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介绍
意图 ：避免请求发送者与接收者耦合在一起，让多个对象都有可能接收请求，将这些对象连接成一条链，并且沿着这条链传递请求，直到有对象处理它为止。
主要解决 ：职责链上的处理者负责处理请求，客户只需要将请求发送到职责链上即可，无须关心请求的处理细节和请求的传递，所以职责链将请求的发送者和请求的处理者解耦了。
何时使用 ：在处理消息的时候以过滤很多道。
如何解决 ：拦截的类都实现统一接口。
实现
实现一
创建抽象类 AbstractLogger，带有详细的日志记录级别。然后我们创建三种类型的记录器，都扩展了 AbstractLogger。每个记录器消息的级别是否属于自己的级别，如果是则相应地打印出来，否则将不打印并把消息传给下一个记录器。
public abstract class AbstractLogger { public static int INFO = 1; public static int DEBUG = 2; public static int ERROR = 3; protected int level; // 责任链中的下一个元素 protected AbstractLogger nextLogger; public void setNextLogger(AbstractLogger nextLogger) { this.nextLogger = nextLogger; } public void logMessage(int level, String message) { if (this.level <= level) { write(message); } if (nextLogger != null) { nextLogger.logMessage(level, message); } } abstract protected void write(String message); } public class ConsoleLogger extends AbstractLogger { public ConsoleLogger(int level) { this.level = level; } [@Override](https://my.oschina.net/u/1162528) protected void write(String message) { System.out.println("Standard Console::Logger: " + message); } } public class ErrorLogger extends AbstractLogger { public ErrorLogger(int level) { this.level = level; } [@Override](https://my.oschina.net/u/1162528) protected void write(String message) { System.out.println("Error Console::Logger: " + message); } } public class FileLogger extends AbstractLogger { public FileLogger(int level) { this.level = level; } [@Override](https://my.oschina.net/u/1162528) protected void write(String message) { System.out.println("File::Logger: " + message); } } public class ChainPatternDemo { private static AbstractLogger getChainOfLoggers() { AbstractLogger errorLogger = new ErrorLogger(AbstractLogger.ERROR); AbstractLogger fileLogger = new FileLogger(AbstractLogger.DEBUG); AbstractLogger consoleLogger = new ConsoleLogger(AbstractLogger.INFO); errorLogger.setNextLogger(fileLogger); fileLogger.setNextLogger(consoleLogger); return errorLogger; } public static void main(String[] args) { AbstractLogger loggerChain = getChainOfLoggers(); loggerChain.logMessage(AbstractLogger.INFO, "This is an information."); System.out.println(); loggerChain.logMessage(AbstractLogger.DEBUG, "This is an debug level information."); System.out.println(); loggerChain.logMessage(AbstractLogger.ERROR, "This is an error information."); System.out.println(); } }
运行结果 Standard Console::Logger: This is an information. File::Logger: This is an debug level information. Standard Console::Logger: This is an debug level information. Error Console::Logger: This is an error information. File::Logger: This is an error information. Standard Console::Logger: This is an error information.
实现二
模拟Mail邮件处理 以及 Enum 的运用来演示责任链模式 public class Enums { private static Random rand = new Random(47); public static <T extends Enum<T>> T random(Class<T> ec) { return random(ec.getEnumConstants()); } public static <T> T random(T[] values) { return values[rand.nextInt(values.length)]; } } class Mail { // The NO's lower the 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++; 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; public Iterator<Mail> iterator() { return new Iterator<Mail>() { public boolean hasNext() { return n-- > 0; } public Mail next() { return randomMail(); } public void remove() { // Not implemented throw new UnsupportedOperationException(); } }; } }; } } public class PostOffice { enum MailHandler { GENERAL_DELIVERY { boolean handle(Mail m) { switch (m.generalDelivery) { case YES: System.out.println("Using general delivery for " + m); return true; default: return false; } } }, MACHINE_SCAN { 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 { 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 { 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("*****"); } } }
运行结果： 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 *****
参考及引用
1.《Java编程思想（第四版）》
2.责任链模式