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2013年3月

IntroducingtheSpringFramework

TheSpringFramework:

apopularopensourceapplicationframeworkthataddressesmanyoftheissuesoutlinedinthisbook.ThischapterwillintroducethebasicideasofSpringanddis-cussthecentral“beanfactory”lightweightInversion-of-Control（IoC）containerindetail.

Springmakesitparticularlyeasytoimplementlightweight,yetextensible,J2EEarchi-tectures.Itprovidesanout-of-the-boximplementationofthefundamentalarchitecturalbuildingblockswerecommend.Springprovidesaconsistentwayofstructuringyourapplications,andprovidesnumerousmiddletierfeaturesthatcanmakeJ2EEdevelopmentsignificantlyeasierandmoreflexiblethanintraditionalapproaches.

ThebasicmotivationsforSpringare:

Toaddressareasnotwellservedbyotherframeworks.TherearenumerousgoodsolutionstospecificareasofJ2EEinfrastructure:

webframeworks,persistencesolutions,remotingtools,andsoon.However,integratingthesetoolsintoacomprehensivearchitecturecaninvolvesignificanteffort,andcanbecomeaburden.Springaimstoprovideanend-to-endsolution,integratingspe-cializedframeworksintoacoherentoverallinfrastructure.Springalsoaddressessomeareasthatotherframeworksdon’t.Forexample,fewframeworksaddressgenerictransactionmanagement,dataaccessobjectimplementation,andgluingallthosethingstogetherintoanapplication,whilestillallowingforbest-of-breedchoiceineacharea.HencewetermSpringanapplicationframework,ratherthanawebframework,IoCorAOPframework,orevenmiddletierframework.

Toallowforeasyadoption.Aframeworkshouldbecleanlylayered,allowingtheuseofindi-vidualfeatureswithoutimposingawholeworldviewontheapplication.ManySpringfeatures,suchastheJDBCabstractionlayerorHibernateintegration,canbeusedinalibrarystyleoraspartoftheSpringend-to-endsolution.

Todelivereaseofuse.Aswe’venoted,J2EEoutoftheboxisrelativelyhardtousetosolvemanycommonproblems.Agoodinfrastructureframeworkshouldmakesimpletaskssimpletoachieve,withoutforcingtradeoffsforfuturecomplexrequirements（likedistributedtransactions）ontheapplicationdeveloper.ItshouldallowdeveloperstoleverageJ2EEservicessuchasJTAwhereappropriate,buttoavoiddependenceonthemincaseswhentheyareunnecessarilycomplex.

Tomakeiteasiertoapplybestpractices.Springaimstoreducethecostofadheringtobestpracticessuchasprogrammingtointerfaces,ratherthanclasses,almosttozero.However,itleavesthechoiceofarchitecturalstyletothedeveloper.

Non-invasiveness.Applicationobjectsshouldhaveminimaldependenceontheframework.IfleveragingaspecificSpringfeature,anobjectshoulddependonlyonthatparticularfeature,whetherbyimplementingacallbackinterfaceorusingtheframeworkasaclasslibrary.IoCandAOParethekeyenablingtechnologiesforavoidingframeworkdependence.

Consistentconfiguration.Agoodinfrastructureframeworkshouldkeepapplicationconfigurationflexibleandconsistent,avoidingtheneedforcustomsingletonsandfactories.Asinglestyleshouldbeapplicabletoallconfigurationneeds,fromthemiddletiertowebcontrollers.

Easeoftesting.Testingeitherwholeapplicationsorindividualapplicationclassesinunittestsshouldbeaseasyaspossible.Replacingresourcesorapplicationobjectswithmockobjectsshouldbestraightforward.

Toallowforextensibility.BecauseSpringisitselfbasedoninterfaces,ratherthanclasses,itiseasytoextendorcustomizeit.ManySpringcomponentsusestrategyinterfaces,allowingeasycustomization.

ALayeredApplicationFramework

Chapter6introducedtheSpringFrameworkasalightweightcontainer,competingwithIoCcontainerssuchasPicoContainer.WhiletheSpringlightweightcontainerforJavaBeansisacoreconcept,thisisjustthefoundationforasolutionforallmiddlewarelayers.

BasicBuildingBlocks

pringisafull-featuredapplicationframeworkthatcanbeleveragedatmanylevels.Itconsistsofmulti-plesub-frameworksthatarefairlyindependentbutstillintegratecloselyintoaone-stopshop,ifdesired.Thekeyareasare:

Beanfactory.TheSpringlightweightIoCcontainer,capableofconfiguringandwiringupJava-BeansandmostplainJavaobjects,removingtheneedforcustomsingletonsandadhocconfigura-tion.Variousout-of-the-boximplementationsincludeanXML-basedbeanfactory.ThelightweightIoCcontaineranditsDependencyInjectioncapabilitieswillbethemainfocusofthischapter.

Applicationcontext.ASpringapplicationcontextextendsthebeanfactoryconceptbyaddingsupportformessagesourcesandresourceloading,andprovidinghooksintoexistingenviron-ments.Variousout-of-the-boximplementationsincludestandaloneapplicationcontextsandanXML-basedwebapplicationcontext.

AOPframework.TheSpringAOPframeworkprovidesAOPsupportformethodinterceptiononanyclassmanagedbyaSpringlightweightcontainer.Itsupportseasyproxyingofbeansinabeanfactory,seamlesslyweavingininterceptorsandotheradviceatruntime.Chapter8dis-cussestheSpringAOPframeworkindetail.ThemainuseoftheSpringAOPframeworkistoprovidedeclarativeenterpriseservicesforPOJOs.

Auto-proxying.SpringprovidesahigherlevelofabstractionovertheAOPframeworkandlow-levelservices,whichofferssimilarease-of-useto.NETwithinaJ2EEcontext.Inparticular,theprovisionofdeclarativeenterpriseservicescanbedrivenbysource-levelmetadata.

Transactionmanagement.Springprovidesagenerictransactionmanagementinfrastructure,withpluggabletransactionstrategies（suchasJTAandJDBC）andvariousmeansfordemarcat-ingtransactionsinapplications.Chapter9discussesitsrationaleandthepowerandflexibilitythatitoffers.

DAOabstraction.Springdefinesasetofgenericdataaccessexceptionsthatcanbeusedforcre-atinggenericDAOinterfacesthatthrowmeaningfulexceptionsindependentoftheunderlyingpersistencemechanism.Chapter10illustratestheSpringsupportforDAOsinmoredetail,examiningJDBC,JDO,andHibernateasimplementationstrategies.

JDBCsupport.SpringofferstwolevelsofJDBCabstractionthatsignificantlyeasetheeffortofwritingJDBC-basedDAOs:

theorg.springframework.jdbc.corepackage（atemplate/

callbackapproach）andtheorg.springframework.jdbc.objectpackage（modelingRDBMSoperationsasreusableobjects）.UsingtheSpringJDBCpackagescandelivermuchgreaterpro-ductivityandeliminatethepotentialforcommonerrorssuchasleakedconnections,comparedwithdirectuseofJDBC.TheSpringJDBCabstractionintegrateswiththetransactionandDAOabstractions.

IntegrationwithO/Rmappingtools.SpringprovidessupportclassesforO/RMappingtoolslikeHibernate,JDO,andiBATISDatabaseLayertosimplifyresourcesetup,acquisition,andrelease,andtointegratewiththeoveralltransactionandDAOabstractions.TheseintegrationpackagesallowapplicationstodispensewithcustomThreadLocalsessionsandnativetransac-tionhandling,regardlessoftheunderlyingO/Rmappingapproachtheyworkwith.

WebMVCframework.SpringprovidesacleanimplementationofwebMVC,consistentwiththeJavaBeanconfigurationapproach.TheSpringwebframeworkenableswebcontrollerstobeconfiguredwithinanIoCcontainer,eliminatingtheneedtowriteanycustomcodetoaccessbusinesslayerservices.ItprovidesagenericDispatcherServletandout-of-the-boxcontrollerclassesforcommandandformhandling.Request-to-controllermapping,viewresolution,localeresolutionandotherimportantservicesareallpluggable,makingtheframeworkhighlyextensi-ble.ThewebframeworkisdesignedtoworknotonlywithJSP,butwithanyviewtechnology,suchasVelocity—withouttheneedforadditionalbridges.Chapter13discusseswebtierdesignandtheSpringwebMVCframeworkindetail.

Remotingsupport.Springprovidesathinabstractionlayerforaccessingremoteserviceswithouthard-codedlookups,andforexposingSpring-managedapplicationbeansasremoteservices.Out-of-the-boxsupportisincludedforRMI,Caucho’sHessianandBurlapwebserviceprotocols,andWSDLWebServicesviaJAX-RPC.Chapter11discusseslightweightremoting.

WhileSpringaddressesareasasdiverseastransactionmanagementandwebMVC,itusesaconsistentapproacheverywhere.Onceyouhavelearnedthebasicconfigurationstyle,youwillbeabletoapplyitinmanyareas.Resources,middletierobjects,andwebcomponentsareallsetupusingthesamebeanconfigurationmechanism.Youcancombineyourentireconfigurationinonesinglebeandefinitionfileorsplititbyapplicationmodulesorlayers;thechoiceisuptoyouastheapplicationdeveloper.Thereisnoneedfordiverseconfigurationfilesinavarietyofformats,spreadoutacrosstheapplication.

SpringonJ2EE

AlthoughmanypartsofSpringcanbeusedinanykindofJavaenvironment,itisprimarilyaJ2EEapplicationframework.Forexample,thereareconvenienceclassesforlinkingJNDIresourcesintoabeanfactory,suchasJDBCDataSourcesandEJBs,andintegrationwithJTAfordistributedtransactionmanagement.Inmostcases,applicationobjectsdonotneedtoworkwithJ2EEAPIsdirectly,improvingreusabilityandmeaningthatthereisnoneedtowriteverbose,hard-to-test,JNDIlookups.

ThusSpringallowsapplicationcodetoseamlesslyintegrateintoaJ2EEenvironmentwithoutbeingunnecessarilytiedtoit.YoucanbuilduponJ2EEserviceswhereitmakessenseforyourapplication,andchooselighter-weightsolutionsifthere