【设计模式】责任链模式

使用频率：★★★☆☆

一、什么是责任链模式

为请求创建了一个接收者对象的链，每个接收者都包含对另一个接收者的引用，当某个接受者不能处理该请求时，会将该请求转给下一个接受者处理；

二、补充说明

请求发送者与请求接受者解耦

应用例子：struts的拦截器，servlet的过滤器

三、角色

抽象请求处理者

具体请求处理者：包含下一个具体请求处理者的引用

客户端：请求发送者

四、例子，JAVA实现

说明：模拟一个村、镇、县的责任链关系请求

抽象处理者

package com.pichen.dp.behavioralpattern.chain;

public abstract class Handler {

    protected Handler next;

    public abstract void handleRequest(String value);

    public Handler next() {
        return this.next;
    }

    public void setNext(Handler next) {
        this.next = next;
    }
}

具体村、镇、县处理者

package com.pichen.dp.behavioralpattern.chain;

public class VillageHandler extends Handler {

    /**
     * @see com.pichen.dp.behavioralpattern.chain.Handler#handleRequest()
     */
    @Override
    public void handleRequest(String value) {
        if ("village".equals(value)) {
            System.out.println("VillageHandler: handled~");
        } else {
            System.out.println("VillageHandler: pass~");
            this.next.handleRequest(value);
        }
    }

}

package com.pichen.dp.behavioralpattern.chain;

public class TownHandler extends Handler {

    @Override
    public void handleRequest(String value) {
        if ("town".equals(value)) {
            System.out.println("VillageHandler: handled~");
        } else {
            System.out.println("Town: pass~");
            this.next.handleRequest(value);
        }
    }
}

package com.pichen.dp.behavioralpattern.chain;

public class CountyHandler extends Handler {

    @Override
    public void handleRequest(String value) {
        if ("county".equals(value)) {
            System.out.println("County: handled~");
        } else if (this.next == null) {
            System.out
                    .println("no next Handler, this request can not be handle~");
        } else {
            System.out.println("County: pass~");
            this.next.handleRequest(value);
        }
    }
}

客户端：

package com.pichen.dp.behavioralpattern.chain;

public class Main {

    public static void main(String[] args) {
        Handler villageHandler = new VillageHandler();
        Handler townHandler = new TownHandler();
        Handler countyHandler = new CountyHandler();
        
        villageHandler.setNext(townHandler);
        townHandler.setNext(countyHandler);
        
        System.out.println("test county request:");
        villageHandler.handleRequest("county");
        
        System.out.println("\ntest city request:");
        villageHandler.handleRequest("city");
    }
}

结果：

test county request:
VillageHandler: pass~
Town: pass~
County: handled~

test city request:
VillageHandler: pass~
Town: pass~
no next Handler, this request can not be handle~