Logging in Java with slf4j文档格式.docx

Logging in Java with slf4j文档格式.docx

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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（"

）;

4:

}

5:

Thisshouldbenamed"

HelloWorld.java"

andiscompilablefromthecommandlinewith"

javacHelloWorld.java"

andthecompiledclasscanthenbeinvokedwith"

javaHelloWorld"

. 

Line1declaresthistobetheclassnamedHelloWorld-thereisno"

package"

linesoitisjustHelloWorldinsteadofe.g.com.bigcoorporation.HelloWorld. 

Line2isthemagicwordsneededtobeabletoinvoketheclassfilefromthecommandlineas"

Line3printstheline"

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{

Loggerlog=LoggerFactory.getLogger（HelloWorld.class）;

log.info（"

}

===== 

andgeneratesthefollowingoutput:

====

2009-06-0710:

38:

21demo.HelloWorldmain

INFO:

HelloWorld

Thefirstlinecontainsadateandtimestamp,thenameofthelogger-here"

demo.HelloWorld"

-andthenameofthemethodcontainingthelog.infostatement-here"

main"

Thesecondlinecontainstheimportancelevel-here"

INFO"

-andthemessagepassed 

-here"

Notehowmuchofthisinformationwasderivedautomatically!

Ifweaddamethodcallwithawait,wecanseethatthetimestampchanges,andthatthemethodnamesinthelogcorrectlyindicatesifthelogstatementisdonefrom"

or"

waitTwoSeconds"

publicclassHelloWorld2{

publicstaticvoidmain（String[]args）throwsException{

Loggerlog=LoggerFactory.getLogger（HelloWorld2.class）;

waitTwoSeconds（log）;

Done"

privatestaticvoidwaitTwoSeconds（Loggerlog）throwsException{

Thread.sleep（2000）;

Slept2seconds"

=====

withthefollowingoutput:

53:

52demo.HelloWorld2main

54demo.HelloWorld2waitTwoSeconds

Slept2seconds

54demo.HelloWorld2main

Done

Ifyouwereshownthislog,youwouldbeabletodeterminethattheprogramhadrunasexpectedtothestatementlogging"

inthemainmethodofdemo.HelloWorld2andyoushouldnotlookforproblemshappeningbeforethatstatement. 

Logginginformationstoredinvariables

Ifyouneedtoaddinformationstoredinavariable,slf4jallowyoutospecifythiswith{}.（Note:

ExampleworkswithJava5andlater） 

publicclassZeroToFour{

Loggerlog=LoggerFactory.getLogger（ZeroToFour.class）;

for（inti=0;

i<

5;

i++）{

i={},i*i={}"

i,i*i）;

whichoutputs 

2009-06-0713:

23:

57demo.ZeroToFourmain

i=0,i*i=0

i=1,i*i=1

i=2,i*i=4

i=3,i*i=9

i=4,i*i=16

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

Loggingproblems

Allprogramsregardlessofprogramminglanguagemus