and instability is software design稳定和不稳定的软件设计课件

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1、StabilityandVolatilityisSoftwareDesignH.RahnamaTutorialinsoftwareengineeringGoodSoftwareDesign:Testability,ReusabilityOnalargescaledecompositionofthesystemrepresentssignificantdesigndecisionsLargescalemodulesinJava:.JAR,C#:DLL,C++:SOAllsystemsshouldbedesign

2、edsothattheycouldbebrokendownintoINDEPENDENTLYdepolyablemodules.Moduleindependence:BetterdesignReusabilityandTestabilityReusabilityandtestabilitydependofdesignsthatsupportsindependentdeployabilityModularDependence:TheymakebuildorderambiguousCausetransitived

3、ependencyPreventindependentreuseofmodulesCyclicGraphs!ModuleAdependsonModuleBModuleBdependsonModuleCModuleCdependsonModuleANoneofthemodulescanbereusedindependentlyGoodDesign:ADPAcyclicDependenciesPrincipleDependenciesamongmodulesmustbearrangedinaDirectedAcy

4、clicGraphsNocyclesofdependenciesamongmodulesEliminatingcycles:FirststepincreatingamodulardesignADPPrincipleThesemodulesaredependentDocumentprocessordependsontwosaversADPPrincipleUsePolymorphisminsteadof“If”UsingtheInterfaceDocumentProcessorhasnodirectdepend

5、enceonDatabaseSaverorFileSystemSaverSaversareVOLATILEVolatilityHowchangeableamoduleis.SavermoduleismorestableThislackofvolatilityallowstheotherstodependonitOthersaversaresafefromchangesCommonClosurePrinciple(CCP)Codeswithdifferentkindsofvolatilityshouldbepl

6、acedindifferentmodulesModulesshouldnotdependonmoduleswhoaremorevolatilethantheyare.Theyshoulddependonlessvolatileones.Volatilityvs.StabilityStability:Howdifficultamoduleistochange.ModulesthataredifficulttochangearelikelytobeinvolatileGooddesign:VolatileCode

7、sgotoinstablemodulesandinvolatilecodesintostablemodules.ModuleXIsModuleXStableorInstable?ManyothermodulesdependonitIfitschanges,othersmayneedtobechangedThemoreincomingdependenciesamodulehas,theharderthatmoduleistochange,thusitsmorestable.ModuleYThemoduleise

8、asytochange:NoothermoduledependsonitSoitsinstableYhassomanyreasonstochange,becauseitdependsonmanyothermodules.GooddesignVolatilecodesintomoduleslikeYInvolatilecodesintomoduleslikeXModuleslikeXshouldnev