Java – How to rotate log files based on time rather than size in Log4j

You can add/remove Appender programmatically to Log4j:

  ConsoleAppender console = new ConsoleAppender(); //create appender
  //configure the appender
  String PATTERN = "%d [%p|%c|%C{1}] %m%n";
  console.setLayout(new PatternLayout(PATTERN)); 
  console.setThreshold(Level.FATAL);
  console.activateOptions();
  //add appender to any Logger (here is root)
  Logger.getRootLogger().addAppender(console);

  FileAppender fa = new FileAppender();
  fa.setName("FileLogger");
  fa.setFile("mylog.log");
  fa.setLayout(new PatternLayout("%d %-5p [%c{1}] %m%n"));
  fa.setThreshold(Level.DEBUG);
  fa.setAppend(true);
  fa.activateOptions();

  //add appender to any Logger (here is root)
  Logger.getRootLogger().addAppender(fa);
  //repeat with all other desired appenders

I'd suggest you put it into an init() somewhere, where you are sure, that this will be executed before anything else. You can then remove all existing appenders on the root logger with

 Logger.getRootLogger().getLoggerRepository().resetConfiguration();

and start with adding your own. You need log4j in the classpath of course for this to work.

Remark:
You can take any Logger.getLogger(...) you like to add appenders. I just took the root logger because it is at the bottom of all things and will handle everything that is passed through other appenders in other categories (unless configured otherwise by setting the additivity flag).

If you need to know how logging works and how is decided where logs are written read this manual for more infos about that.
In Short:

  Logger fizz = LoggerFactory.getLogger("com.fizz")

will give you a logger for the category "com.fizz".
For the above example this means that everything logged with it will be referred to the console and file appender on the root logger.
If you add an appender to Logger.getLogger("com.fizz").addAppender(newAppender) then logging from fizz will be handled by alle the appenders from the root logger and the newAppender.
You don't create Loggers with the configuration, you just provide handlers for all possible categories in your system.

Achieving this is not possible with the standard Log4j library, but it is implemented in the Log4j Extras Companion library, which is a separate JAR file and can be downloaded from here.

Standard RollingFileAppender performs log rotation (or rolling) based on the log file size. If the log file reached the specified size limit, a new log file is started and the old log file will be renamed to have a _1 suffix. Existing archive logs will also be rotated with _2, _3, etc. suffixes.

What you need is to override this default log file naming policy to use current date in archive log file names instead of a counter. This can be configured with the TimeBasedRollingPolicy filter (here is the documentation).

Modify your Log4J configuration file (log4j.xml):

<appender name="FILE" class="org.apache.log4j.rolling.RollingFileAppender"> 
    <param name="Threshold" value="DEBUG"/>   
    <param name="File" value="engine.log"/>
    <param name="Append" value="true"/>
    <rollingPolicy class="org.apache.log4j.rolling.TimeBasedRollingPolicy">
        <param name="FileNamePattern" value="engine_%d.log" />
    </rollingPolicy>
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern" value="%d{ISO8601} %t %-5p [%c{1}] %m%n "/>
    </layout>
</appender>

I've taken this example configuration from the official Log4J wiki (see Filter configuration and More examples sections at the bottom of the page).

Update:

The RollingFileAppender class from the extras library does not have the appropriate attribute for setting the maximum log file size as the old RollingFileAppender had. This property can be specified using a triggering policy, in this case the SizeBasedTriggeringPolicy should be used (documentation is here). The configuration of the triggering policy is the same as for the rolling policy (I show only the relevant part):

<appender ...>
    ...
    <triggeringPolicy class="org.apache.log4j.rolling.SizeBasedTriggeringPolicy">
        <param name="MaxFileSize" value="10000000" />
    </triggeringPolicy>
    ...
</appender>