3. 行为型模式 (5) 责任链模式

3. 行为型模式 (5) 责任链模式

责任链模式（Chain of Responsibility Pattern）为请求创建了一个接收者对象的链。这种模式给予请求的类型，对请求的发送者和接收者进行解耦。

通常每个接收者都包含对另一个接收者的引用。如果一个对象不能处理该请求，那么它会把相同的请求传给下一个接收者，依此类推。

优点：
1. 降低耦合度。它将请求的发送者和接收者解耦。
2. 简化了对象。使得对象不需要知道链的结构。
3. 增强给对象指派职责的灵活性。通过改变链内的成员或者调动它们的次序，允许动态地新增或者删除责任。
4. 增加新的请求处理类很方便。

缺点：
1. 不能保证请求一定被接收。
2. 系统性能将受到一定影响，而且在进行代码调试时不太方便，可能会造成循环调用。
3. 可能不容易观察运行时的特征，有碍于除错。

1. 责任链模式

package com.andrew.pattern0305.chain.model01;
public abstract class AbstractLogger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;
    protected int level;
    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);
}

package com.andrew.pattern0305.chain.model01;
public class ConsoleLogger extends AbstractLogger {
    public ConsoleLogger(int level) {
        this.level = level;
    }
    @Override
    protected void write(String message) {
        System.out.println("Standard Console::Logger: " + message);
    }
}

package com.andrew.pattern0305.chain.model01;
public class ErrorLogger extends AbstractLogger {
    public ErrorLogger(int level) {
        this.level = level;
    }
    @Override
    protected void write(String message) {
        System.out.println("Error Console::Logger: " + message);
    }
}

package com.andrew.pattern0305.chain.model01;
public class FileLogger extends AbstractLogger {
    public FileLogger(int level) {
        this.level = level;
    }
    @Override
    protected void write(String message) {
        System.out.println("File Console::Logger: " + message);
    }
}

package com.andrew.pattern0305.chain.model01;
/**
 * 1. 责任链模式
 * 
 * @author andrew
 * @date 2018/08/15
 */
public class Client {
    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.");
        loggerChain.logMessage(AbstractLogger.DEBUG, "This is an debug level information.");
        loggerChain.logMessage(AbstractLogger.ERROR, "This is an error information.");
    }
}
运行结果：
Standard Console::Logger: This is an information.
File Console::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 Console::Logger: This is an error information.
Standard Console::Logger: This is an error information.

2. 责任链模式

package com.andrew.pattern0305.chain.model02;
public abstract class Handler {
    protected Handler successor;
    public abstract void handleRequest();
    public Handler getSuccessor() {
        return successor;
    }
    public void setSuccessor(Handler successor) {
        this.successor = successor;
    }
}

package com.andrew.pattern0305.chain.model02;
public class ConcreteHandler extends Handler {
    @Override
    public void handleRequest() {
        if (getSuccessor() != null) {
            System.out.println("放过请求");
            getSuccessor().handleRequest();
        } else {
            System.out.println("处理请求");
        }
    }
}

package com.andrew.pattern0305.chain.model02;
/**
 * 2. 责任链模式
 * 
 * @author andrew
 * @date 2018/08/15
 */
public class Client {
    public static void main(String[] args) {
        Handler handler1 = new ConcreteHandler();
        Handler handler2 = new ConcreteHandler();
        handler1.setSuccessor(handler2);
        handler1.handleRequest();
    }
}
运行结果：
放过请求
处理请求