How to configure checkstyle for log4j

I want to disallow code like this:

private static final Logger LOGGER = LogManager.getLogger(...)
...
...

LOGGER.info("User(%s) not found.", userId);

It is better to do like this:

LOGGER.info("User({}) not found.", userId);

how to configure checkstyle for catching these errors?

There is likely no standard check to disallow this (at least in the official page).

For 99% of cases the following custom check based on regex (RegexpSingleLineJava) will work:

<module name="RegexpSinglelineJava">
  <property name="format" value="(LOGGER|LOG|logger|log)\.[a-z]+\(.*%s"/>
  <property name="ignoreComments" value="true"/>
  <property name="message" value="Formatting with %s in Log4J log strings is not allowed, use {} instead" />
</module>

The regexp locates a logger followed by a dot, multiple letters (i.e. info, warn, error, etc.), opening brace and %s after some arbitrary characters. The comments are ignored.

Limitations:

• if %s is not at the same line as LOGGER, checkstyle won't catch error (however, the format string is usually at the same line as LOGGER, unlike parameters);
• if logger instance has some exotic name (not like (LOGGER|LOG|logger|log)), checkstyle won't catch error;
• if there are multiple statements on the same line, checkstyle may catch a false-positive:

LOGGER.warn("This is an extremely awful code style."); String str = String.format("%s", "someText");

Writing a custom Checkstyle check may help to overcome these limitations however I'm not sure that it worth it in real projects. If the goal is to tell your developers to stop using %s in logs rather than finding every single occurrence of this situation, then using single-line regex check should be totally fine.