Log4j to write json array to disk

Question

Im creating a log file system. the log file will be in json format so a server can read it after but i dont think thats too important. What i need to know is can log4j be configured to write into to a file but without any tags like info,debug, timestamp etc in the file. I have looked here

but this polutes the file with with other things. I want ONLY the data i write to show up in the file. I'd also like to set some kind of file rotation on the file if it gets too big after a max size is reached.

Answer 1

This is relatively easy, using a log4j.properties configuration file (place it at the top of your classpath, and Log4j will 'just find it'):

# This is the default logger, simply logs to console
log4j.logger.com.foo.bar=DEBUG,A1
log4j.appender.A1=org.apache.log4j.ConsoleAppender
log4j.appender.A1.layout=org.apache.log4j.PatternLayout

# Note the Pattern here, emits a lot of stuff - btw, don't use this in production
# %C is expensive - see the Javadoc for ConversionPattern for the meaning of all
# the % modifiers:
log4j.appender.A1.layout.ConversionPattern=%d{MMM dd, HH:mm:ss} [%C{2}] %-5p - %m%n

# Logging to file can be enabled by using this one
log4j.logger.com.example=DEBUG, R

log4j.appender.R=org.apache.log4j.RollingFileAppender
log4j.appender.R.File=/var/log/generic.log
log4j.appender.R.MaxFileSize=100KB
# Keep one backup file
log4j.appender.R.MaxBackupIndex=1

# This is the most minimalist layout you can have: just the 'm'essage is emitted
# (and a \n newline):
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%m%n

All the classes in the com.foo.bar package (and subpackages) will log to console, those in com.example (and below) will log to /var/log/generic.log.

To emit JSON, just use Jackson (com.fasterxml) convert your data to a JSON object and write it out as a string.

Answer 2

What you want is the PatternLayout with %m%n only, and combine with the answer to previously asked question here

Answer 3

You should be able to write a custom log appender, I have a vague recollection of there being an example to do this with MongoDB. MongoDD stores its data as JSON, so it have converted the log to json format before inserting it.

I would start by searching for a MondgoDB appender, and looking at the file appender that is shipped, that should give you a starting point for an appender if one doesnt already exist.

http://log4mongo.org/display/PUB/Home