SMTPAppender for log4j is a type of appender which sends emails via an SMTP server. It’s very useful for applications released in production where you’d definitely need to know of all application errors logged. Of course every caring developer should look at the server logs every now and then, but if you’ve got hundreds of them (applications) then it becomes a full time job in itself.

Sometimes a fresh release of a high traffic website may produce hundreds or thousands of ERROR level log events. Many times this may be something minor which is being logged deep inside your code. Until the bug is fixed and a new release is deployed, your inbox and the mail server may suffer heavily.

What follows is an extension of SMTPAppender which limits the amount of emails sent in a specified period of time. It features sensible defaults which of course can be configured externally via the log4j configuration file.

package com.cherouvim; import org.apache.log4j.Logger; import org.apache.log4j.net.SMTPAppender; public class LimitedSMTPAppender extends SMTPAppender { private int limit = 10; // max at 10 mails ... private int cycleSeconds = 3600; // ... per hour public void setLimit(int limit) { this.limit = limit; } public void setCycleSeconds(int cycleSeconds) { this.cycleSeconds = cycleSeconds; } private int lastVisited; private long lastCycle; protected boolean checkEntryConditions() { final long now = System.currentTimeMillis(); final long thisCycle = now - (now % (1000L*cycleSeconds)); if (lastCycle!=thisCycle) { lastCycle = thisCycle; lastVisited = 0; } lastVisited++; return super.checkEntryConditions() && lastVisited<=limit; } }

The configuration would look something like this:

log4j.appender.???=com.cherouvim.LimitedSMTPAppender log4j.appender.???.limit=3 log4j.appender.???.cycleSeconds=60 log4j.appender.???.BufferSize=25 log4j.appender.???.SMTPHost=${mail.smtp.host} log4j.appender.???.From=${mail-sender} log4j.appender.???.To=${sysadmin.email} log4j.appender.???.Subject=An error occured log4j.appender.???.layout=org.apache.log4j.PatternLayout log4j.appender.???.layout.ConversionPattern=%d{ISO8601} %-5p (%F:%L) - %m%n log4j.appender.???.threshold=ERROR

The above configuration will limit the mail dispatch to only 3 emails per minute. Any further errors in that minute will not be emailed. The limit and cycleSeconds setting lines can be omitted and the defaults will be applied.

Happy logging!