exceptions in my JSF web application using log4j. I read this post Log runtime Exceptions in Java using log4j and add a class that implements Thread.UncaughtExceptionHandler. but the method is not fired.

What is the correct way to achieve this?

Answer 1

add a class that implements Thread.UncaughtExceptionHandler. but the method is not fired

There's usually no means of an uncaught exception in the HTTP request thread managed by a Java EE web application as the server ultimately catches any exception coming from the web application and displays it in case of synchronous HTTP requests in a HTTP 500 error page (in case of asynchronous (ajax) requests, this is in turn framework-dependent). Moreover, if there was really an uncaught exception which escaped the server's attention, it would have killed the server's runtime thread causing the whole server to stop. This is clearly not an ideal real world scenario.

JSF offers you the ExceptionHandler API to get hand of those exceptions and if necessary control them.

Here's a kickoff example:

public class YourExceptionHandler extends ExceptionHandlerWrapper {

    private ExceptionHandler wrapped;

    public YourExceptionHandler(ExceptionHandler wrapped) {
        this.wrapped = wrapped;
    }

    @Override
    public void handle() throws FacesException {
        FacesContext facesContext = FacesContext.getCurrentInstance();

        for (Iterator<ExceptionQueuedEvent> iter = getUnhandledExceptionQueuedEvents().iterator(); iter.hasNext();) {
            Throwable exception = iter.next().getContext().getException(); // There it is!

            // Now do your thing with it. As per the question, you want to log it using log4j. 
            logger.error("An exception occurred!", exception);
        }

        getWrapped().handle();
    }

    @Override
    public ExceptionHandler getWrapped() {
        return wrapped;
    }

}

In order to get it to run, create a custom ExceptionHandlerFactory as follows:

public class YourExceptionHandlerFactory extends ExceptionHandlerFactory {

    private ExceptionHandlerFactory parent;

    public YourExceptionHandlerFactory(ExceptionHandlerFactory parent) {
        this.parent = parent;
    }

    @Override
    public ExceptionHandler getExceptionHandler() {
        return new YourExceptionHandler(parent.getExceptionHandler());
    }

}

Which needs to be registered in faces-config.xml as follows:

<factory>
    <exception-handler-factory>com.example.YourExceptionHandlerFactory</exception-handler-factory>
</factory>

See also:

• Exception handling in JSF ajax requests