有興趣可看此處原文及相關(guān)討論: 總結(jié)一下最近關(guān)于domain object以及相關(guān)的討論
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在最近的圍繞domain object的討論中浮現(xiàn)出來了三種模型,(還有一些其他的旁枝,不一一分析了),經(jīng)過一番討論,各種問題逐漸清晰起來,在這里我試圖做一個總結(jié),便于大家了解和掌握。
第一種模型:只有g(shù)etter/setter方法的純數(shù)據(jù)類,所有的業(yè)務(wù)邏輯完全由business object來完成(又稱TransactionScript),這種模型下的domain object被Martin Fowler稱之為“貧血的domain object”。下面用舉一個具體的代碼來說明,代碼來自Hibernate的caveatemptor,但經(jīng)過我的改寫:
一個實體類叫做Item,指的是一個拍賣項目
一個DAO接口類叫做ItemDao
一個DAO接口實現(xiàn)類叫做ItemDaoHibernateImpl
一個業(yè)務(wù)邏輯類叫做ItemManager(或者叫做ItemService)
- public ? class ?Item? implements ?Serializable?{??
- ???? private ?Long?id?=? null ;??
- ???? private ? int ?version;??
- ???? private ?String?name;??
- ???? private ?User?seller;??
- ???? private ?String?description;??
- ???? private ?MonetaryAmount?initialPrice;??
- ???? private ?MonetaryAmount?reservePrice;??
- ???? private ?Date?startDate;??
- ???? private ?Date?endDate;??
- ???? private ?Set?categorizedItems?=? new ?HashSet();??
- ???? private ?Collection?bids?=? new ?ArrayList();??
- ???? private ?Bid?successfulBid;??
- ???? private ?ItemState?state;??
- ???? private ?User?approvedBy;??
- ???? private ?Date?approvalDatetime;??
- ???? private ?Date?created?=? new ?Date();??
- ???? //??getter/setter方法省略不寫,避免篇幅太長 ??
- }??
public class Item implements Serializable { private Long id = null; private int version; private String name; private User seller; private String description; private MonetaryAmount initialPrice; private MonetaryAmount reservePrice; private Date startDate; private Date endDate; private Set categorizedItems = new HashSet(); private Collection bids = new ArrayList(); private Bid successfulBid; private ItemState state; private User approvedBy; private Date approvalDatetime; private Date created = new Date(); // getter/setter方法省略不寫,避免篇幅太長 }
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- public ? interface ?ItemDao?{??
- ???? public ?Item?getItemById(Long?id);??
- ???? public ?Collection?findAll();??
- ???? public ? void ?updateItem(Item?item);??
- }??
public interface ItemDao { public Item getItemById(Long id); public Collection findAll(); public void updateItem(Item item); }
ItemDao定義持久化操作的接口,用于隔離持久化代碼。
- public ? class ?ItemDaoHibernateImpl? implements ?ItemDao? extends ?HibernateDaoSupport?{??
- ???? public ?Item?getItemById(Long?id)?{??
- ???????? return ?(Item)?getHibernateTemplate().load(Item. class ,?id);??
- ????}??
- ???? public ?Collection?findAll()?{??
- ???????? return ?(List)?getHibernateTemplate().find( "from?Item" );??
- ????}??
- ???? public ? void ?updateItem(Item?item)?{??
- ????????getHibernateTemplate().update(item);??
- ????}??
- }??
public class ItemDaoHibernateImpl implements ItemDao extends HibernateDaoSupport { public Item getItemById(Long id) { return (Item) getHibernateTemplate().load(Item.class, id); } public Collection findAll() { return (List) getHibernateTemplate().find("from Item"); } public void updateItem(Item item) { getHibernateTemplate().update(item); } }
ItemDaoHibernateImpl完成具體的持久化工作,請注意,數(shù)據(jù)庫資源的獲取和釋放是在ItemDaoHibernateImpl 里面處理的,每個DAO方法調(diào)用之前打開Session,DAO方法調(diào)用之后,關(guān)閉Session。(Session放在ThreadLocal中,保證 一次調(diào)用只打開關(guān)閉一次)
- public ? class ?ItemManager?{??
- ???? private ?ItemDao?itemDao;??
- ???? public ? void ?setItemDao(ItemDao?itemDao)?{? this .itemDao?=?itemDao;}??
- ???? public ?Bid?loadItemById(Long?id)?{???
- ????????itemDao.loadItemById(id);??
- ????}??
- ???? public ?Collection?listAllItems()?{??
- ???????? return ??itemDao.findAll();??
- ????}??
- ???? public ?Bid?placeBid(Item?item,?User?bidder,?MonetaryAmount?bidAmount,??
- ????????????????????????Bid?currentMaxBid,?Bid?currentMinBid)? throws ?BusinessException?{??
- ???????????? if ?(currentMaxBid?!=? null ?&&?currentMaxBid.getAmount().compareTo(bidAmount)?>? 0 )?{??
- ???????? throw ? new ?BusinessException( "Bid?too?low." );??
- ????}??
- ??????
- ???? //?Auction?is?active ??
- ???? if ?(?!state.equals(ItemState.ACTIVE)?)??
- ???????? throw ? new ?BusinessException( "Auction?is?not?active?yet." );??
- ??????
- ???? //?Auction?still?valid ??
- ???? if ?(?item.getEndDate().before(? new ?Date()?)?)??
- ???????? throw ? new ?BusinessException( "Can't?place?new?bid,?auction?already?ended." );??
- ??????
- ???? //?Create?new?Bid ??
- ????Bid?newBid?=? new ?Bid(bidAmount,?item,?bidder);??
- ??????
- ???? //?Place?bid?for?this?Item ??
- ????item.getBids().add(newBid);??
- ????itemDao.update(item);????? //??調(diào)用DAO完成持久化操作 ??
- ???? return ?newBid;??
- ????}??
- }??
public class ItemManager { private ItemDao itemDao; public void setItemDao(ItemDao itemDao) { this.itemDao = itemDao;} public Bid loadItemById(Long id) { itemDao.loadItemById(id); } public Collection listAllItems() { return itemDao.findAll(); } public Bid placeBid(Item item, User bidder, MonetaryAmount bidAmount, Bid currentMaxBid, Bid currentMinBid) throws BusinessException { if (currentMaxBid != null && currentMaxBid.getAmount().compareTo(bidAmount) > 0) { throw new BusinessException("Bid too low."); } // Auction is active if ( !state.equals(ItemState.ACTIVE) ) throw new BusinessException("Auction is not active yet."); // Auction still valid if ( item.getEndDate().before( new Date() ) ) throw new BusinessException("Can't place new bid, auction already ended."); // Create new Bid Bid newBid = new Bid(bidAmount, item, bidder); // Place bid for this Item item.getBids().add(newBid); itemDao.update(item); // 調(diào)用DAO完成持久化操作 return newBid; } }
事務(wù)的管理是在ItemManger這一層完成的,ItemManager實現(xiàn)具體的業(yè)務(wù)邏輯。除了常見的和CRUD有關(guān)的簡單邏輯之外,這里還有一個placeBid的邏輯,即項目的競標。
以上是一個完整的第一種模型的示例代碼。在這個示例中,placeBid,loadItemById,findAll等等業(yè)務(wù)邏輯統(tǒng)統(tǒng)放在ItemManager中實現(xiàn),而Item只有g(shù)etter/setter方法。
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第二種模型,也就是Martin Fowler指的rich domain object是下面這樣子的:
一個帶有業(yè)務(wù)邏輯的實體類,即domain object是Item
一個DAO接口ItemDao
一個DAO實現(xiàn)ItemDaoHibernateImpl
一個業(yè)務(wù)邏輯對象ItemManager
- public ? class ?Item? implements ?Serializable?{??
- ???? //??所有的屬性和getter/setter方法同上,省略 ??
- ???? public ?Bid?placeBid(User?bidder,?MonetaryAmount?bidAmount,??
- ????????????????????????Bid?currentMaxBid,?Bid?currentMinBid)??
- ???????? throws ?BusinessException?{??
- ??????
- ???????? //?Check?highest?bid?(can?also?be?a?different?Strategy?(pattern)) ??
- ???????? if ?(currentMaxBid?!=? null ?&&?currentMaxBid.getAmount().compareTo(bidAmount)?>? 0 )?{??
- ???????????? throw ? new ?BusinessException( "Bid?too?low." );??
- ????????}??
- ??????
- ???????? //?Auction?is?active ??
- ???????? if ?(?!state.equals(ItemState.ACTIVE)?)??
- ???????????? throw ? new ?BusinessException( "Auction?is?not?active?yet." );??
- ??????
- ???????? //?Auction?still?valid ??
- ???????? if ?(? this .getEndDate().before(? new ?Date()?)?)??
- ???????????? throw ? new ?BusinessException( "Can't?place?new?bid,?auction?already?ended." );??
- ??????
- ???????? //?Create?new?Bid ??
- ????????Bid?newBid?=? new ?Bid(bidAmount,? this ,?bidder);??
- ??????
- ???????? //?Place?bid?for?this?Item ??
- ???????? this .getBids.add(newBid);?? //?請注意這一句,透明的進行了持久化,但是不能在這里調(diào)用ItemDao,Item不能對ItemDao產(chǎn)生依賴! ??
- ??????
- ???????? return ?newBid;??
- ????}??
- }??
public class Item implements Serializable { // 所有的屬性和getter/setter方法同上,省略 public Bid placeBid(User bidder, MonetaryAmount bidAmount, Bid currentMaxBid, Bid currentMinBid) throws BusinessException { // Check highest bid (can also be a different Strategy (pattern)) if (currentMaxBid != null && currentMaxBid.getAmount().compareTo(bidAmount) > 0) { throw new BusinessException("Bid too low."); } // Auction is active if ( !state.equals(ItemState.ACTIVE) ) throw new BusinessException("Auction is not active yet."); // Auction still valid if ( this.getEndDate().before( new Date() ) ) throw new BusinessException("Can't place new bid, auction already ended."); // Create new Bid Bid newBid = new Bid(bidAmount, this, bidder); // Place bid for this Item this.getBids.add(newBid); // 請注意這一句,透明的進行了持久化,但是不能在這里調(diào)用ItemDao,Item不能對ItemDao產(chǎn)生依賴! return newBid; } }
競標這個業(yè)務(wù)邏輯被放入到Item中來。請注意this.getBids.add(newBid);? 如果沒有Hibernate或者JDO這種O/R Mapping的支持,我們是無法實現(xiàn)這種透明的持久化行為的。但是請注意,Item里面不能去調(diào)用ItemDAO,對ItemDAO產(chǎn)生依賴!
ItemDao和ItemDaoHibernateImpl的代碼同上,省略。
- public ? class ?ItemManager?{???
- ???? private ?ItemDao?itemDao;???
- ???? public ? void ?setItemDao(ItemDao?itemDao)?{? this .itemDao?=?itemDao;}???
- ???? public ?Bid?loadItemById(Long?id)?{???
- ????????itemDao.loadItemById(id);???
- ????}???
- ???? public ?Collection?listAllItems()?{???
- ???????? return ??itemDao.findAll();???
- ????}???
- ???? public ?Bid?placeBid(Item?item,?User?bidder,?MonetaryAmount?bidAmount,???
- ????????????????????????????Bid?currentMaxBid,?Bid?currentMinBid)? throws ?BusinessException?{???
- ????????item.placeBid(bidder,?bidAmount,?currentMaxBid,?currentMinBid);??
- ????????itemDao.update(item);???? //?必須顯式的調(diào)用DAO,保持持久化 ??
- ????}??
- }??
public class ItemManager { private ItemDao itemDao; public void setItemDao(ItemDao itemDao) { this.itemDao = itemDao;} public Bid loadItemById(Long id) { itemDao.loadItemById(id); } public Collection listAllItems() { return itemDao.findAll(); } public Bid placeBid(Item item, User bidder, MonetaryAmount bidAmount, Bid currentMaxBid, Bid currentMinBid) throws BusinessException { item.placeBid(bidder, bidAmount, currentMaxBid, currentMinBid); itemDao.update(item); // 必須顯式的調(diào)用DAO,保持持久化 } }
在第二種模型中,placeBid業(yè)務(wù)邏輯是放在Item中實現(xiàn)的,而loadItemById和findAll業(yè)務(wù)邏輯是放在 ItemManager中實現(xiàn)的。不過值得注意的是,即使placeBid業(yè)務(wù)邏輯放在Item中,你仍然需要在ItemManager中簡單的封裝一 層,以保證對placeBid業(yè)務(wù)邏輯進行事務(wù)的管理和持久化的觸發(fā)。
這種模型是Martin Fowler所指的真正的domain model。在這種模型中,有三個業(yè)務(wù)邏輯方法:placeBid,loadItemById和findAll,現(xiàn)在的問題是哪個邏輯應(yīng)該放在Item 中,哪個邏輯應(yīng)該放在ItemManager中。在我們這個例子中,placeBid放在Item中(但是ItemManager也需要對它進行簡單的封 裝),loadItemById和findAll是放在ItemManager中的。
切分的原則是什么呢? Rod Johnson提出原則是“case by case”,可重用度高的,和domain object狀態(tài)密切關(guān)聯(lián)的放在Item中,可重用度低的,和domain object狀態(tài)沒有密切關(guān)聯(lián)的放在ItemManager中。
我提出的原則是:看業(yè)務(wù)方法是否顯式的依賴持久化。
Item的placeBid這個業(yè)務(wù)邏輯方法沒有顯式的對持久化ItemDao接口產(chǎn)生依賴,所以要放在Item中。
請注意,如果脫離了Hibernate這個持久化框架,Item這個domain object是可以進行單元測試的,他不依賴于Hibernate的持久化機制。它是一個獨立的,可移植的,完整的,自包含的域?qū)ο?
。
而loadItemById和findAll這兩個業(yè)務(wù)邏輯方法是必須顯式的對持久化ItemDao接口產(chǎn)生依賴,否則這個業(yè)務(wù)邏輯就無法完成。 如果你要把這兩個方法放在Item中,那么Item就無法脫離Hibernate框架,無法在Hibernate框架之外獨立存在。
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第三種模型印象中好像是firebody或者是Archie提出的(也有可能不是,記不清楚了),簡單的來說,這種模型就是把第二種模型的domain object和business object合二為一了。所以ItemManager就不需要了,在這種模型下面,只有三個類,他們分別是:
Item:包含了實體類信息,也包含了所有的業(yè)務(wù)邏輯
ItemDao:持久化DAO接口類
ItemDaoHibernateImpl:DAO接口的實現(xiàn)類
由于ItemDao和ItemDaoHibernateImpl和上面完全相同,就省略了。
- public ? class ?Item? implements ?Serializable?{??
- ???? //??所有的屬性和getter/setter方法都省略 ??
- ??? private ? static ?ItemDao?itemDao;??
- ???? public ? void ?setItemDao(ItemDao?itemDao)?{ this .itemDao?=?itemDao;}??
- ??????
- ???? public ? static ?Item?loadItemById(Long?id)?{??
- ???????? return ?(Item)?itemDao.loadItemById(id);??
- ????}??
- ???? public ? static ?Collection?findAll()?{??
- ???????? return ?(List)?itemDao.findAll();??
- ????}??
- ??
- ???? public ?Bid?placeBid(User?bidder,?MonetaryAmount?bidAmount,??
- ????????????????????Bid?currentMaxBid,?Bid?currentMinBid)??
- ???? throws ?BusinessException?{??
- ??????
- ???????? //?Check?highest?bid?(can?also?be?a?different?Strategy?(pattern)) ??
- ???????? if ?(currentMaxBid?!=? null ?&&?currentMaxBid.getAmount().compareTo(bidAmount)?>? 0 )?{??
- ???????????? throw ? new ?BusinessException( "Bid?too?low." );??
- ????????}??
- ??????????
- ???????? //?Auction?is?active ??
- ???????? if ?(?!state.equals(ItemState.ACTIVE)?)??
- ???????????? throw ? new ?BusinessException( "Auction?is?not?active?yet." );??
- ??????????
- ???????? //?Auction?still?valid ??
- ???????? if ?(? this .getEndDate().before(? new ?Date()?)?)??
- ???????????? throw ? new ?BusinessException( "Can't?place?new?bid,?auction?already?ended." );??
- ??????????
- ???????? //?Create?new?Bid ??
- ????????Bid?newBid?=? new ?Bid(bidAmount,? this ,?bidder);??
- ??????????
- ???????? //?Place?bid?for?this?Item ??
- ???????? this .addBid(newBid);??
- ????????itemDao.update( this );?????? //??調(diào)用DAO進行顯式持久化 ??
- ???????? return ?newBid;??
- ????}??
- }??
public class Item implements Serializable { // 所有的屬性和getter/setter方法都省略 private static ItemDao itemDao; public void setItemDao(ItemDao itemDao) {this.itemDao = itemDao;} public static Item loadItemById(Long id) { return (Item) itemDao.loadItemById(id); } public static Collection findAll() { return (List) itemDao.findAll(); } public Bid placeBid(User bidder, MonetaryAmount bidAmount, Bid currentMaxBid, Bid currentMinBid) throws BusinessException { // Check highest bid (can also be a different Strategy (pattern)) if (currentMaxBid != null && currentMaxBid.getAmount().compareTo(bidAmount) > 0) { throw new BusinessException("Bid too low."); } // Auction is active if ( !state.equals(ItemState.ACTIVE) ) throw new BusinessException("Auction is not active yet."); // Auction still valid if ( this.getEndDate().before( new Date() ) ) throw new BusinessException("Can't place new bid, auction already ended."); // Create new Bid Bid newBid = new Bid(bidAmount, this, bidder); // Place bid for this Item this.addBid(newBid); itemDao.update(this); // 調(diào)用DAO進行顯式持久化 return newBid; } }
在這種模型中,所有的業(yè)務(wù)邏輯全部都在Item中,事務(wù)管理也在Item中實現(xiàn)。
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轉(zhuǎn)自:http://www.iteye.com/topic/11712
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