使用 enum 的職責鏈 · 《Java 編程思想》第五版

### 使用 enum 的職責鏈在職責鏈（Chain of Responsibility）設計模式中，程序員以多種不同的方式來解決一個問題，然后將它們鏈接在一起。當一個請求到來時，它遍歷這個鏈，直到鏈中的某個解決方案能夠處理該請求。通過常量相關的方法，我們可以很容易地實現一個簡單的職責鏈。我們以一個郵局的模型為例。郵局需要以盡可能通用的方式來處理每一封郵件，并且要不斷嘗試處理郵件，直到該郵件最終被確定為一封死信。其中的每一次嘗試可以看作為一個策略（也是一個設計模式），而完整的處理方式列表就是一個職責鏈。我們先來描述一下郵件。郵件的每個關鍵特征都可以用 enum 來表示。程序將隨機地生成 Mail 對象，如果要減小一封郵件的 GeneralDelivery 為 YES 的概率，那最簡單的方法就是多創建幾個不是 YES 的 enum 實例，所以 enum 的定義看起來有點古怪。我們看到 Mail 中有一個 randomMail() 方法，它負責隨機地創建用于測試的郵件。而 generator() 方法生成一個 Iterable 對象，該對象在你調用 next() 方法時，在其內部使用 randomMail() 來創建 Mail 對象。這樣的結構使程序員可以通過調用 Mail.generator() 方法，很容易地構造出一個 foreach 循環： ```java // enums/PostOffice.java // Modeling a post office import java.util.*; import onjava.*; class Mail { // The NO's reduce probability of random selection: enum GeneralDelivery {YES,NO1,NO2,NO3,NO4,NO5} enum Scannability {UNSCANNABLE,YES1,YES2,YES3,YES4} enum Readability {ILLEGIBLE,YES1,YES2,YES3,YES4} enum Address {INCORRECT,OK1,OK2,OK3,OK4,OK5,OK6} enum ReturnAddress {MISSING,OK1,OK2,OK3,OK4,OK5} GeneralDelivery generalDelivery; Scannability scannability; Readability readability; Address address; ReturnAddress returnAddress; static long counter = 0; long id = counter++; @Override public String toString() { return "Mail " + id; } public String details() { return toString() + ", General Delivery: " + generalDelivery + ", Address Scanability: " + scannability + ", Address Readability: " + readability + ", Address Address: " + address + ", Return address: " + returnAddress; } // Generate test Mail: public static Mail randomMail() { Mail m = new Mail(); m.generalDelivery = Enums.random(GeneralDelivery.class); m.scannability = Enums.random(Scannability.class); m.readability = Enums.random(Readability.class); m.address = Enums.random(Address.class); m.returnAddress = Enums.random(ReturnAddress.class); return m; } public static Iterable<Mail> generator(final int count) { return new Iterable<Mail>() { int n = count; @Override public Iterator<Mail> iterator() { return new Iterator<Mail>() { @Override public boolean hasNext() { return n-- > 0; } @Override public Mail next() { return randomMail(); } @Override public void remove() { // Not implemented throw new UnsupportedOperationException(); } }; } }; } } public class PostOffice { enum MailHandler { GENERAL_DELIVERY { @Override boolean handle(Mail m) { switch(m.generalDelivery) { case YES: System.out.println( "Using general delivery for " + m); return true; default: return false; } } }, MACHINE_SCAN { @Override boolean handle(Mail m) { switch(m.scannability) { case UNSCANNABLE: return false; default: switch(m.address) { case INCORRECT: return false; default: System.out.println( "Delivering "+ m + " automatically"); return true; } } } }, VISUAL_INSPECTION { @Override boolean handle(Mail m) { switch(m.readability) { case ILLEGIBLE: return false; default: switch(m.address) { case INCORRECT: return false; default: System.out.println( "Delivering " + m + " normally"); return true; } } } }, RETURN_TO_SENDER { @Override boolean handle(Mail m) { switch(m.returnAddress) { case MISSING: return false; default: System.out.println( "Returning " + m + " to sender"); return true; } } }; abstract boolean handle(Mail m); } static void handle(Mail m) { for(MailHandler handler : MailHandler.values()) if(handler.handle(m)) return; System.out.println(m + " is a dead letter"); } public static void main(String[] args) { for(Mail mail : Mail.generator(10)) { System.out.println(mail.details()); handle(mail); System.out.println("*****"); } } } ``` 輸出為： ``` Mail 0, General Delivery: NO2, Address Scanability: UNSCANNABLE, Address Readability: YES3, Address Address: OK1, Return address: OK1 Delivering Mail 0 normally ***** Mail 1, General Delivery: NO5, Address Scanability: YES3, Address Readability: ILLEGIBLE, Address Address: OK5, Return address: OK1 Delivering Mail 1 automatically ***** Mail 2, General Delivery: YES, Address Scanability: YES3, Address Readability: YES1, Address Address: OK1, Return address: OK5 Using general delivery for Mail 2 ***** Mail 3, General Delivery: NO4, Address Scanability: YES3, Address Readability: YES1, Address Address: INCORRECT, Return address: OK4 Returning Mail 3 to sender ***** Mail 4, General Delivery: NO4, Address Scanability: UNSCANNABLE, Address Readability: YES1, Address Address: INCORRECT, Return address: OK2 Returning Mail 4 to sender ***** Mail 5, General Delivery: NO3, Address Scanability: YES1, Address Readability: ILLEGIBLE, Address Address: OK4, Return address: OK2 Delivering Mail 5 automatically ***** Mail 6, General Delivery: YES, Address Scanability: YES4, Address Readability: ILLEGIBLE, Address Address: OK4, Return address: OK4 Using general delivery for Mail 6 ***** Mail 7, General Delivery: YES, Address Scanability: YES3, Address Readability: YES4, Address Address: OK2, Return address: MISSING Using general delivery for Mail 7 ***** Mail 8, General Delivery: NO3, Address Scanability: YES1, Address Readability: YES3, Address Address: INCORRECT, Return address: MISSING Mail 8 is a dead letter ***** Mail 9, General Delivery: NO1, Address Scanability: UNSCANNABLE, Address Readability: YES2, Address Address: OK1, Return address: OK4 Delivering Mail 9 normally ***** ``` 職責鏈由 enum MailHandler 實現，而 enum 定義的次序決定了各個解決策略在應用時的次序。對每一封郵件，都要按此順序嘗試每個解決策略，直到其中一個能夠成功地處理該郵件，如果所有的策略都失敗了，那么該郵件將被判定為一封死信。