Project Lombok: Clean, Concise Java Code - Oracle

The Lombok library was created to make Java an easier language in which to code. It takes some of the most common boilerplate headaches out of the developer's ... ClicktoviewourAccessibilityPolicy Skiptocontent home nav Oracle Close Search Search Products Industries Resources Support Events Developer Partners ViewAccountsSignIn Back OracleAccount CloudAccount SignintoCloud SignUpforFreeCloudTier Sign-In CreateanAccount Help SignOut ContactSales Noresultsfound Yoursearchdidnotmatchanyresults. Wesuggestyoutrythefollowingtohelpfindwhatyou’relookingfor: Checkthespellingofyourkeywordsearch. Usesynonymsforthekeywordyoutyped,forexample,try“application”insteadof“software.” Tryoneofthepopularsearchesshownbelow. Startanewsearch. TrendingQuestions Close   AboutOracle   JavaDeveloper|Libraries ProjectLombok:Clean,ConciseJavaCode AddLomboktoyourprojectandgetridofmostofyourboilerplatecode. ByJoshJuneau ImaginethatyouarecodingaJavaapplicationandcreatingaplainoldJavaobject(POJO),aJavaclasswithseveralprivatefieldsthatwillrequiregetterandsettermethodstoprovideaccess.Howmanylinesofcodewillbeneededtogenerate gettersandsettersforeachofthefields?Moreover,addingaconstructorandatoString()methodwillcauseevenmorelinesofcodeandclutter.Thatisalotofboilerplatecode.HowaboutwhenyouareutilizingJavaobjectsthatneedtobeclosedafteruse,soyouneedtocodeafinallyblockorusetry-with-resourcestoensurethattheobjectclosingoccurs?Addingfinallyblockboilerplatetocloseobjectscanaddasignificantamountofcluttertoyourcode. ProjectLombokisamaturelibrarythatreducesboilerplatecode.ThecasesmentionedabovecoverjustafewofthosewhereProjectLombokcanbeagreatbenefit.Thelibraryreplacesboilerplatecodewitheasy-to-useannotations.Inthisarticle,IexamineseveralusefulfeaturesthatProjectLombokprovides—makingcodeeasiertoreadandlesserror-proneandmakingdevelopersmoreproductive.Bestofall,thelibraryworkswellwithpopularIDEsandprovidesautilityto“delombok”yourcodebyreverting—thatis,addingbackalltheboilerplatethatwasremovedwhentheannotationswereadded. CheckforNulls Let’sstartwithoneofthemostbasicutilitiesthatLombokhastooffer.The@NonNullannotation,whichshouldnotbeconfusedwiththeBeanValidationannotation,canbeusedtogenerateanullcheckonasetterfield.ThecheckthrowsaNullPointerExceptioniftheannotatedclassfieldcontainsanullvalue.Simplyapplyittoafieldtoenforcetherule: @NonNull@Setter privateStringemployeeId; Thiscodegeneratesthefollowingcode: publicidsetEmployeeId(@NonNullfinalStringemployeeId) { if(employeeId==null)throw newjava.lang.NullPointerException("employeeId"); this.employeeId=employeeId; [email protected],awarningisissuedandnonullcheckisgenerated. ConciseDataObjects WritingaPOJOcanbelaborious,especiallyiftherearemanyfields.IfyouaredevelopingaPOJO,youshouldalwaysprovideprivateaccessdirectlytotheclassfields,whilecreatingaccessormethods—gettersandsetters—toreadfromandwritetothosefields.Althoughdevelopingaccessormethodsiseasy,theygenerallyarejustboilerplatecode.Lombokcantakecareofgeneratingthesemethodsifafieldisannotatedwith@[email protected],thefollowingtwocodelistingsprovidetheexactsamefunctionality. WithoutProjectLombok: privateStringcolumnName; publicStringgetColumnName(){ returnthis.columnName; } publicvoidsetColumnName(StringcolumnName){ this.columnName=columnName; } UsingProjectLombok: @Getter@SetterprivateStringcolumnName; Asyoucansee,Lomboknotonlymakesthecodemoreconcise,butitalsomakesthecodeeasiertoreadandlesserror-prone.Theseannotationsalsoacceptanoptionalparametertodesignatetheaccesslevelifneeded.Moregoodnews:@Getterand@Setterrespectthepropernamingconventions,sogeneratedcodeforaBooleanfieldresultsinaccessormethodsbeginningwithisratherthanget.Iftheyareappliedattheclasslevel,gettersandsettersaregeneratedforeachnonstaticfieldwithintheclass.  Lombokcantakecareoftheloggerdeclarationifyouplacethe@Logannotation(oranannotationpertainingtoyourchoiceofloggingAPI)onanyclassthatrequiresloggingcapability.  Inmanycases,dataobjectsalsoshouldcontaintheequals(),hashCode(),andtoString()methods.Thisboilerplatecanbetakencareofbyannotatingaclasswiththe@EqualsAndHashCodeand@ToStringannotations,respectively.TheseannotationscauseLomboktogeneratetherespectivemethods,andtheyarecustomizablesothatyoucanspecifyfieldexclusionsandotherfactors.Bydefault,anynonstaticornontransientfieldsareincludedinthelogicthatisusedtocomposethesemethods.Theseannotationsusetheattributeexcludetospecifymethodsthatshouldnotbeincludedinthelogic.ThecallSuperattributeacceptsatrueorfalse,anditindicateswhethertousetheequals()methodofthesuperclasstoverifyequality.Thefollowingcodedemonstratestheuseoftheseannotations. @EqualsAndHashCode @ToString(exclude={"columnLabel"}) publicclassColumnBean{ privateBigDecimalid; privateStringcolumnName; privateStringcolumnLabel; } The@Dataannotationcanbeusedtoapplyfunctionalitybehindalltheannotationsdiscussedthusfarinthissection.Thatis,simplyannotatingaclasswith@DatacausesLomboktogenerategettersandsettersforeachofthenonstaticclassfieldsandaclassconstructor,aswellasthetoString(),equals(),andhashCode()methods.Italsocreatesaconstructorthatacceptsanyfinalfieldsorthoseannotatedwith@NonNullasarguments.Finally,itgeneratesdefaulttoString(),equals(),andhashCode()methodsthattakeallclassfieldsandmethodsintoconsideration.ThismakesthecodingofaPOJOveryeasy,anditismuchthesameassomealternativelanguages,suchasGroovy,thatoffersimilarfeatures.Listing1(alllistingsforthisarticlecanbefoundinJavaMagazine’sdownloadsection)showsthefullJavacodeforthePOJOthatisgeneratedbythefollowingcode: @Data publicclassColumnBean{ @NonNull privateBigDecimalid; @NonNull privateStringcolumnName; @NonNull privateStringcolumnLabel; } Notethatifyoucreateyourowngettersorsetters,Lombokdoesnotgeneratethecodeeveniftheannotationsarepresent.Thiscanbehandyifyouwishtodevelopacustomgetterorsetterforoneormoreoftheclassfields. Ifyouaremerelyinterestedinhavingconstructorsgeneratedautomatically,@AllArgsConstructorand@NoArgsConstructormightbeofuse.@AllArgsConstructorcreatesaconstructorfortheclassusingallthefieldsthathavebeendeclared.Ifafieldisaddedorremovedfromtheclass,thegeneratedconstructorisrevisedtoaccommodatethischange.Thisbehaviorcanbeconvenientforensuringthataclassconstructoralwaysacceptsvaluesforeachoftheclassfields.Thedisadvantageofusingthisannotationisthatreorderingtheclassfieldscausestheconstructorargumentstobereorderedaswell,whichcouldintroducebugsifthereiscodethatdependsuponthepositionofargumentswhengeneratingtheobject.@NoArgsConstructorsimplygeneratesano-argumentconstructor. The@Valueannotationissimilartothe@Dataannotation,butitgeneratesanimmutableclass.Theannotationisplacedattheclasslevel,anditinvokestheautomaticgenerationofgettersforallprivatefinalfields.Nosettersaregenerated,andtheclassismarkedasfinal.Lastly,thetoString(),equals(),andhashCode()methodsaregenerated,andaconstructorisgeneratedthatcontainsargumentsforeachofthefields. Can’tMyIDEDoThat? Youmightbeaskingyourself,“Can’tmyIDEalreadydothatsortofrefactoring?”MostmodernIDEs—suchasNetBeans,Eclipse,andIntelliJ—offerfeaturessuchasencapsulationoffieldsandauto-generationofcode.Theseabilitiesaregreatbecausetheycansignificantlyincreaseproductivity.However,thesecapabilitiesdonotreducecodeclutter,sotheycanleadtorefactoringdowntheroad.Let’ssayyourJavaobjecthas10fields.ToconformtoaJavaBean,itwillcontain20accessormethods(onegetterandsetterpairperfield).That’salotofclutter.Also,whathappenswhenyoudecidetochangeoneofyourfieldnames?You’llhavetodosomerefactoringinordertochangeitcleanly.Ifyou’reusingLombok,yousimplychangethefieldnameandmoveonwithyourlife. BuilderObjects Sometimesitisusefultohavetheabilitytodevelopabuilderobject,whichallowsobjectstobeconstructedusingastep-by-steppatternwithcontrolledconstruction.Forexample,insomecaseslargeobjectsrequireseveralfieldstobepopulated,whichcanbeproblematicwhensuchanobjectisimplementedviaaconstructor. LombokmakesitsimpletocreatebuilderobjectsinmuchthesamewaythatitenableseasyPOJOcreation.Annotatingaclasswith@Builderproducesaclassthatadherestothebuilderpattern—thatis,aninnerbuilderclassisproducedthatcontainseachoftheclassfields.(“Builder”isprecededbythenameoftheclass.SoaclassnamedFoohasaFooBuilderclassgenerated.)Thegeneratedbuilderclasscontainsa“setter”methodforeachoftheclassfields,butthenamesofthemethodsdonotincludetheusual“set”prefix.Themethodsthemselvessetthevaluethatispassedintothemethods,andthentheyreturnthebuilderobject.Listing2inthedownloadablecodedemonstratesaclassthatcontainsabuilder,andListing3demonstratesthesameobjectannotatedwith@Builder. [email protected],theannotationcanbeplacedontheclass,onaconstructor,oronamethod.PlacingtheannotationonaconstructorproducesthesamebuilderobjectshowninListing2,butitgeneratesmethodsforeachoftheconstructor’sargumentsinthebuilder.Thismeansthatyoucanomitaclassfieldfromtheconstructor,oryoucanchoosetoincludeasuperclassfieldintheconstructor.Theonlywaytoincludesuperclassfieldsinabuilderisforanobjecttocontainasuperclass. ThetoBuilderattributeofthe@Builderannotationacceptstrueorfalse,anditcanbeusedtodesignatewhetheratoBuilder()methodisincludedinthegeneratedbuilderobject.Thismethodcopiesthecontentsofanexistingobjectofthesametype. Itispossibletotreatoneofthefieldsasabuildercollectionbyannotatingitwith@Singular.Thiscausestwoaddermethodstobegenerated—onetoaddasingleelementandanothertoaddallelements.Thisannotationalsocausesaclear()methodtobegenerated,whichclearsthecontentsofthecollection. EasyCleanup Lombokmakesiteasytocleanupresourcesaswell.Howoftenhaveyoueitherforgottentoclosearesourceorwrittenlotsofboilerplatetry-catchblockstoaccommodateresourceclosing?Thankstothe@Cleanupannotation,younolongerneedtoworryaboutforgettingtoreleasearesource. AlthoughtheJavalanguagenowcontainsthetry-with-resourcesstatementtohelpcloseresources,@Cleanupcanbeausefulalternativeinsomecases,becauseitcausesatry-finallyblocktobegeneratedaroundthesubsequentcode,andthenitcallstheannotatedresource’sclose()method.Ifthecleanupmethodforagivenresourceisnotnamedclose(),thecleanupmethodnamecanbespecifiedwiththeannotation’svalueattribute.Listing4inthedownloadablecodedemonstratesablockofcodethatcontainssomelinestomanuallyclosetheresource.Listing5demonstratesthesameblockofcodeusing@Cleanup. Itisimportanttonotethatinacasewherecodethrowsanexceptionandthensubsequentcodeinvokedvia@Cleanupalsothrowsanexception,theoriginalexceptionwillbehiddenbythesubsequentlythrownexception. LockingSafely Toensuresafetybyhavingonlyonethreadthatcanaccessaspecifiedmethodatatime,themethodshouldbemarkedassynchronized.Lomboksuppliesanevensaferwaytoensurethatonlyonethreadcanaccessamethodatatime:the@Synchronizedannotation.Thisannotationcanbeusedonlyonstaticandinstancemethods,justlikethesynchronizedkeyword.However,ratherthanlockingonthis,theannotationlocksonaprivatefieldnamed$lockfornonstaticmethodsandon$LOCKforstaticmethods.Thisfieldisauto-generatedifitdoesnotalreadyexist,oryoucancreateityourself.Youcanalsospecifyadifferentlockfieldbyspecifyingitasaparameterto@Synchronized.Thefollowingcodeillustratestheuseof@Synchronized: @Synchronized publicstaticvoidhelloLombok(){ System.out.println("Lombok"); } Thissolutioncanbeasaferalternativetousingthesynchronizedkeyword,becauseitallowsyoutolockonaninstancefieldratherthanonthis. EffortlessLogging Mostloggingrequiressomedeclarationtosetupaloggerwithineachclass.Thiscodeisdefinitelyrepetitiveboilerplatecode.Lombokcantakecareoftheloggerdeclarationifyouplacethe@Logannotation(oranannotationpertainingtoyourchoiceofloggingAPI)onanyclassthatrequiresloggingcapability. Forinstance,ifyouwishtousealoggingAPI—say,Log4j2—eachclassthatusestheloggermustdeclaresomethingsimilartothefollowing: publicclassClassName(){ privatestaticfinalorg.apache.log4j.Loggerlog= org.apache.log4j.Logger.getLogger(ClassName.class); ... //Uselogvariableasneeded } Lombokmakesitpossibletodothefollowinginstead: @Log4j2 publicclassClassName(){ ... //Uselogvariableasneeded } Listing6inthedownloadablecodeshowsanexampleusingLog4j2.Thenameoftheloggerwillautomaticallybethesameasitscontainingclass’name.However,thiscanbecustomizedbyspecifyingthetopicattributeoftherespectiveloggingannotation.ForacompletelistingofsupportedloggingAPIs,refertotheLombokdocumentationandtheLombokJavadoc. OtherUsefulItems ThereareseveralotherusefulfeaturesLombokoffersthatIhaven’tyetcovered.Let’sgothroughacoupleofthemosthighlyused. Informaldeclaration.Thevalkeywordcanbeusedinplaceofanobjecttypewhenyoudeclarealocalfinalvariable,muchlikethevalkeywordthatyouhaveseeninalternativelanguagessuchasGroovyorJython.Takethefollowingcode,forinstance: finalArrayListmyJobs=newArrayList(); Usingthevalkeyword,youcanchangethecodetothefollowing: valmyJobs=newArrayList(); Therearesomeconsiderationsforusingthevalkeyword.First,asmentionedpreviously,itmarksthemethoddeclarationasfinal.Therefore,ifyoulaterneedtochangethevalueofthevariable,usingthevalkeywordisnotpossible.ItalsodoesnotworkcorrectlyinsomeIDEs,soifyouaretryingtomarklocalvariablesasfinalinthoseIDEs,theyareflaggedaserrors. Besneakywithexceptions.Thereareoccasionswhereexceptionhandlingcanbecomeaburden,andI’darguethatthisistypicallythecasewhenyouareworkingwithboilerplateexceptions.Mostofthetime,Javaallowsyoutoeasilyseewhereproblemsexistviatheuseofcheckedexceptions.However,inthosecaseswherecheckedexceptionsareburdensome,youcaneasilyhidethemusingLombok.  ThedelombokutilitycanbeappliedtoyourcodetoconvertcodethatusesLombokbacktovanillaJava.  The@SneakyThrowsannotationcanbeplacedonamethodtoessentially“swallow”theexceptions,allowingyoutoomitthetry-catchblockcompletely.Theannotationallowsamethodtohandleallexceptionsquietly,oryoucanspecifyexactlywhichexceptionstoignorebypassingtheexceptionclassestotheannotationasattributes.Listing7inthedownloadablecodedemonstratestheuseof@SneakyThrowsspecifyingwhichexceptionstoswallow. IwanttoreiteratethatthisLombokfeatureshouldbeusedwithcaution,becauseitcanbecomearealissueiftoomanyexceptionsareignored. Lazygetters.Itispossibletoindicatethatafieldshouldhaveagettercreatedonce,andthentheresultshouldbecachedforsubsequentinvocations.Thiscanbeusefulifyourgettermethodisexpensiveasfarasperformancegoes.Forinstance,ifyouneedtopopulatealistfromadatabasequery,oryouneedtoaccessawebservicetoobtainthedataforyourfieldonthefirstaccess,itmightmakesensetocachetheresultforsubsequentcalls.Tousethisfeature,aprivatefinalvariablemustbegeneratedandinitializedwiththeexpensiveexpression.Youcanthenannotatethefieldwith@Getter(lazy=true)toimplementthisfunctionality. IDEcompatibility.LombokplayswellwiththemajorIDEs,sosimplyincludingLombokinyourprojectandannotatingaccordinglytypicallydoesnotgenerateerrorsincodeorcauseerrorswhenthegeneratedmethodsarecalled.Infact,inNetBeanstheclassNavigatorispopulatedwiththegeneratedmethodsafterannotationsareplacedandthecodeissaved,eventhoughthemethodsdonotappearinthecode.Auto-completionworksjustasifthemethodsweretypedintotheclass,evenwhengeneratedpropertiesareaccessedfromawebviewinexpressionlanguage. Evenmore-conciseJavaEE.Overthepastfewyears,JavaEEhasbeenmakinggoodheadwayonbecomingaveryproductiveandconciseplatform.ThoseofyouwhorecallthelaboriousJ2EEplatformcancertainlyattesttothegreatnumberofimprovementsthathavebeenmade.IwasveryhappytolearnthatLombokplaysnicelywithsomeJavaEEAPIs,suchasJavaPersistenceAPI(JPA).Thismeansitisveryeasytodevelopconstructssuchasentityclasseswithoutwritingalltheboilerplate,whichmakestheclassesmuchmoreconciseandlesserror-prone.I’vedevelopedentireJavaEEapplicationswithoutanygettersorsettersinmyentityclasses,justbyannotatingthemwith@Data.Isuggestyouplayaroundwithitandseewhatworksbestforyou. Usecautionandrollback.Aswiththeuseofanylibrary,therearesomecaveatstokeepinmind.Thisisespeciallytruewhenyouarethinkingaboutfuturemaintenanceormodificationstothecodebase.Lombokgeneratescodeforyou,butthatmightcauseaproblemwhenitcomestorefactoring.Itisdifficulttorefactorcodethatdoesnotexistuntilcompiletime,sobecautiouswithrefactoringcodethatusesLombok.Youalsoneedtothinkaboutreadability.Lombokannotationsmightmaketroubleshootingamysteryforsomeonewhoisnotfamiliarwiththelibrary—andevenforthosewhoare—ifsomethingsuchas@SneakyThrowsishidinganexception. Fortunately,Lombokmakesiteasytorollbackifyouneedto.ThedelombokutilitycanbeappliedtoyourcodetoconvertcodethatusesLombokbacktovanillaJava.ThisutilitycanbeusedviaAntorthecommandline. Conclusion TheLomboklibrarywascreatedtomakeJavaaneasierlanguageinwhichtocode.Ittakessomeofthemostcommonboilerplateheadachesoutofthedeveloper’stasklist.Itcanbeusefulformakingyourcodemoreconcise,reducingthechanceforbugs,andspeedingupdevelopmenttime.TryaddingLomboktooneofyourapplicationsandseehowmanylinesofcodeyoucancutout. ThisarticleoriginallywaspublishedinJavaMagazine. JoshJuneau(@javajuneau)isaJavaChampionandamemberoftheNetBeansDreamTeam.Heworksasanapplicationdeveloper,systemanalyst,anddatabaseadministrator.HeisafrequentcontributortoOracleTechnologyNetworkandJavaMagazine.JuneauhaswrittenseveralbooksonJavaandJavaEEforApress,andheisaJCPExpertGroupmemberforJSR372(JavaServerFaces[JSF]2.3)andJSR378(Portlet3.0BridgeforJSF2.2). 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