设计模式漫谈：职责链（Chain of Responsibility）

一、概念

职责链是一种行为型模式，通过创建一条链来组织请求处理者和处理行为，请求发送者无须知道请求在何时、何处以及如何被处理，实现了请求发送者与处理者的解耦。

由客户端负责链的创建，当系统中新增处理者或修改处理链路时，无须修改系统代码，只需要在客户端重新建链即可，具有良好的灵活性，符合开闭原则。

二、应用场景

典型的应用场景包括：

1、Web应用中的过滤器（Filter）

2、工作流引擎中的审批流程

三、典型代码

Classgram of pattern of chain of responsibility

Handler.java

package patterns.chain_of_responsibility.definition;

public abstract class Handler {
	
	protected Handler successor;
	
	public void setSuccessor(Handler successor) {
		this.successor = successor;
	}
	
	public abstract void handleRequest(Object request);
	
}

ConcreteHandler.java

package patterns.chain_of_responsibility.definition;

public class ConcreteHandler extends Handler {

	@Override
	public void handleRequest(Object request) {
		if("one".equals(request)) { //满足条件
			//处理代码
		} else {
			this.successor.handleRequest(request);  //转发请求
		}
	}
}

四、示例：

审批项目场景：

50万元（不含）以内的项目：部门经理有审批权；

50万元（含）~200万元（不含）：公司总监有审批权；

200万元（含）~500万元（不含）：公司总裁有审批权；

500万元（含）以上：召开董事会决议。

1、请求类：ProjectRequest.java

package patterns.chain_of_responsibility.example;

//请求类：项目申请
public class ProjectRequest {
	private double amount;  //项目金额
	private String id;     //项目编号
	private String desc; //项目说明
	
	
	public ProjectRequest(double amount, String id, String desc) {
		super();
		this.amount = amount;
		this.id = id;
		this.desc = desc;
	}
	
	public double getAmount() {
		return amount;
	}
	
	public void setAmount(double amount) {
		this.amount = amount;
	}
	
	public String getId() {
		return id;
	}
	
	public void setId(String id) {
		this.id = id;
	}
	
	public String getDesc() {
		return desc;
	}
	
	public void setDesc(String desc) {
		this.desc = desc;
	}
	
}

2、抽象审批者类：Approver.java

package patterns.chain_of_responsibility.example;

public abstract class Approver {
	protected Approver successor;    //后继审批者
	protected String name;           //审批者姓名
	
	public Approver(String name) {
		super();
		this.name = name;
	}

	//设置后继者
	public void setSuccessor(Approver successor) {
		this.successor = successor;
	}
	
	//抽象请求处理方法
	public abstract void processRequest(ProjectRequest request);
	
}

3、具体审批者类：经理（Manager.java）、总监（Director.java）、总裁（President.java）、董事会（Congress.java）

package patterns.chain_of_responsibility.example;

//经理
public class Manager extends Approver {

	public Manager(String name) {
		super(name);
	}

	@Override
	public void processRequest(ProjectRequest request) {
		if(request.getAmount() < 500000) {
			System.out.println(this.name + "（经理）审批项目申请单：" + request.getId() + "，项目金额：" 
					+ request.getAmount() + "元，项目说明：" + request.getDesc() + "。");  
		} else {
			this.successor.processRequest(request);
		}
	}

}

package patterns.chain_of_responsibility.example;

//总监
public class Director extends Approver {

	public Director(String name) {
		super(name);
	}

	@Override
	public void processRequest(ProjectRequest request) {
		if(request.getAmount() < 2000000) {
			System.out.println(this.name + "（总监）审批项目申请单：" + request.getId() + "，项目金额：" 
					+ request.getAmount() + "元，项目说明：" + request.getDesc() + "。");  
		} else {
			this.successor.processRequest(request);
		}
	}

}

package patterns.chain_of_responsibility.example;

//总裁
public class President extends Approver {

	public President(String name) {
		super(name);
	}

	@Override
	public void processRequest(ProjectRequest request) {
		if(request.getAmount() < 5000000) {
			System.out.println(this.name + "（总裁）审批项目申请单：" + request.getId() + "，项目金额：" 
					+ request.getAmount() + "元，采购说明：" + request.getDesc() + "。");  
		} else {
			this.successor.processRequest(request);
		}
	}

}

package patterns.chain_of_responsibility.example;

public class Congress extends Approver {

	public Congress(String name) {
		super(name);
	}

	@Override
	public void processRequest(ProjectRequest request) {
		System.out.println("董事会审议项目申请：" + request.getId() + "，项目金额：" 
				+ request.getAmount() + "元，项目说明：" + request.getDesc() + "。");  
	}

}

4、客户端：Client.java

package patterns.chain_of_responsibility.example;

public class Client {
	public static void main(String[] args) {
		Approver manager = new Manager("张飞");
		Approver director = new Director("关羽");
		Approver president = new President("刘备");
		Approver congress = new Congress("董事会");
		
		//创建职责链
		manager.setSuccessor(director);
		director.setSuccessor(president);
		president.setSuccessor(congress);
		
	    //创建项目申请单
		ProjectRequest pr1 = new ProjectRequest(499000, "P0001", "买酒");
		manager.processRequest(pr1);
		
		ProjectRequest pr2 = new ProjectRequest(1898000, "P0002", "建粮仓");
		manager.processRequest(pr2);
		
		ProjectRequest pr3 = new ProjectRequest(4660000, "P0003", "建军马场");
		manager.processRequest(pr3);
		
		ProjectRequest pr4 = new ProjectRequest(8825000, "P0004", "研发新兵器");
		manager.processRequest(pr4);
		
	}

}

【运行结果】

张飞（经理）审批项目申请单：P0001，项目金额：499000.0元，项目说明：买酒。

关羽（总监）审批项目申请单：P0002，项目金额：1898000.0元，项目说明：建粮仓。

刘备（总裁）审批项目申请单：P0003，项目金额：4660000.0元，采购说明：建军马场。

董事会审议项目申请：P0004，项目金额：8825000.0元，项目说明：研发新兵器。