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。

java">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;
    }

    @Override
    protected void write(String message) {
        System.out.println("Standard Console::Logger: " + message);
    }
}

class ErrorLogger extends Logger {
    public ErrorLogger(int level) {
        this.level = level;
    }

    @Override
    protected void write(String message) {
        System.err.println("Error Console::Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level) {
        this.level = level;
    }

    @Override
    protected 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。

java">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 {
    @Override
    public 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 {
    @Override
    public 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 {
    @Override
    public 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);
    }
}