Making a log4j console appender use different colors for different threads

I am tracking down some concurrency issues and it would be very helpful to have the output lines from each thread in a different color when logging to a console. I am on OS X. Could this be done using a conversion pattern to output some control codes or would it need a custom appender? Anyone know how?

2011-10-21 12:14:42,859 ["http-bio-8080"-exec-9] DEBUG ...
2011-10-21 12:14:43,198 ["http-bio-8080"-exec-10] DEBUG ...

The lines for exec-9 and exec-10 should be in different colors.

You can use MulticolorLayout from jcabi-log. Add this dependency to the project:

<dependency>
  <groupId>com.jcabi</groupId>
  <artifactId>jcabi-log</artifactId>
  <version>0.17.1</version>
</dependency>

And then configure it in log4j.properties:

log4j.rootLogger=INFO, CONSOLE
log4j.appender.CONSOLE=org.apache.log4j.ConsoleAppender
log4j.appender.CONSOLE.layout=com.jcabi.log.MulticolorLayout
log4j.appender.CONSOLE.layout.ConversionPattern=[%color{%p}] %c: %m%n

Same in log4j.xml:

<appender name="CONSOLE" class="org.apache.log4j.ConsoleAppender">
    <param name="Target" value="System.out" />
    <layout class="com.jcabi.log.MulticolorLayout">
        <param name="ConversionPattern" value="[%color{%p}] %m%n" />
    </layout>
</appender>

In this example, %p will be replaced by DEBUG, INFO, ERROR, etc. and then painted into the color that is relevant to the logging level. Besides that you are able to use your own colors or predefined colors, for example:

log4j.appender.CONSOLE.layout.ConversionPattern=[%p] $color-cyan{%c}: %color-red{%m}%n
log4j.appender.CONSOLE.layout.ConversionPattern=[%p] $color-0;0;31{%c}: %m%n

More documentation about ANSI colors.

You can extend PatternLayout and override format(ILoggingEvent). There you could look at LoggingEvent.getThreadName() to get some color based on the thread name (odd/even, maybe?).

In order to output color to the console, you'll need to use an ANSI Escape Sequence.

For instance, to output a red text:

  "\u001b["  // Prefix - see [1]
+ "0"        // Brightness
+ ";"        // Separator
+ "31"       // Red foreground
+ "m"        // Suffix
+ text       // the text to output
+ "\u001b[m " // Prefix + Suffix to reset color

Here some examples:

• ColoredPatternLayout implementation by Ingo Thon.
• Colour-coded Console Logging with Log4J blog post.

Just to add, maybe you could also achieve this by setting in the MDC a variable "randColor" with a random ANSI color code, for instance, in a Filter, and using it in the conversionPattern of a standard org.apache.log4j.PatternLayout in your log4j's console appender configuration:

<appender name="consoleAppender" class="org.apache.log4j.ConsoleAppender">
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern"
               value="\u001b[0;%X{randColor}m ....... \u001b[m" />
    </layout>
</appender>

[1] What does "\u001B[J" represent?

See PatternLayout, configuration property "highlight": https://logging.apache.org/log4j/2.x/manual/layouts.html#PatternLayout