Log4j: How to configure simplest possible file logging?

Question

My story:

I want to make a thing which is as simple as a simplest possible log4j logger that logs rows to a file. I have found several examples with some fun

Answer 1

Here is a log4j.properties file that I've used with great success.

logDir=/var/log/myapp

log4j.rootLogger=INFO, stdout
#log4j.rootLogger=DEBUG, stdout

log4j.appender.stdout=org.apache.log4j.DailyRollingFileAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{MM/dd/yyyy hh:mm:ss a}|%-5p|%-30c{1}| %m%n
log4j.appender.stdout.DatePattern='.'yyyy-MM-dd
log4j.appender.stdout.File=${logDir}/myapp.log
log4j.appender.stdout.append=true

The DailyRollingFileAppender will create new files each day with file names that look like this:

myapp.log.2017-01-27
myapp.log.2017-01-28
myapp.log.2017-01-29
myapp.log  <-- today's log

Each entry in the log file will will have this format:

01/30/2017 12:59:47 AM|INFO |Component1   | calling foobar(): userId=123, returning totalSent=1
01/30/2017 12:59:47 AM|INFO |Component2   | count=1 > 0, calling fooBar()

Set the location of the above file by using -Dlog4j.configuration, as mentioned in this posting:

java -Dlog4j.configuration=file:/home/myapp/config/log4j.properties com.foobar.myapp

In your Java code, be sure to set the name of each software component when you instantiate your logger object. I also like to log to both the log file and standard output, so I wrote this small function.

private static final Logger LOGGER = Logger.getLogger("Component1");

public static void log(org.apache.log4j.Logger logger, String message) {

    logger.info(message);
    System.out.printf("%s\n", message);
}

public static String stackTraceToString(Exception ex) {
    StringWriter sw = new StringWriter();
    PrintWriter pw = new PrintWriter(sw);
    ex.printStackTrace(pw);
    return sw.toString();
}

And then call it like so:

LOGGER.info(String.format("Exception occurred: %s", stackTraceToString(ex)));