Vaadin Servlet

CustomUIServlet.java

import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.cdi.server.VaadinCDIServlet;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;

/**
 * PM custom Vaadin servlet.
 * <p>Vaading config <code>productionMode</code> is set by default to <code>true</code>.
 * You may change it by setting the system property <code>productionMode</code> (done in {@link #init(ServletConfig)})
 */
@WebServlet(urlPatterns = "/*")
@VaadinServletConfiguration(ui = AddressbookUI.class, productionMode = true)
public class CustomUIServlet extends VaadinCDIServlet {

    private static final String PRODUCTION_MODE = "productionMode";

    @Override
    public void init(ServletConfig servletConfig) throws ServletException {
        setupProductionMode(servletConfig);
        super.init(servletConfig);
    }

    @Override
    protected void servletInitialized() throws ServletException {
        getService().addSessionInitListener(new ErrorHandlerSessionListener());
    }

    private void setupProductionMode(ServletConfig servletConfig) {

        String productionMode = System.getProperty(PRODUCTION_MODE);

        if (productionMode != null) {
            servletConfig.getServletContext().setInitParameter(PRODUCTION_MODE, productionMode);
        }
    }

}

ErrorHandlerSessionListener.java

import com.vaadin.server.*;
import com.vaadin.ui.Notification;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Date;

/**
 * The error handler shows an error notification and logs the error identifier.
 */
public class ErrorHandlerSessionListener implements SessionInitListener {

    private static final Logger LOGGER = LoggerFactory.getLogger(ErrorHandlerSessionListener.class);

    @Override
    public void sessionInit(SessionInitEvent event) throws ServiceException {
        VaadinSession session = event.getSession();
        session.setErrorHandler(new DefaultErrorHandler() {
            @Override
            public void error(ErrorEvent event) {
                Throwable relevantThrowable = DefaultErrorHandler.findRelevantThrowable(event.getThrowable());
                String errorIdentifier = createErrorIdentifier();
                LOGGER.error("Error - {}", errorIdentifier, relevantThrowable);
                Notification.show("Error", "An error has occurred. The error identifier is: " + errorIdentifier, Notification.Type.ERROR_MESSAGE);
            }
        });
    }

    /**
     * Creates an error identifier, it is last 12 digits of timestamp formatted to four digits chunks, e.g. 4413-7312-5282
     * @return
     */
    private String createErrorIdentifier() {

        String errorIdString = Long.toString(new Date().getTime());
        errorIdString = errorIdString.substring(errorIdString.length() - 12);

        StringBuilder builder = new StringBuilder(15);
        for (int i = 0; i < errorIdString.length(); i++) {
            builder.append(errorIdString.charAt(i));
            if (i % 4 == 3 && i != errorIdString.length() - 1) {
                builder.append("-");
            }
        }

        return builder.toString();
    }

}