设计模式学习－责任链模式

1.定义

避免请求发送者与接收者耦合在一起，让多个对象都有可能接收请求，将这些对象连接成一条链，并且沿着这条链传递请求，直到有对象处理它为止。

2.类图

3.代码示例 

package com.zhaoyangwoo.chainOfResponsibility;

/**
 * Created by john on 16/6/16.
 * 职责链模式,经费审批场景
 */
public class Responsibility {

    public static void main(String[] args) {

        //zhaoyanghoo报销100元
        Request request = new Request();
        request.setName("zhaoyanghoo");
        request.setLevel(new Level(100));

        ApprovalUtil.handleApproval(request).print();

        //zhaoyanghoo报销100元
        Request request2 = new Request();
        request2.setName("xiaohong");
        request2.setLevel(new Level(10000));

        ApprovalUtil.handleApproval(request2).print();


        //zhaoyanghoo报销100元
        Request request3 = new Request();
        request3.setName("xiaoming");
        request3.setLevel(new Level(10000000));
        ApprovalUtil.handleApproval(request3).print();

    }

}

/**
 * 用这个工具类封装内部关系,client只需要知道ApprovalUtil,而不需要跟manage/director等关联
 */
class ApprovalUtil {
    static Approval manage, director, cfo;

    static {
        manage = new Manager();
        director = new Director();
        cfo = new CFO();

        manage.setSuccessor(director);
        director.setSuccessor(cfo);
    }

    static Response handleApproval(Request request) {
        return manage.handleApproval(request);
    }
}


/**
 * 抽象类,审批人
 */
abstract class Approval {

    public void setSuccessor(Approval successor) {
        this.successor = successor;
    }

    protected abstract Level getLevel();

    //后继审批人
    protected Approval successor;


    public final Response handleApproval(Request request) {
        if (request == null || request.getLevel() == null) return null;
        if (this.getLevel() != null && this.getLevel().higherThen(request.getLevel())) {
            return this.hanlder(request);
        } else if (this.successor != null) {
            return this.successor.handleApproval(request);
        } else
            return new Response("无人能处理" + request.getName() + "的报销请求");
    }

    protected abstract Response hanlder(Request request);
}

/**
 * 经理只审批500元内经费
 */
class Manager extends Approval {

    //最多审批500元
    private final static Level level = new Level(500);

    @Override
    protected Level getLevel() {
        return level;
    }

    @Override
    public Response hanlder(Request request) {
        return new Response(String.format("经理审批了%s的%d元报销", request.getName(), request.getLevel().getAmount()));
    }
}

/**
 * 总监审批1000元内的报销
 */
class Director extends Approval {

    //最多审批1000元
    private final static Level level = new Level(1000);

    @Override
    protected Level getLevel() {
        return level;
    }

    @Override
    public Response hanlder(Request request) {
        return new Response(String.format("总监审批了%s的%d元报销", request.getName(), request.getLevel().getAmount()));
    }
}

/**
 * CFO审批100000
 */
class CFO extends Approval {

    //最多审批500元
    private final static Level level = new Level(100000);

    @Override
    protected Level getLevel() {
        return level;
    }

    @Override
    public Response hanlder(Request request) {
        return new Response(String.format("CFO审批了%s的%d元报销", request.getName(), request.getLevel().getAmount()));
    }
}

//请求模拟类
class Request {
    public Level getLevel() {
        return level;
    }

    public void setLevel(Level level) {
        this.level = level;
    }

    //请求的级别
    Level level;

    //姓名
    String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

//请求级别类
class Level {

    public Level(int amount) {
        this.amount = amount;
    }

    public int getAmount() {
        return amount;
    }

    //报销金额
    private final int amount;

    //如果高于当前金额,则无法处理
    public boolean higherThen(Object obj) {
        if (obj == null) return false;
        if (obj == this) {
            return true;
        } else if (obj instanceof Level) {
            return this.amount > ((Level) obj).amount;
        }
        return false;
    }
}

//相应模拟类
class Response {
    public Response(String message) {
        this.message = message;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    //审批结果
    private String message;

    //打印审批结果
    public void print() {
        System.out.println(message);
    }
}

4.应用场景举例

• 有多个对象可以处理同一个请求，具体哪个对象处理该请求由运行时刻自动确定。
• 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。
• 可动态指定一组对象处理请求

5.JDK源码中的模式实现

　　java.util.logging.Logger类应用了责任链模式

public void log(LogRecord record) {
        if (!isLoggable(record.getLevel())) {
            return;
        }
        Filter theFilter = filter;
        if (theFilter != null && !theFilter.isLoggable(record)) {
            return;
        }

        // Post the LogRecord to all our Handlers, and then to
        // our parents' handlers, all the way up the tree.

        Logger logger = this;
        while (logger != null) {
            final Handler[] loggerHandlers = isSystemLogger
                ? logger.accessCheckedHandlers()
                : logger.getHandlers();

            for (Handler handler : loggerHandlers) {
                handler.publish(record);
            }

            final boolean useParentHdls = isSystemLogger
                ? logger.useParentHandlers
                : logger.getUseParentHandlers();

            if (!useParentHdls) {
                break;
            }

            logger = isSystemLogger ? logger.parent : logger.getParent();
        }
    }



    public boolean isLoggable(Level level) {
        if (level.intValue() < levelValue || levelValue == offValue) {
            return false;
        }
        return true;
    } 

6.思考

• 如何控制链的长度，以保证性能

　  可以在抽象类中添加一个static final的常量，标志最长的链有MAX个元素，一个static的set类型变量，setSuccessor方法中将handler添加到set变量中，比较MAX和Set.size()。

7.参考

　　1.《JAVA与模式》之责任链模式

　　2.java.util.logging.Logger使用详解