设计模式：责任链模式示例

责任链模式可以应用于多种场景，下面是几个不同场景的例子，每个例子都包括完整的代码。

示例1：日志处理系统

在日志处理系统中，日志消息可以根据其严重性（错误、警告、信息）被不同级别的日志处理器处理。

abstract class Logger {public static int INFO = 1;public static int DEBUG = 2;public static int ERROR = 3;protected int level;protected Logger nextLogger;public void setNextLogger(Logger 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);
}class ConsoleLogger extends Logger {public ConsoleLogger(int level) {this.level = level;}@Overrideprotected void write(String message) {System.out.println("Standard Console::Logger: " + message);}
}class ErrorLogger extends Logger {public ErrorLogger(int level) {this.level = level;}@Overrideprotected void write(String message) {System.err.println("Error Console::Logger: " + message);}
}class FileLogger extends Logger {public FileLogger(int level) {this.level = level;}@Overrideprotected void write(String message) {System.out.println("File::Logger: " + message);}
}public class ChainPatternDemo {private static Logger getChainOfLoggers() {Logger errorLogger = new ErrorLogger(Logger.ERROR);Logger fileLogger = new FileLogger(Logger.DEBUG);Logger consoleLogger = new ConsoleLogger(Logger.INFO);errorLogger.setNextLogger(fileLogger);fileLogger.setNextLogger(consoleLogger);return errorLogger;}public static void main(String[] args) {Logger loggerChain = getChainOfLoggers();loggerChain.logMessage(Logger.INFO, "This is an information.");loggerChain.logMessage(Logger.DEBUG, "This is a debug level information.");loggerChain.logMessage(Logger.ERROR, "This is an error information.");}
}

示例2：支付处理系统

在一个支付处理系统中，一个支付请求可以通过多种支付方式进行处理。

abstract class PaymentHandler {protected PaymentHandler successor;public void setSuccessor(PaymentHandler successor) {this.successor = successor;}public abstract void handlePayment(PaymentRequest request);
}class PaymentRequest {private double amount;public PaymentRequest(double amount) {this.amount = amount;}public double getAmount() {return amount;}
}class PaypalHandler extends PaymentHandler {@Overridepublic void handlePayment(PaymentRequest request) {if (request.getAmount() < 1000) {System.out.println("Paypal processed $" + request.getAmount());} else if (successor != null) {successor.handlePayment(request);}}
}class CreditCardHandler extends PaymentHandler {@Overridepublic void handlePayment(PaymentRequest request) {if (request.getAmount() < 5000) {System.out.println("Credit Card processed $" + request.getAmount());} else if (successor != null) {successor.handlePayment(request);}}
}class BankTransferHandler extends PaymentHandler {@Overridepublic void handlePayment(PaymentRequest request) {System.out.println("Bank Transfer processed $" + request.getAmount());}
}public class PaymentChainDemo {public static void main(String[] args) {PaymentHandler paypal = new PaypalHandler();PaymentHandler creditCard = new CreditCardHandler();PaymentHandler bankTransfer = new BankTransferHandler();paypal.setSuccessor(creditCard);creditCard.setSuccessor(bankTransfer);PaymentRequest request = new PaymentRequest(450);paypal.handlePayment(request);request = new PaymentRequest(1200);paypal.handlePayment(request);request = new PaymentRequest(6500);paypal.handlePayment(request);}
}