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Convert Static to Dynamic Construction -- 静态到动态构造转换

2013-03-17 23:31 288 查看

RefactoringcontributedbyGerardM.Davison

Youhaveclassesthathavestaticcompiletimedependenciesonclassesthatcanonlybebuiltonaspecificplatform.

你有一些平台独立的静态编译时期的类

Makeuseofthejava.lang.reflecttobreakthestaticdependency.

利用java反射打破静态依赖。

importorg.davison.data.jdbc.JDBCProvider;

.
.
.

DataProviderdp=newJDBCProvider();

try
{
DataProviderdp=(DataProvider)
Class.forName("org.davison.data.jdbc.JDBCProvider").newInstance();
}
catch(IllegalAccessExceptioniae)
{
//Convertexceptiontoerrortopresevetheinterface.
//

thrownewIllegalAccessError(iae.getMessage());
}
catch(InstantiationExceptionie)
{
//Convertexceptiontoerrortopresevetheinterface.
//

thrownewInstantiationError(ie.getMessage());
}
catch(ClassNotFoundExceptioncnfe)
{
//Convertexceptiontoerrortopresevetheinterface.
//

thrownewNoClassDefFoundError(cnfe.getMessage());
}

Motivation

Insomecasesyouhavetoprovidedriverstoprovidedifferentfunctionalitydependingonthesituation.Youmighthaveclassesthatonagivenplatformneedtoaccesssomesun.*classestoperformaspecificfunction.
ForexampleaclasstoaccessWinHelpneedstogetthewindowhandleviathesun.awt.windows.WWindowPeerclasses.
有时候你会根据情况提供不同功能的驱动程序。
Theremightalsobethedesiretoonlyprovidecertianfunctionalitytoagivensub-setofusers.Thisistheequivalentofbeingabletocompileoutcodewithouthavingtoalterthesourceorusepre-compilers.
你可能期望给予用户不同的功能子集，这就相当于不用修改源码或者预编译就能达到这种目的。
Thismethodcanbeusedwithgoodeffectwithapplicationframeworkswhereyoudonotknowwhichclassneedtobeusedatcompiletime.
在应用程序框架中使用反射是一种的好的方式，譬如在编译期你不知道到底需要选择哪个类时。

Mechanics

Identifytheplacewherethedifferentclassesareinstantiated.Ifthereismorethanoneplace,youmightliketomakeuseofcodeconsolidationrefactoringstocreateoneentrypoint.
Makesurethatthereisacommoninterfaceandmethodofconstruction.UtiliseExtractInterfaceoruseExtractSuperclasstocreateasuitableabstractiontoworkwith.
Replacetheclassselectionprocesswithonethatinsteadselectsforthestringnameoftheclassinquestion.
Removeanyimportstatementsthatreferethetheclassesbeingconstructed.
Addcodeintoloadandinstantiatetheclass.Formoreparameterisedcontructors,makeuseofthejava.lang.reflectpackage.
Dealwithanyexcpetiongenerated.
Youarenowreadytocompileandtestthecode.Makesurethatanydependantcodeisalsotestedproperly.
Therefactoringiscomplete,butyoushouldmakesurethatthisrun-timedependencyisproperlydocumentedandthatalldevelopersknowtoinstantiatetheclassesusingtheneworalteredmethod.

Example

Startwiththiscode:

importorg.davison.data.jdbc.JDBCProvider;

.
.
.

DataProviderdp=newJDBCProvider();

Firstwehavetochangetheselectioncodetoreturnaclassnameratherthananactualinstance.Wecansafelyremovetheimportstatementthatreferestothisclass.

Class.forName("org.davison.data.jdbc.JDBCProvider")

Wecannowinstantiatetheclass,inthissimplecasetheconstructorhasnoparameters,sowedonothavetomakeuseoftheextendedjava.lang.reflectpackage.

DataProviderdp=(DataProvider)
Class.forName("org.davison.data.jdbc.JDBCProvider").newInstance();

Wenowhavetoaddthecodetodealwiththepossibleexceptioncases.HereIhavechosentoconvertthemtotheequivalentJavaErrorsinordertomaintaintheinterface.Ifthisisnotrequiredthensimplercodeis
possible.

try
{
DataProviderdp=(DataProvider)
Class.forName("org.davison.data.jdbc.JDBCProvider").newInstance();
}
catch(IllegalAccessExceptioniae)
{
//Convertexceptiontoerrortopresevetheinterface.
//

thrownewIllegalAccessError(iae.getMessage());
}
catch(InstantiationExceptionie)
{
//Convertexceptiontoerrortopresevetheinterface.
//

thrownewInstantiationError(ie.getMessage());
}
catch(ClassNotFoundExceptioncnfe)
{
//Convertexceptiontoerrortopresevetheinterface.
//

thrownewNoClassDefFoundError(cnfe.getMessage());
}

Compileandtestatthispointaswehavecodethatiscomplete.
Whenthisisfinishedandalldependentclassesarere-tested,therefactoringiscomplete.

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标签： 重构 refactoring

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