设计模式（13）——责任链（Chain of Responsibility)

责任链是什么？

责任链条模式使多个对象都有机会处理请求。打个比方说，责任就是得分，多个对象就是篮球队员，在三分线的远投的时候，球由投手投篮。带球突破则由后卫负责。。。

责任链模式的适用范围

1. 有多个对象可以处理一个请求，哪个对象处理该请求运行时刻自动确定。

2. 在不明确指定接收者的情况下向多个对象中的一个提交一个请求。

3. 可处理的一个请求对象集合应被动态指定。

责任链模式的例子

abstract class Logger {    public static int ERR = 3;    public static int NOTICE = 5;    public static int DEBUG = 7;    private int mask;     // The next element in the chain of responsibility    private Logger next;     public Logger(int mask) {        this.mask = mask;    }     public void setNext(Logger logger) {        next = logger;    }     public void message(String msg, int priority) {        if (priority <= mask) {            writeMessage(msg);        }        if (next != null) {            next.message(msg, priority);        }    }     abstract protected void writeMessage(String msg);} class StdoutLogger extends Logger {    public StdoutLogger(int mask) {        super(mask);    }     protected void writeMessage(String msg) {        System.out.println("Writing to stdout: " + msg);    }} class EmailLogger extends Logger {    public EmailLogger(int mask) {        super(mask);    }     protected void writeMessage(String msg) {        System.out.println("Sending via e-mail: " + msg);    }} class StderrLogger extends Logger {    public StderrLogger(int mask) {        super(mask);    }     protected void writeMessage(String msg) {        System.err.println("Sending to stderr: " + msg);    }} public class ChainOfResponsibilityExample {     private static Logger createChain() {        // Build the chain of responsibility         Logger logger = new StdoutLogger(Logger.DEBUG);         Logger logger1 = new EmailLogger(Logger.NOTICE);        logger.setNext(logger1);         Logger logger2 = new StderrLogger(Logger.ERR);        logger1.setNext(logger2);         return logger;    }     public static void main(String[] args) {        Logger chain = createChain();         // Handled by StdoutLogger (level = 7)        chain.message("Entering function y.", Logger.DEBUG);         // Handled by StdoutLogger and EmailLogger (level = 5)        chain.message("Step1 completed.", Logger.NOTICE);         // Handled by all three loggers (level = 3)        chain.message("An error has occurred.", Logger.ERR);    }}/*The output is:   Writing to stdout:   Entering function y.   Writing to stdout:   Step1 completed.   Sending via e-mail:  Step1 completed.   Writing to stdout:   An error has occurred.   Sending via e-mail:  An error has occurred.   Sending to stderr:   An error has occurred.*/