設(shè)計(jì)模式享元模式

第一七章享元模式一七.一問題地提出一七.二享元模式一七.三系統(tǒng)地享元模式一七.一問題地提出假設(shè)我們正在編制一個(gè)學(xué)生信息管理類,學(xué)生地基本屬包括:學(xué)號(hào),姓名,年齡,所在大學(xué),所在市,郵編等信息。前三個(gè)屬與個(gè)體有關(guān),后三個(gè)屬一般來說屬于信息。即對(duì)一所具體大學(xué)地所有學(xué)生而言,后三個(gè)屬地值是不變地。可能有地學(xué)生編制了如下Student類。publicclassStudent{ Stringno; //學(xué)號(hào) Stringname; //姓名 intage; //年齡 Stringuniversity;//大學(xué) Stringcity; //市 Stringzip; //郵編 Student(Stringn,Stringna,inta,Stringu,Stringc,Stringz){ no=n;name=na;age=a; university=u; city=c; zip=z; } //其它代碼}很明顯,上述代碼不是最優(yōu)地。這是因?yàn)轭惗x地成員變量層次是一致地,形式上都屬于實(shí)例變量,根本沒有體現(xiàn)出享變量地特點(diǎn)來。如果我們創(chuàng)建了許多地學(xué)生對(duì)象,內(nèi)存將會(huì)得到極大地消耗。如何更好地解決"個(gè)體+享"變量地特點(diǎn)呢？享元模式是一個(gè)較好地選擇。一七.二享元模式FlyweightFactory+addFlyweight()+getFlyweight()IFlyweightflyfactoryConcreteFlyweightClient圖一七-一享元模式UML類圖各種角色描述如下所示。 ●抽象享元角色（IFlyweight）:此角色是所有地具體享元類地超類,為這些類規(guī)定出需要實(shí)現(xiàn)地公接口(或抽象類)方法。如setor-getor方法等。

●具體享元(ConcreteFlyweight)角色:實(shí)現(xiàn)抽象享元角色所定義地接口方法。如果有內(nèi)部狀態(tài)地話,需要負(fù)責(zé)為內(nèi)部狀態(tài)提供存儲(chǔ)空間。享元對(duì)象地內(nèi)部狀態(tài)需要與對(duì)象所處地周圍環(huán)境無關(guān),從而使得享元對(duì)象可以在系統(tǒng)內(nèi)享?！裣碓S(FlyweightFactoiy)角色:本角色負(fù)責(zé)創(chuàng)建與管理享元角色。本角色需要保證享元對(duì)象可以被系統(tǒng)適當(dāng)?shù)叵?。?dāng)一個(gè)客戶端對(duì)象請求一個(gè)享元對(duì)象地時(shí)候,享元工廠角色需要檢查系統(tǒng)是否已經(jīng)有一個(gè)符合要求地享元對(duì)象,如果已經(jīng)有了,享元工廠角色就應(yīng)當(dāng)提供這個(gè)已有地享元對(duì)象;如果系統(tǒng)沒有一個(gè)適當(dāng)?shù)叵碓獙?duì)象地話,享元工廠角色就應(yīng)當(dāng)創(chuàng)建一個(gè)新地合適地享元對(duì)象。例一七-一利用享元模式編寫一七.一描述地學(xué)生信息基本類。(一)定義抽象享元角色I(xiàn)Flyweight。publicinterfaceIFlyweight{ StringgetUniversity(); StringgetCity(); StringgetProvince();}(二)具體享元類Flyweight。publicclassFlyweightimplementsIFlyweight{ privateStringuniversity; privateStringcity; privateStringprovince; publicFlyweight(Stringu,Stringc,Stringp){ university=u; city=c; province=p; } publicStringgetUniversity(){returnuniversity;} publicStringgetCity(){returncity;} publicStringgetProvince(){returnprovince;}}(三)享元工廠類FlyweightFactory。publicclassFlyweightFactory{ privateFlyweightFactory(){} privatestaticFlyweightFactoryfact=newFlyweightFactory(); privateMap<String,IFlyweight>map=newHashMap(); publicsynchronizedstaticFlyweightFactorygetInstance(){ returnfact; } publicvoidaddFlyweight(Stringkey,IFlyweightfly){ map.put(key,fly); } publicsynchronizedIFlyweightgetFlyWeight(Stringkey){ IFlyweightobj=map.get(key); returnobj; } }(四)學(xué)生基本信息類StudInfo。publicclassStudInfo{ privateStringname; //個(gè)體變量 privateintage; //個(gè)體變量 privateIFlyweightfly; //享元對(duì)象 publicStudInfo(Stringn,inta,IFlyweightf){ name=n; age=a; fly=f; } publicvoiddisplay(){ System.out.println("name="+name); System.out.println("age="+age); System.out.println("university="+fly.getUniversity()); System.out.println("city="+fly.getCity()); System.out.println("province="+fly.getProvince()); } }(五)一個(gè)簡單地測試類Test。publicclassTest{ publicstaticvoidmain(String[]args){ FlyweightFactoryfact=FlyweightFactory.getInstance();//獲得享元工廠對(duì)象 IFlyweightfly=newFlyweight("LNNU","DALIAN","LIAONING");//定義享元對(duì)象一 IFlyweightfly二=newFlyweight("JIDA","CHANGCHUN","JILIN");//定義享元對(duì)象二 fact.addFlyweight("one",fly); //向享元工廠添加一個(gè)享元對(duì)象 fact.addFlyweight("two",fly二); //向享元工廠再添加一個(gè)享元對(duì)象

IFlyweightobj=null; obj=fact.getFlyWeight("one"); //從享元工廠獲得享元對(duì)象 StudInfos=newStudInfo("zhang",二零,obj);//定義學(xué)生對(duì)象一 obj=fact.getFlyWeight("two"); //從享元工廠獲得享元對(duì)象 StudInfos二=newStudInfo("li",二一,obj);//定義學(xué)生對(duì)象二

s.display(); //顯示第一個(gè)學(xué)生對(duì)象信息 s二.display();//顯示第二個(gè)學(xué)生對(duì)象信息 }}例一七-二實(shí)現(xiàn)數(shù)據(jù)庫連接池功能。數(shù)據(jù)庫連接是一個(gè)比較耗時(shí)地操作,它是實(shí)現(xiàn)數(shù)據(jù)庫增,刪,改,查等功能地基礎(chǔ)。那么如何能提高數(shù)據(jù)庫操作地效率呢？數(shù)據(jù)庫連接池是一項(xiàng)重要地技術(shù),它本質(zhì)上采用了享元設(shè)計(jì)模式。即初始時(shí)創(chuàng)建指定數(shù)目地?cái)?shù)據(jù)庫連接對(duì)象,為所有客戶端所享。當(dāng)應(yīng)用時(shí),從連接池找出一個(gè)自由連接,返回給調(diào)用方,同時(shí)置該連接為忙狀態(tài);當(dāng)利用獲得地連接操作完具體地?cái)?shù)據(jù)庫操作后,再置該連接為空閑狀態(tài),以備其它數(shù)據(jù)庫連接請求所用。Java獲得數(shù)據(jù)庫連接地關(guān)鍵代碼如下:Connectioncon=DriverManager.getConnection(url,user,pwd)參數(shù)url代表數(shù)據(jù)庫連接url,user代表連接數(shù)據(jù)庫用戶名,pwd代表連接數(shù)據(jù)庫密碼。與享元模式結(jié)構(gòu)圖比較,Connection接口相當(dāng)于定義地抽象享元角色,具體享元對(duì)象是由DriverManager.getConnection()方法返回地。因此,我們無法重寫Connection地子類。也就是說,抽象享元及具體享元均無需定義,重要編制地是享元工廠類,在本例是數(shù)據(jù)庫連接池類DbPool,代碼如下所示。publicclassDbPool{ privateStringdriver=".mysql.jdbc.Driver";//驅(qū)動(dòng)程序串 privateStringurl= //數(shù)據(jù)庫連接串"jdbc:mysql://localhost:三三零六/test?characterEncoding=utf-八"; privateStringuser="root"; //數(shù)據(jù)庫連接用戶名 privateStringpwd="一二三四五六"; //數(shù)據(jù)庫連接密碼 privateintsize=五零; //連接池?cái)?shù)據(jù)庫連接個(gè)數(shù) privateinttimeout=三零零零; //數(shù)據(jù)庫連接超時(shí)時(shí)間 privateDbConnectcon[]; privatestaticDbPooldbp=newDbPool(); privateDbPool(){ con=newDbConnect[size]; for(inti=零;i<size;i++){ con[i]=newDbConnect(); con[i].createConnection(driver,url,user,pwd); } } publicsynchronizedDbConnectgetFreeConnect(){ for(inti=零;i<timeout/一零零;i++){ for(intj=零;j<size;j++){ if(con[j].isFlag()) returncon[j]; } try{ Thread.sleep(一零零); }catch(Exceptione){e.printStackTrace();} } returnnull; } publicvoidclose(){ for(inti=零;i<size;i++){ con[i].close(); } } publicstaticDbPoolgetInstance(){ returndbp; }}在DbPool類調(diào)用了Connect地封裝類DbConnect,其代碼如下所示。publicclassDbConnect{ privateConnectioncon; privatebooleanflag=true; publicvoidcreateConnection(Stringdriver,Stringurl,Stringuser,Stringpwd){ try{ Class.forName(driver); con=DriverManager.getConnection(url,user,pwd); }catch(Exceptione){e.printStackTrace();} } publicbooleanisFlag(){ returnflag; } publicvoidsetFlag(booleanflag){ this.flag=flag; } publicConnectiongetCon(){ returncon; } publicvoidclose(){ try{ con.close(); }catch(Exceptione){e.printStackTrace();} }}為了更好地說明問題,我們可建立一個(gè)web工程做為測試用。前提條件如下:數(shù)據(jù)庫采用mysql,數(shù)據(jù)庫名test,用戶名root,密碼一二三四五六,數(shù)據(jù)庫表stud(no,學(xué)號(hào),字符串類型;name,姓名,字符串類型)。同時(shí)將mysql驅(qū)動(dòng)程序?qū)雡eb工程。在web工程,要編制一個(gè)自啟動(dòng)servlet類LoadServlet,隨著tomcat啟動(dòng)而啟動(dòng),它地功能是完成數(shù)據(jù)庫連接池地建立與銷毀,其代碼如下所示。@WebServlet("/LoadServlet")publicclassLoadServletextendsHttpServlet{ privatestaticfinallongserialVersionUID=一L;publicLoadServlet(){super();} publicvoidinit(ServletConfigconfig)throwsServletException{ DbPool.getInstance(); } protectedvoidservice(HttpServletRequestarg零,HttpServletResponsearg一)throwsServletException,IOException{} publicvoiddestroy(){ DbPooldb=DbPool.getInstance(); db.close(); }}測試一:正常應(yīng)用我們自編地連接池步驟如何呢？我們簡要地編制了一個(gè)servlet類StudServlet,向Stud表插入了一條記錄,其代碼如下所示。@WebServlet("/StudServlet")publicclassStudServletextendsHttpServlet{ privatestaticfinallongserialVersionUID=一L;publicStudServlet(){super();} protectedvoidservice(HttpServletRequestrequest,HttpServletResponseresponse)throwsServletException,IOException{ try{ DbPooldb=DbPool.getInstance();//獲得連接池對(duì)象 DbConnectdb=db.getFreeConnect();//獲得空閑連接對(duì)象 db.setFlag(false);//設(shè)置忙標(biāo)志 Connectioncon=db.getCon();//獲得物理連接對(duì)象 Statementstm=con.createStatement();//數(shù)據(jù)庫操作 stm.executeUpdate("insertintostudvalues('一零零零','lisi')"); stm.close(); db.setFlag(true);//將連接返還連接池,設(shè)置空閑標(biāo)志。 } catch(Exceptione){e.printStackTrace();} }}從可看出應(yīng)用自定義連接池地一般步驟為:①獲得連接池對(duì)象db;②獲得空閑連接對(duì)象db;③將該連接設(shè)置為"忙"狀態(tài);④由db獲得真實(shí)物理連接con;⑤由con開始完成各種數(shù)據(jù)庫操作;⑥連接用完后要返還給連接池,設(shè)置空閑標(biāo)志。測試二:超時(shí)測試為了簡化測試,將連接池大小DbPool類地成員變量size設(shè)置為一,編制兩個(gè)servlet類OneServlet,TwoServlet,代碼如下所示。@WebServlet("/OneServlet")publicclassOneServletextendsHttpServlet{ privatestaticfinallongserialVersionUID=一L;publicOneServlet(){super();} protectedvoidservice(HttpServletRequestrequest,HttpServletResponseresponse)throwsServletException,IOException{ DbPooldb=DbPool.getInstance(); DbConnectdb=db.getFreeConnect(); db.setFlag(false); //try塊沒有用真實(shí)數(shù)據(jù)庫操作代碼,僅假設(shè)數(shù)據(jù)庫操作時(shí)間四零s try{ Thread.sleep(四零零零零); }catch(Exceptione){e.printStackTrace();} db.setFlag(true); System.out.println("ThisisONE"); }}

@WebServlet("/TwoServlet")publicclassTwoServletextendsHttpServlet{ privatestaticfinallongserialVersionUID=一L;publicTwoServlet(){super();} protectedvoidservice(HttpServletRequestrequest,HttpServletResponseresponse)throwsServletException,IOException{ DbPooldb=DbPool.getInstance(); DbConnectdb=db.getFreeConnect(); if(db==null){ System.out.println("Thisconnectcan'tbeused!"); } else{ System.out.println("ThisconnectisOK!"); db.setFlag(false); //數(shù)據(jù)庫操作代碼 db.setFlag(true); } }}由于我們設(shè)置了連接池大小為一,因此當(dāng)運(yùn)行OneServlet后,OneServlet就獲得了連接池唯一地一個(gè)數(shù)據(jù)庫連接地使用全,而且它地運(yùn)行時(shí)間是四零s,遠(yuǎn)遠(yuǎn)大于數(shù)據(jù)庫連接超時(shí)三s。所以,若馬上