Logging in Java with slf4j.docx

Logging in Java with slf4j.docx

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LogginginJavawithslf4j

LogginginJavawithslf4j

ThisdocumentisanintroductorytextteachingnewJavaprogrammershowtologinformationabouttheirprograms,andhowtoreadtheinformationloggedfordebuggingpurposes. ThisisdemonstratedwithfullyfunctionalprogramsusingthesimplestpossibleconfigurationofSimpleLoggingFacadeforJavafrontend.  Evensothemethologiesshownaresuitableforfullscaleproduction. 

Pleaseprovidefeedbackifyouliketo ravn@ 

--ThorbjørnRavnAndersen 

Tellingyourgrandmotherwhatyoudo

Welcometotheworldofprogramming!

Hopefullyyou'llenjoyyourstay,andperhapsevenwanttolivehere.    Ifyoudoso,andyoueverneedtotellyourgrandmotherwhatyoudoforalivingthendon'tdespair. Justaskherwhat 

"Bindoff6（7,7,9）stsatbegofnext2rows. DecRow（RS）:

 K2,k2tog,kacrosstolast4sts,ssk,k2.RepDecRoweveryotherrow10（12,13,14）timesmore--38（40,46,48）sts.Continueeventoapprox211/2（22,221/2,23）inchesfrombegendingwithaWSrow."

means-mostgrandmothersknowthat-andtellheryouwritethosebutforcomputers. She'llnodandunderstandcompletely. 

IfyouarelikemostlearningprogrammersyouhaveNOideawhatthequoteabovemeansatall. Well,itispartofaknittingrecipeforasweater[ 1 ]andthatiswhatprogrammingisbasicallyallabout-writingknittingrecipesforcomputers. 

Programmingisacraftittakesyearstomaster

PeterNorvigwritesinhisonlinearticle TeachYourselfProgramminginTenYears -arecommendedread-thatittakes10yearsorperhaps10000hoursofdeliberatepracticetodevelopexpertiseinanarea.  Forprogrammersthatmeanswritingprogramsandreadingprograms,butunfortunatelythereisnotyetageneraltraditionofexperiencedprogrammersmentoringnewprogrammersinperson,soyouwillhavetolearnalotonyourown.  

Byusingthetechniquesdescribedinthisdocumentyouwillhaveagreaterchancetoavoidsomeveryembarrassingandexhaustingexperiencesinyourfuturejobs,andhopefullynothavingtounlearnahabitortwo. 

Ifyouneedtogetexperiencewhilestudyingorpreparingtogetajob,IwouldrecommendthatyoulocateanOpenSourceprojectwhichyouaregenuinelyinterestedin-forinstanceifyouneeditinyourownprojects-andhelpthem. Thiscanbothbeprogramming,butalsowritingdocumentation-asgooddocumentationrequiresthoroughunderstandingofthesoftwarebeingdocumented. Thisalsogivesyouexperienceworkingwithateamwhichcommunicateelectronicallyandusetoolsthatallowmultipledeveloperstoworkonthesameprojectwithoutdisturbingeachothersfiles. 

BychoosinganOpenSourceprojectyoucandemonstratewhatyouhavedonetoafutureemployeraspartoftheinterviewiftheyaskwhatexperienceyouhave. 

"Andyoutryandtelltheyoungpeopleoftodaythat.....theywon'tbelieveyou."

JavawasdesignedbySunMicrosystems,whichhappenedtomakealivingatthetimesellingcomputersrunningtheSundialectoftheUnixoperatingsystem. ItisnotasurprisethenthatJavaprogramstendtolooklikeotherUnixprograms,whichweredesignedtoworkonthecomputeryouseeinthepicturebelow. 

PictureshowsKenThompsonandDennisRitchieworkingatthefirstrealUnixmachine.  Pleasenotethatthereisrealpaperinthe"typewriters",theclosetbehindthemenis the computer,andthatthecircularcontainersontopofthecomputercontainprogramanddataonpapertape. Thingsweredifferentin1973.

UnderUnixeveryprogramhasthreeinput/outputconnectionsavailable-namely 

∙standardinput -thisprovidesthethingstheusertypetotheprogram. （Java:

 System.in）

∙standardoutput -thisisnormalinformation（Java:

 System.out）

∙standarderror -thisisinformationabouterrorsituations（Java:

 System.err）

TheUnixphilosphywasstatedbyMcIlroy[ 3 ]tobe" Writeprogramsthatdoonethinganddoitwell.Writeprogramstoworktogether.Writeprogramstohandletextstreams,becausethatisauniversalinterface. ".Thismeansyoucanlettheoutputofoneprogrambetheinputofanotherprogram-calledapipe -encouragingsmallspecializedprogramsbeingcombinedtocompletealargertask,likecreatingalistofwordsinafilebyfirstextractingallwords,sortingthem,andtheneliminateduplicates. UnfortunatelythishasnotcaughtoninJava,mostlybecausetheJavaenvironmenttakestoolongtostartforsmallprogramstobereallyusable. Theconceptsarestillthesame,however. 

SothesimplestpossibleJavaprogramusesSystem.outtoprintouttheline"HelloWorld":

1:

publicclassHelloWorld{

2:

publicstaticvoidmain（String[]args）{

3:

System.out.println（"HelloWorld"）;

4:

}

5:

}

Thisshouldbenamed"HelloWorld.java",andiscompilablefromthecommandlinewith"javacHelloWorld.java",andthecompiledclasscanthenbeinvokedwith"javaHelloWorld". 

Line1declaresthistobetheclassnamedHelloWorld-thereisno"package"linesoitisjustHelloWorldinsteadofe.g.com.bigcoorporation.HelloWorld. Line2isthemagicwordsneededtobeabletoinvoketheclassfilefromthecommandlineas"javaHelloWorld".  Line3printstheline"HelloWorld"tostandardoutput. 

Now,whydid that happen?

Oneoftheunfortunatesideeffectsofprogrammingforalivingisthatpeopleareusingyourprograms,andtheytendtofindthatyourprogramsdonotworkas they expect-thisistraditionallycalleda"bug"[ 2 ]-anditisthenyourresponsibilitytoidentifyandcorrecttheproblem. 

Inmostcases,allyouhavetoworkwithistheuserdescribingwhathappenedandwhatwasexpected. Theprogramhasendedandisforallpracticalpurposesunavailabletoyou.  Henceawayofsavinginformationfromwithintheprogramforlaterisneeded,muchinthesamewaythatlogbooksareusedonshipstoaidinnavigation.  Theterm"logging"isusedingeneralfortheprocessofsystematicallysavinginformationforlater. 

SeveralloggingsolutionsareavailabletoJavaprogrammers. Allofthesegenerallysharetheabilitytolog:

∙Theareaofthecodecontainingthelogstatement.

∙Thetimewhenthelogstatementwasinvoked.

∙Animportancelevelofthislogstatement.

∙Anexception（withaccompanyingstacktrace）.

∙Alogmessage.

Thishasproventobeveryhelpfulforlogsintendedforthehumanreader. Facilitiesavailableinsomeloggingframeworksinclude:

∙storingeventsinadatabase

∙alertusersbyemail,instantmessagingoranyotherelectronicmeans

（butthatistooadvancedtobecoveredinthisintroduction-itisjusttogiveyouanideaofwhatispossible）

TheHelloWorldprogramrewrittentousetheSLF4Jloggingsolution（fullinstructionsareinappendixA）andputinthe"demo"package,lookslike:

==== 

packagedemo;

importorg.slf4j.Logger;

importorg.slf4j.LoggerFactory;

publicclassHelloWorld{

publicstaticvoidmain（String[]args）{

Loggerlog=LoggerFactory.getLogger（HelloWorld.class）;

log.info（"HelloWorld"）;

}

}

===== 

andgeneratesthefollowingoutput:

====

2009-06-0710:

38:

21demo.HelloWorldmain

INFO:

HelloWorld

===== 

Thefirstlinecontainsadateandtimestamp,thenameofthelogger-here"demo.HelloWorld"-andthenameofthemethodcontainingthelog.infostatement-here"main".  Thesecondlinecontainstheimportancelevel-here"INFO"-andthemessagepassed -here"HelloWorld". Notehowmuchofthisinformationwasderivedautomatically!

Ifweaddamethodcallwithawait,wecanseethatthetimestampchanges,andthatthemethodnamesinthelogcorrectlyindicatesifthelogstatementisdonefrom"main"or"waitTwoSeconds". 

====

packagedemo;

importorg.slf4j.Logger;

importorg.slf4j.LoggerFactory;

publicclassHelloWorld2{

publicstaticvoidmain（String[]args）throwsException{

Loggerlog=LoggerFactory.getLogger（HelloWorld2.class）;

log.info（"HelloWorld"）;

waitTwoSeconds（log）;

log.info（"Done"）;

}

privatestaticvoidwaitTwoSeconds（Loggerlog）throwsException{

Thread.sleep（2000）;

log.info（"Slept2seconds"）;

}

}

=====

withthefollowingoutput:

=====

2009-06-0710:

53:

52demo.HelloWorld2main

INFO:

HelloWorld

2009-06-0710:

53:

54demo.HelloWorld2waitTwoSeconds

INFO:

Slept2seconds

2009-06-0710:

53:

54demo.HelloWorld2main

INFO:

Done

===== 

Ifyouwereshownthislog,youwouldbeabletodeterminethattheprogramhadrunasexpectedtothestatementlogging"Done"inthemainmethodofdemo.HelloWorld2andyoushouldnotlookforproblemshappeningbeforethatstatement. 

Logginginformationstoredinvariables

Ifyouneedtoaddinformationstoredinavariable,slf4jallowyoutospecifythiswith{}.（Note:

ExampleworkswithJava5andlater） 

====

packagedemo;

importorg.slf4j.Logger;

importorg.slf4j.LoggerFactory;

publicclassZeroToFour{

publicstaticvoidmain（String[]args）throwsException{

Loggerlog=LoggerFactory.getLogger（ZeroToFour.class）;

for（inti=0;i<5;i++）{

log.info（"i={},i*i={}",i,i*i）;

}

}

}

==== 

whichoutputs 

====

2009-06-0713:

23:

57demo.ZeroToFourmain

INFO:

i=0,i*i=0

2009-06-0713:

23:

57demo.ZeroToFourmain

INFO:

i=1,i*i=1

2009-06-0713:

23:

57demo.ZeroToFourmain

INFO:

i=2,i*i=4

2009-06-0713:

23:

57demo.ZeroToFourmain

INFO:

i=3,i*i=9

2009-06-0713:

23:

57demo.ZeroToFourmain

INFO:

i=4,i*i=16

==== 

Allobjectscanbeloggedinthisway,andthetoString（）methodiscalledtogetthestringtolog.Notethatint,long,double,floats,charsarenotobjects,butthatinJava5andlatertheyareinvisiblyconvertedcorrectly. 

Loggingproblems

Allprogramsregardlessofprogramminglanguagemus