OpenHelper是安卓打開數(shù)據(jù)庫的幫助類,我們通常需要繼承這個類并重寫里面onCreateonUpgrade方法,因為它是一個抽象類.onCreate方法只會被調(diào)用一次,在數(shù)據(jù)庫創(chuàng)建時被調(diào)用,通常需要在里面做數(shù)據(jù)庫的初始操作,比如說執(zhí)行建表語句等,畢竟一個空的數(shù)據(jù)庫是什么都做不了的.當(dāng)我們需要更新數(shù)據(jù)庫版本時,onUpgrade方法中進(jìn)行更新的操作我們可以通過OpenHelper的getReadableDatabase()和getWritableDatabase()方法來獲取數(shù)據(jù)庫Database對象,通過這個對象對數(shù)據(jù)庫進(jìn)行增刪查改操作.通過方法的名字,我們很容易去認(rèn)為getReadableDatabase()是獲取一個只讀的數(shù)據(jù)庫,但實Createand/oropena Thiswillbethesameobjectreturnedby{@linkunlesssomeproblem,suchasafulldisk,requiresthedatabasetobeopenedread-only.Inthatcase,aread-onlydatabaseobjectwillbeIftheproblemisfixed,afuturecallto{@link#getWritableDatabase}maysucceed,inwhichcasetheread-onlydatabaseobjectwillbeclosedandtheread/writeobjectwillbereturnedinthefuture.Like{@link#getWritableDatabase},thismethodmaytakealongtimetoreturn,soyoushouldnotcallitfromtheapplicationmainthread,includingfrom{@linkandroid.content.ContentProvider#onCreate Exceptionifthedatabasecannotbe@returnadatabaseobjectvaliduntil{@link#getWritableDatabase}or{@link#close}iscalled.用(getReadableDatabase()getWritableDatabase(),先調(diào)用的數(shù)據(jù)庫 DatabasegetReadableDatabase()synchronized(this)return}}Createand/oropenadatabasethatwillbeusedforreadingandThefirsttimethisiscalled,thedatabasewillbeopened{@link#onCreate},{@link#onUpgrade}and/or{@link#onOpen}willbeonCreateonUpgrade和/(如果打開成功,onOpen Onceopenedsuccessfully,thedatabaseiscached,soyoucancallthismethodeverytimeyouneedtowritetothedatabase.(Makesuretocall{@link#close}whenyounolongerneedtheErrorssuchasbadpermissionsorafulldiskmaycausethismethodtofail,butfutureattemptsmaysucceediftheproblemisfixed.Databaseupgrademaytakealongtime,youshouldnotcallthismethodfromtheapplicationmainthread,includingfrom{@linkandroid.content.ContentProvider#onCreateContentProvider.onCreate()}. Exceptionifthedatabasecannotbeopenedforwriting@returnaread/writedatabaseobjectvaliduntil{@link#close}is DatabasegetWritableDatabase()synchronized(this)return}}通過上面的注釋和代碼,我們可以知道getReadableDatabase()的數(shù)據(jù)庫時才會打開一個只讀的數(shù)據(jù)庫,這兩個方法內(nèi)部實現(xiàn)基本相同,都調(diào)用了這樣的一個方法:getDatabaseLocked(booleanwritable),他們是通過這個方法的參數(shù)來進(jìn)行區(qū)分的,getReadableDatabase()方法傳入的參數(shù)是false,而getWritableDatabase()方法傳入的參數(shù)是DatabasegetDatabaseLocked(booleanwritable)DatabasegetDatabaseLocked(booleanwritable)if(mDatabase!=null)//mDatabase如果不等于null,說明之前已經(jīng)創(chuàng)建過數(shù)據(jù)庫,也就是說 ,獲取的數(shù)據(jù)庫對象會被緩存if(!mDatabase.isOpen())//Darn!Theuserclosedthedatabasebycalling//open狀態(tài)的也就是說調(diào)用過close()方法mDatabase=null;}elseif(!writable||!mDatabase.isReadOnly())//Thedatabaseisalreadyopenfor//elseif說明數(shù)據(jù)庫可用,writablefalse,getReadableDatabase()方法那么直接返回該數(shù)據(jù)庫對象因為該數(shù)據(jù)庫對象既然可用,那么無論它是只讀還是可讀可寫,都可以讀,就可以在這里直接返回getWritableDatabase()方法那么只有該數(shù)據(jù)庫對象不是只讀的才可以返回,否則就要新創(chuàng)建一個.return}}if(mIsInitializing)thrownewIllegalStateException("getDatabasecalled}Databasedb= 1.db=trymIsInitializing=if(db!=null)if(writable&&db.isReadOnly()){}}elseif(mName==null)Databasecreate()方法創(chuàng)建一個數(shù)據(jù)庫CursorFactorynull,該db }elsetryif(DEBUG_STRICT_READONLY&&!writable)//如果是OpenHelperDEBUG_STRICT_READONLY并且調(diào)getReadableDatabase().,寫數(shù)據(jù)庫失敗true也就是如果之前創(chuàng)建的是一個只讀的數(shù)finalStringpath=db= Database.openDatabase(path,mFactory,}elsedb=mContext.openOrCreateDatabase(mName,mEnableWriteAheadLogging?Context.MODE_ENABLE_WRITE_AHEAD_LOGGING:0,mFactory,}}catch Exceptionex)if(writable)(getReadableDatabase()數(shù)據(jù)庫創(chuàng)建失敗那么應(yīng)該嘗試創(chuàng)建只讀的數(shù)據(jù)throw}//走到這里說明創(chuàng)建數(shù)據(jù)庫失敗而且調(diào)用的是getReadableBase(),Log.e(TAG,"Couldn'topen"+mName+"forwriting(willtryread-only):",finalStringpath=db Database.openDatabase(path,Database.OPEN_READONLY,}}//db的配置如果走到這里db創(chuàng)建成功那么就可以調(diào)用這個方法finalintversion=if(version!=mNewVersion)if(db.isReadOnly())thrownew Exception("Can'tupgraderead-onlydatabasefromversion"+db.getVersion()+"to"+mNewVersion+":+}//開啟事務(wù),這里表明了 OpenHelper的onCreate(),onUpgrade()方法是在事務(wù)中進(jìn)行的,也就是不必?fù)?dān)心這兩個方法中的代碼只有部分被執(zhí)行.tryif(version==0)}elseif(version>mNewVersion)onDowngrade(db,version,}else}}

onUpgrade(db,version,

}finally}}//onOpen方法因為數(shù)據(jù)庫剛創(chuàng)建open的所以可以在這里調(diào)用這個方if(db.