Advantage of log4j

问题:

What's the advantage of log4j over set System.out and System.err to output to a log file?

回答1:

At a high level, the win from Log4j over manual logging is that you can decouple your logging code from what you actually want to log and where and how you want to log it. Details about logging verbosity/filtering, formatting, log location, and even log type (files, network, etc.) are handled declaratively using configuration and extensibly via custom appenders, rather you having to code that flexibility yourself.

This is critically important because it's often hard for developers to predict how logging needs will change once their software is in production. Operations teams managing that software may need less verbose logs, may need mulitple logs, may need to ship those logs to multiple servers, may need to sometimes get really verbose data for troubleshooting, etc. And it's usually impossible for operations teams, if they need to change how logging works, to convince the developer to make big code changes. This often leads to production downtime, friction between operations and development, and wasted time all around.

From the developer's point of view, Log4j insulates you from having to make code changes to support logging, and insulates you from being pestered by people who want logging changes. It enables people managing your code to scratch their own itch rather than bugging you!

Also, since Log4j is the de-facto standard for Java logging, there are lots of tools available which can do cool things with Log4j-- furthermore preventing you and your operations teams from re-inventing the wheel.

My favorite feature is the ability to easily write appenders send data to non-file sources, like SYSLOG, Splunk, etc. which makes it easy to your app's custom logging into operations management tools your IT department is already using.

回答2:

Actually, you should look into the slf4j facade these days, as it allows you to use {}-placeholders for the most concise statements. You can then use the appropriate logging framework behind slf4j to handle the actual treatment of your log statements. This could be log4j or the slf4j-simple which just prints out all of INFO, WARN and ERROR, and discards the rest.

The crucial observation you need to make is that the WRITING of log statements is done when the code is written, and the DECISION of what is needed is done when the code is deployed, which may be years after the code was written and tested. System.out.println requires you to physically change your code to get rid of them, which is unacceptable in a rigid write-test-deploy cycle. IF the code changes, it must be retested. With slf4j you just enable those you want to see.

We have full logging in the test phase, and rather verbose logging in the initial period of a production deployment, after which we go down to information only. This gives us full information in a scenario where debugging a case is very rarely possible.

You might find this article I wrote interesting. The target audience is beginning Java programmers, with my intention of giving them good habits from the start. http://runjva.appspot.com/logging101/index.html

回答3:

my favorites (not all)

• Ability to set parameters of logging in config, without recompiling
• Ability to set the way log is written (from text file to SMTP sender)
• Ability to filter by severity

回答4:

Levels, formatting, logging to multiple files... A logging framework (even if it's java.util.logging) is really beneficial if there's a chance anything may go wrong while your code is running.

回答5:

log4j allows you to log to various resources e.g. event log, email, file system etc while allowing your application to remain decoupled from all of these resources. Furthermore, you get to use a common interface to log to all of the various resources without having to learn or integrate thier corresponding APIs.

回答6:

Log4j offers the ability to rotate your log files based on size and delete them based on quantity (logrotate), so your servers don't fill up their disks. Personally I think that is one of the more valuable features in Log4j.

Also Log4j is popular and understood by many developers. The last three companies I've worked at have all used Log4j in most projects.

回答7:

Take a look and you will understand the power of log4j :

log4j.properties I used once for a project :

# ALL < DEBUG < INFO < WARN < ERROR < FATAL < OFF

# No appenders for rootLogger 
log4j.rootLogger=OFF

folder=..
prefix=
fileExtension=.log
htmlExtension=${fileExtension}.html
datestamp=yyyy-MM-dd/HH:mm:ss.SSS/zzz
layout=%d{${datestamp}} ms=%-4r [%t] %-5p %l %n%m %n%n

# myLogger logger
log4j.logger.myLogger=ALL, stdout, infoFile, infoHtml, errorFile

# stdout 
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=${layout}
# infoFile 
log4j.appender.infoFile=org.apache.log4j.FileAppender
log4j.appender.infoFile.File=${folder}/${prefix}_info${fileExtension}
log4j.appender.infoFile.layout=org.apache.log4j.PatternLayout
log4j.appender.infoFile.layout.ConversionPattern=${layout}
# infoHtml 
log4j.appender.infoHtml=org.apache.log4j.FileAppender
log4j.appender.infoHtml.File=${folder}/${prefix}_info${htmlExtension}
log4j.appender.infoHtml.layout=org.apache.log4j.HTMLLayout
log4j.appender.infoHtml.layout.Title=Logs
log4j.appender.infoHtml.layout.LocationInfo=true
# errorFile 
log4j.appender.errorFile=org.apache.log4j.FileAppender
log4j.appender.errorFile.File=${folder}/${prefix}_error${fileExtension}
log4j.appender.errorFile.layout=org.apache.log4j.PatternLayout
log4j.appender.errorFile.layout.ConversionPattern=${layout}

# APPENDERS SETTINGS
log4j.appender.stdout.Threshold = ALL
log4j.appender.infoFile.Threshold = INFO
log4j.appender.infoHtml.Threshold = INFO
log4j.appender.errorFile.Threshold = WARN.

To change the variables in your java code you can do :

Loading Configuration

Log4j will automatically load the configuration if it is stored in a file called "log4j.properties" and is present on the classpath under "" (e.g. WEB-INF/classes/log4j.properties).

I don't like that approach and prefer to load the configuration explicitly by calling:

PropertyConfigurator.configure( Config.ETC + "/log4j.properties" ); This way I can reload the configuration at any time as long as my application is still running. I like to add a button to an administrative jsp, "Reload Log4J".

Dynamic Log File Location

Many people complain that Log4j forces you to hard-code the location where your logs will be kept. Actually, it is possible to dynamically choose the log-file location, especially if you use the ${log.dir} property substitution technique above. Here's how:

String dynamicLog = // log directory somehow chosen... 
Properties p = new Properties( Config.ETC + "/log4j.properties" ); 
p.put( "log.dir", dynamicLog ); // overwrite "log.dir" 
PropertyConfigurator.configure( p );

回答8:

• logging (Document historical business events that occur, you can check old logs)
• track the application (project flow)
• debugging the application (Detailed information what occurs in a method at granular level //data, value and all inside methods)
• error handling (information about specific error that occur)