When to use log4j over system out

Question

Why is it a good practice to use logger.debug() instead of System.out.println()?

I understand that log4j allows you to control what you wish to print (DEBUG,FATAL,INFO, etc), but apart from that, is there any other benefit?

I'd also recommend looking at slf4j; it's a lot more flexible and can output to log4j on the backend if needed. (It's the rewrite from the same author.) — chrylis -cautiouslyoptimistic-, Sep 05 '13 at 14:32
@chrylis slf4j is not a rewrite as it is not about the same thing. slf4j is a logging facade framework, such as commons-logging, whereas log4j is a logging implementation. For logging implementations, you might want to look logback, log4j2 etc. — eis, Sep 05 '13 at 14:41
@eis Technically, log4j is a facade as well, over the appenders. ;-) — chrylis -cautiouslyoptimistic-, Sep 05 '13 at 14:44
well, even if it is, it doesn't do the same stuff as log4j :) — eis, Sep 05 '13 at 14:55

score 5 · Accepted Answer · edited May 23 '17 at 10:25

A Logger adds so much more value to your logging.

Log levels: In a production environment you don't care about some levels (ex: debug info), only serious errors and system crashes.
Log filtering: Filter what components are allowed to log.
Log formatting: Control the information and its format contained in the log. You can setup defaults (like time and thread id) or add more specific ones per statement.
The log destination: You can set up your logger to write to a file, to emails, to databases, etc.
A policy for keeping and archiving logs: Let the logger manage the logs, keep some history, delete older

Also, Loggers are usually synchronized, so your print statements aren't written over eachother. You cannot guarantee that with System.out.

You can implement all this with System.out, but it would be so complex and tedious.

score 0 · Answer 2 · answered Sep 05 '13 at 14:31

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If your code is ever used in a web service or a system service, using debug statements will ensure that they appear in the services logs. If not you risk them being printed to a hidden console window and being lost.

answered Sep 05 '13 at 14:31

OldCurmudgeon

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score 0 · Answer 3 · answered Sep 05 '13 at 14:34

Specialized loggers offer a number of benefits over plain standard output:

logical separation of logging, in at least two orthogonal areas: subsystem and importance
flexibility - there are many predefined ways to output and store log data, and you can roll you own, if you really want
configurability - you can precisely control what and where to log, and all of this is external to the code that actually uses logging, making it simple and inobtrusive
formatting - you don't need to manually append all the bookkeeping information, such as the class name, time etc. Logger does that for you, in an entirely configurable way, allowing you to write simple, clear logging code and retain all the benefits, like filtering and precise information.
speed - loggers are optimized not to introduce much overhead

When to use log4j over system out

3 Answers3