Logback SMTPAppender send only one email at a particular time with all the exceptions

Question

Is there a way to configure the SMTPAppender in LogBack to meet the following criteria?

1. Group all the exceptions into one message
2. Only send

Answer 1

One simple solution is to log those errors to a file and have a script on your server/machine that reads the file once a day and sends an email.

If you want to use an appender, it seems to me that you would need to roll your own as I don't think the standard SMTPAppender enables you to send emails once a day:

• extend SMTPAppender
• override the sendBuffer method that is in SMTPAppenderBase so that it simply adds the log message to a collection
• add a ScheduledExecutorService to your appender that runs a sendEmail method once a day
• the sendEmail method would synchronize on this for thread safety, check if the collection is empty, send an email with all the errors and clear the collection

A basic implementation could look like the class below (I have not tested it - I'm using Java 8 syntax but you can replace it by anonymous classes if required). Note that I only keep the event that caused the exception, you may also want to keep the content of the CyclicBuffer in the sendBuffer method and/or add some error separators between errors in the sendEmail method. This can become quite complex and is to be fine-tuned depending on your requirements.

public class ScheduledSMTPAppender extends SMTPAppender {
  private final ThreadFactory tf = r -> {
    Thread t = new Thread(r, "ScheduledSMTPAppender Thread");
    t.setDaemon(true); //make daemon or it will prevent your program to exit
    return t;
  };
  private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1, tf);
  private final List<ILoggingEvent> events = new ArrayList<> ();

  private int maxMessages = 10;

  public ScheduledSMTPAppender() { super(); }
  public ScheduledSMTPAppender(EventEvaluator<ILoggingEvent> eventEvaluator) { super(eventEvaluator); }

  @Override public void start() {
    super.start();
    scheduler.scheduleAtFixedRate(this::sendEmail, 1, 1, TimeUnit.DAYS);
  }

  @Override protected void sendBuffer(CyclicBuffer<ILoggingEvent> cb, ILoggingEvent lastEventObject) {
    events.add(lastEventObject);
    if (events.size() > maxMessages) sendEmail();
  }

  //needs to be synchronized for thread safety
  private synchronized void sendEmail() {
    try {
      if (events.isEmpty()) return;
      ILoggingEvent lastEvent = events.get(events.size() - 1);
      events.remove(events.size() - 1);
      CyclicBuffer<ILoggingEvent> cb;
      if (events.isEmpty()) {
        cb = new CyclicBuffer<>(1);
      } else {
        cb = new CyclicBuffer<>(events.size());
        for (ILoggingEvent e : events) cb.add(e);
      }
      super.sendBuffer(cb, lastEvent);
      events.clear();
    } catch (Exception e) {
      //Important to have a catch all here or the scheduled task will die
      addError("Error occurred while sending e-mail notification.", e);
    }
  }

  //this allows to make "maxMessages" a parameter of your appender
  public int getMaxMessages() { return maxMessages; }
  public void setMaxMessages(int maxMessages) { this.maxMessages = maxMessages; }
}

Your logback configuration file then looks like:

<appender name="Email" class="your.package.ScheduledSMTPAppender">
    <smtpHost>SERVER</smtpHost>
    <smtpPort>PORT</smtpPort>
    <asynchronousSending>false</asynchronousSending>
    <from>SENDER</from>
    <to>RECIPIENT</to>
    <subject>SUBJECT</subject>
    <maxMessages>10</maxMessages>

    <layout class="ch.qos.logback.classic.PatternLayout">
        <pattern>%d{HH:mm:ss.SSS} %-55(%X{user} %level [%thread] %logger{20}) - %msg%n</pattern>
    </layout>
</appender>

To go further, you could add parameters, such as the time of the day when this is sent, the number of emails per day etc.