Using log4j's FallbackErrorHandler

Our applications currently use a DailyRollingFileAppender for logging, but since they run on NFS across a number of different servers, we quite often get errors due to stale NFS file handles, when log4j tries to write to the files. We sometimes also get errors if the logging mount point is missing on some of the servers.

I've been trying to find a way to switch to a different appender (such as a ConsoleAppender), if log4j fails to write to the log files. At first I thought of writing my own custom appender, to wrap up a FileAppender and a ConsoleAppender, and to switch to the ConsoleAppender if the FileAppender threw an IOException, but then I came across the FallbackErrorHandler, which allows you to configure a backup appender, which takes over if the primary appender fails for whatever reason.

This is how you can set up your log4j.xml file to use a FallbackErrorHandler:

1. Create a backup appender:
The backup appender will be used if the primary appender fails. My backup is a ConsoleAppender:

  <appender name="console" class="org.apache.log4j.ConsoleAppender">
    <param name="Target" value="System.out"/>
    <layout class="org.apache.log4j.PatternLayout">
      <param name="ConversionPattern" value="%d %-5p %30.30c - %m%n"/>
    </layout>
  </appender>

2. Add a FallbackErrorHandler to your primary appender:
My primary appender is a DailyRollingFileAppender. Add a FallbackErrorHandler to it and tell it to use the "console" (backup) appender, using the appender-ref tag. The root-ref tag refers to the logger that is currently using that appender. If you have a different logger use the logger-ref tag to refer to it instead.

  <appender name="file" class="org.apache.log4j.DailyRollingFileAppender">
  <errorHandler class="org.apache.log4j.varia.FallbackErrorHandler">
       <root-ref/>
       <appender-ref ref="console"/>
  </errorHandler>
    <param name="File" value="C:/temp/test.log"/>
    <layout class="org.apache.log4j.PatternLayout">
      <param name="ConversionPattern" value="%d %-5p %30.30c - %m%n"/>
    </layout>
  </appender>

3. Trying it out:
To test this works, make your log file read-only, or change the path of the file to one which doesn't exist. When you run your application, you will see log4j print an error to stderr, and start logging to console, instead of file. If you turn log4j debug on you will see the message: "FB: INITIATING FALLBACK PROCEDURE." before console logging begins.

The complete log4j.xml configuration:
Here is my complete config file. (I tried setting up a log4j.properties file, but ran into problems and wasn't able to.)

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
    <appender name="console" class="org.apache.log4j.ConsoleAppender">
        <param name="Target" value="System.out" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d %-5p %30.30c - %m%n" />
        </layout>
    </appender>
    <appender name="file" class="org.apache.log4j.DailyRollingFileAppender">
        <errorHandler class="org.apache.log4j.varia.FallbackErrorHandler">
            <root-ref />
            <appender-ref ref="console" />
        </errorHandler>
        <param name="File" value="C:/temp/test.log" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d %-5p %30.30c - %m%n" />
        </layout>
    </appender>
    <root>
        <level value="INFO" />
        <appender-ref ref="file" />
    </root>
</log4j:configuration>

Send Email Alerts Using Log4j [Howto]

It's really easy to add error alerting to your java application. If your application logs all errors using log4j and you want these errors emailed out to a support team or to yourself, then all you have to do is add another appender to your log4j.properties file.

Let's say you have the following class which logs an error message: import org.apache.log4j.*; public class Dummy{ private static Logger logger = Logger.getLogger(Dummy.class); public void logError(String message){ logger.error(message); } public static void main(String[] args){ new Dummy().logError("An error has occurred!"); } } And your log4j properties file is: log4j.rootLogger=INFO, R log4j.appender.R=org.apache.log4j.ConsoleAppender log4j.appender.R.layout=org.apache.log4j.PatternLayout log4j.appender.R.layout.ConversionPattern=%d [%t] %-5p %c %x - %m%n Now add a new MAIL appender, so that your properties file looks like this: log4j.rootLogger=INFO, R, MAIL log4j.appender.R=org.apache.log4j.ConsoleAppender log4j.appender.R.layout=org.apache.log4j.PatternLayout log4j.appender.R.layout.ConversionPattern=%d [%t] %-5p %c %x - %m%n log4j.appender.MAIL=org.apache.log4j.net.SMTPAppender log4j.appender.MAIL.BufferSize=10 log4j.appender.MAIL.SMTPHost=post.uk.mail.db.com log4j.appender.MAIL.From=username@abc.com log4j.appender.MAIL.to=username@abc.com log4j.appender.MAIL.Subject=[Dummy] Error Alert log4j.appender.MAIL.layout=org.apache.log4j.PatternLayout log4j.appender.MAIL.layout.ConversionPattern=%d [%t] %-5p %c %x - %m%n Don't forget to place mail.jar and activation.jar on your classpath and then run the application.

Check your inbox and voila, you have an alert without making any source code changes!

NB: By default, an email message will be sent when an ERROR or higher severity message is appended. If you want to email messages with levels less than ERROR (e.g. INFO) then you currently have to configure your own implementation of the TriggeringEventEvaluator. Setting log4j.appender.MAIL.Threshold=INFO will not work.

Further Reading:
SMTPAppender javadoc
SmtpAppender Members and Properties