The PrefixCompositeConverter class is a part of the Logback library that allows you to include a prefix before the output of another converter. It is typically used to group related converters together, making it easy to enable or disable them as a group.

Here is an example of how you might use the PrefixCompositeConverter in a Logback configuration file to prefix all log messages with the current timestamp:

<configuration>
  <conversionRule conversionWord="prefix" converterClass="ch.qos.logback.classic.pattern.PrefixCompositeConverter">
    <prefixConverter class="ch.qos.logback.classic.pattern.DateConverter">
      <pattern>yyyy-MM-dd HH:mm:ss.SSS</pattern>
    </prefixConverter>
    <suffixConverter class="ch.qos.logback.classic.pattern.LevelConverter"/>
    <suffixConverter class="ch.qos.logback.classic.pattern.LoggerConverter"/>
    <suffixConverter class="ch.qos.logback.classic.pattern.MessageConverter"/>
  </conversionRule>

  <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%prefix %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="CONSOLE" />
  </root>
</configuration>

This configuration file sets up a CONSOLE appender that outputs log messages in the format "%prefix %msg%n". The %prefix part of the pattern is where the output of the PrefixCompositeConverter is inserted into the log message. The PrefixCompositeConverter itself is configured to use a DateConverter as its prefix converter, so the current timestamp will be included at the beginning of each log message.

It is also using suffix converter as LevelConverter, LoggerConverter and MessageConverter to add the level, logger name and message respectively.

Please note that, the output will be written to the console in this example, you can use file based appender to output the logs to a file. And you can refer to the logback documentation for more information and options available.