Implementation to create a reporting methodology to report to admin through mail

Question

Constantly monitor a http request which if returns code 200 then no action is taken but if a 404 is returned then the administrator should be alerted via warning or mail.

I wanted to know how to approach it from a Java perspective. The codes available are not very useful.

Answer 1

First of all, you should consider using an existing tool designed for this job (e.g. Nagios or the like). Otherwise you'll likely find yourself rewriting many of the same features. You probably want to send only one email once a problem has been detected, otherwise you'll spam the admin. Likewise you might want to wait until the second or third failure before sending an alert, otherwise you could be sending false alarms. Existing tools do handle these things and more for you.

That said, what you specifically asked for isn't too difficult in Java. Below is a simple working example that should help you get started. It monitors a URL by making a request to it every 30 seconds. If it detects a status code 404 it'll send out an email. It depends on the JavaMail API and requires Java 5 or higher.

public class UrlMonitor implements Runnable {
    public static void main(String[] args) throws Exception {
        URL url = new URL("http://www.example.com/");
        Runnable monitor = new UrlMonitor(url);
        ScheduledExecutorService service = Executors.newScheduledThreadPool(1);
        service.scheduleWithFixedDelay(monitor, 0, 30, TimeUnit.SECONDS);
    }

    private final URL url;

    public UrlMonitor(URL url) {
        this.url = url;
    }

    public void run() {
        try {
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            if (con.getResponseCode() == HttpURLConnection.HTTP_NOT_FOUND) {
                sendAlertEmail();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void sendAlertEmail() {
        try {
            Properties props = new Properties();
            props.setProperty("mail.transport.protocol", "smtp");
            props.setProperty("mail.host", "smtp.example.com");

            Session session = Session.getDefaultInstance(props, null);
            Message message = new MimeMessage(session);
            message.setFrom(new InternetAddress("me@example.com", "Monitor"));
            message.addRecipient(Message.RecipientType.TO,
                    new InternetAddress("me@example.com"));
            message.setSubject("Alert!");
            message.setText("Alert!");

            Transport.send(message);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}