enum的职责链-枚举类型

这个模式还是挺不错的

以前没有听说过，在职责链设计模式总，会用多种不同的方式来解决同一个问题，然后将他们链接在一起。当一个请求到来时，它遍历这个链直到链中的某个解决方案能够处理该请求。其中每次尝试都可以看做为一个策略，而完整的处理方式列表就是一个职责链。

看一个例子

import java.util.Iterator;class Mail {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++;//重写toString方法public String toString() {return "Mail " + id;}//显示详细详细public String details() {return toString() +", GeneralDelivery: " + generalDelivery +", Scannability: " + scannability +", Readability: " + readability +", Address: " + address +", ReturnAddress: " + returnAddress ;}//创建一个随机的Mail对象，内容也随机public static Mail randomMail() {Mail mail = new Mail();//调用一个Enums中的随机选取工具mail.generalDelivery = Enums.random(GeneralDelivery.class);mail.scannability = Enums.random(Scannability.class);mail.readability = Enums.random(Readability.class);mail.address = Enums.random(Address.class);mail.returnAddress = Enums.random(ReturnAddress.class);return mail;}/** * 随机产生制度能够数量的信 * @param count 数量 * @return 以迭代器返回，可以使用for-each遍历 */public static Iterable<Mail> generator(final int count) {return new Iterable<Mail>() {int n = count;@Overridepublic Iterator<Mail> iterator() {return new Iterator<Mail>() {@Overridepublic boolean hasNext() {return n-- > 0;}@Overridepublic Mail next() {return randomMail();}@Overridepublic void remove() {throw new UnsupportedOperationException();}};}};}}public class PostOffice {/* * 定义了四种处理邮件的方式 * 每一种都会返回对应的值true或false * 如果返回false表示该种方式不可行 */enum MailHandler {GENERAL_DELIVERY {@Overrideboolean handle(Mail m) {switch (m.generalDelivery) {case YES:System.out.println("Using general delivery for " + m);return true;default:break;}return false;}},MACHINE_SCAN {@Overrideboolean handle(Mail m) {switch (m.scannability) {case UNSCANNABLE:return false;default:switch (m.address) {case INCORRECT:return false;default:System.out.println("Delivering " + m + " atuomatically");return true;}}}},VISUAL_INSPECTION {@Overrideboolean 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_SENOER {@Overrideboolean handle(Mail m) {switch (m.returnAddress) {case MISSING:return false;default:System.out.println("Returing " + m + " to sender");return true;}}};abstract boolean handle(Mail m);}/** * 按照MailHandler中的几种方法依次处理该邮件， * 如果有一种可以处理则返回成功 * 如果所有的方法都不能处理，判定该邮件为死信 * @param m 邮件 */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) {//随机生成10封信for(Mail mail : Mail.generator(10)) {System.out.println(mail.details());handle(mail);System.out.println("===============================");}}}

输出结果

Mail 0, GeneralDelivery: NO2, Scannability: UNSCANNABLE, Readability: YES3, Address: OK1, ReturnAddress: OK1Delivering Mail 0 normally===============================Mail 1, GeneralDelivery: NO5, Scannability: YES3, Readability: ILLEGIBLE, Address: OK5, ReturnAddress: OK1Delivering Mail 1 atuomatically===============================Mail 2, GeneralDelivery: YES, Scannability: YES3, Readability: YES1, Address: OK1, ReturnAddress: OK5Using general delivery for Mail 2===============================Mail 3, GeneralDelivery: NO4, Scannability: YES3, Readability: YES1, Address: INCORRECT, ReturnAddress: OK4Returing Mail 3 to sender===============================Mail 4, GeneralDelivery: NO4, Scannability: UNSCANNABLE, Readability: YES1, Address: INCORRECT, ReturnAddress: OK2Returing Mail 4 to sender===============================Mail 5, GeneralDelivery: NO3, Scannability: YES1, Readability: ILLEGIBLE, Address: OK4, ReturnAddress: OK2Delivering Mail 5 atuomatically===============================Mail 6, GeneralDelivery: YES, Scannability: YES4, Readability: ILLEGIBLE, Address: OK4, ReturnAddress: OK4Using general delivery for Mail 6===============================Mail 7, GeneralDelivery: YES, Scannability: YES3, Readability: YES4, Address: OK2, ReturnAddress: MISSINGUsing general delivery for Mail 7===============================Mail 8, GeneralDelivery: NO3, Scannability: YES1, Readability: YES3, Address: INCORRECT, ReturnAddress: MISSINGMail 8 is a dead letter.===============================Mail 9, GeneralDelivery: NO1, Scannability: UNSCANNABLE, Readability: YES2, Address: OK1, ReturnAddress: OK4Delivering Mail 9 normally===============================

看起来较为复杂，其实很简单。

就是有很多种处理信的方式，来了一封信就用这些方式依次处理，如果所有的方式都不能处理那就是死信，一旦有一种可以处理就通过。

enum MailHandler 中每一种处理方式都可以看成是一种策略，如果想增加处理方式，直接在enum MailHandler 添加即可，特别方便