设计模式读书笔记之职责链模式(chain of responsibility)

职责链模式：当一个请求有可能被多个对象处理，则将这些对象连成一条链，并沿着这条链传递请求，直到该请求被处理为止。

单看上图, 你肯定看不出职责链模式的特征, 先举一个例子来说明一下. 比如一个员工想加工资, 他首先会告知team leader, 如果在team leader授权范围之内,则他可以办到, 否则,就要请求上级来处理,最后直到老板, 于是很容易写出这样的代码来:

public class Employee {
	private TeamLeader tl;
	private DeptManager deptMgr;
	private GeneralManager gm;
	private Boss boss;
	//some other properties
	public void addSalary(int x){
		if(x<=100){
			tl.addSalary(this, x);
		}
		else if(x<=500){
			deptMgr.addSalary(this, x);
		}
		else if(x<=800){
			gm.addSalary(this, x);
		}
		else{
			boss.addSalary(this, x);
		}
	}
	// some other methods
}

 这个代码中的if else带来了坏味道, 同时, Employee必须知道他的每一个上司,并使Employee和每一个处理者(他的上司)耦合在一起. 其实一个Employee一般来说只需要和他的直接上司打交道就可以了. 就如下时序图所示.

 把Employee的上司连成一条链, 即team leader持有他的上司部门经理, 部门经理持有他的上司总经理, 总经理持有他的上司老板. 则Employee只需要知道team leader即可.

//
public interface Supervisor {
	public void addSalary(Employee e, int added);
}

///////////////////
public class Employee {
	public String name;
	private Supervisor supervisor;
	public Employee(String name){
		this.name = name;
	}
	public void setSupervisor(Supervisor supervisor) {
		this.supervisor = supervisor;
	}
	public void addSalary(int added){
		if(supervisor!=null){
			supervisor.addSalary(this, added);
		}
	}
	
}
///////////////////
public class TeamLeader implements Supervisor{
	private Supervisor successor;
	public TeamLeader(Supervisor s){
		this.successor = s;
	}
	public void addSalary(Employee e, int added) {
		if(added<100){
			System.out.println("Team Leader: " + e.name + " want to add " 
					+ added + " salary, I can process it, done!");
		}
		else if(successor!=null){
			System.out.println("Team Leader: " + e.name + " want to add " 
					+ added + " salary, I can't process it, but my supervisor can do!");
			successor.addSalary(e, added);
		}
	}
}
///////////////////
public class DeptManager implements Supervisor{
	private Supervisor successor;
	public DeptManager(Supervisor s){
		this.successor = s;
	}
	public void addSalary(Employee e, int added) {
		if(added<500){
			System.out.println("Dept Manager: " + e.name + " want to add " 
					+ added + " salary, I can process it, done!");
		}
		else if(successor!=null){
			System.out.println("Dept Manager: " + e.name + " want to add " 
					+ added + " salary, I can't process it, but my supervisor can do!");
			successor.addSalary(e, added);
		}
	}
}
///////////////////////
public class GeneralManager implements Supervisor{
	private Supervisor successor;
	public GeneralManager(Supervisor s){
		this.successor = s;
	}
	public void addSalary(Employee e, int added) {
		if(added<800){
			System.out.println("General Manager: " + e.name + " want to add " 
					+ added + " salary, I can process it, done!");
		}
		else if(successor!=null){
			System.out.println("General Manager: " + e.name + " want to add " 
					+ added + " salary, I can't process it, but my supervisor can do!");
			successor.addSalary(e, added);
		}
	}
}
//////////////////
public class Boss implements Supervisor{
	public Boss(){}
	public void addSalary(Employee e, int added) {
		//boss具有最终处理权限, 但是他打官腔
		System.out.println("Boss: I will process it soon.");
	}
}
/////////////////////////////
//测试用例
public class Test {
	public static void main(String[] args) {
		Employee e = new Employee("Jack");
		Boss b = new Boss();
		GeneralManager gm = new GeneralManager(b);
		DeptManager dm = new DeptManager(gm);
		TeamLeader tl = new TeamLeader(dm);
		e.setSupervisor(tl);
		e.addSalary(1200);
	}
}
/////////////////////////////
//输出
//Team Leader: Jack want to add 1200 salary, I can't process it, but my supervisor can do!
//Dept Manager: Jack want to add 1200 salary, I can't process it, but my supervisor can do!
//General Manager: Jack want to add 1200 salary, I can't process it, but my supervisor can do!
//Boss: I will process it soon.

上面的代码有两个优点:

1 Employee不再依赖于具体的上司,转而依赖于抽象, 而且不必知道所有的上司;

2 如果新增了处理者, 只需要更改客户端代码, 即重新构造责任链就可以了.

基础代码：

package designpattern.chainofresponsibility;

public abstract class Handler {
	//注意修饰符为protected，因为子类要访问他
	protected Handler successor;
	
	public void setSuccessor(Handler successor) {
		this.successor = successor;
	}
	
	public abstract void handleRequest(int request);
}
//
package designpattern.chainofresponsibility;

public class ConcreteHandler1 extends Handler {
	public void handleRequest(int request) {
		if(request>=0&&request<10){
			System.out.println("request handled by ConcreteHandler1");
			return;
		}
		if(successor!=null){
			System.out.println("ConcreteHandler1 can't handle this request, dispatch to next.");
			successor.handleRequest(request);
		}
	}
}
//
package designpattern.chainofresponsibility;

public class ConcreteHandler2 extends Handler {
	public void handleRequest(int request) {
		if(request>=10&&request<20){
			System.out.println("request handled by ConcreteHandler2");
			return;
		}
		if(successor!=null){
			System.out.println("ConcreteHandler2 can't handle this request, dispatch to next.");
			successor.handleRequest(request);
		}
	}
}
//
package designpattern.chainofresponsibility;

public class ConcreteHandler3 extends Handler {
	public void handleRequest(int request) {
		if(request>=20&&request<30){
			System.out.println("request handled by ConcreteHandler3");
			return;
		}
		if(successor!=null){
			System.out.println("ConcreteHandler3 can't handle this request, dispatch to next.");
			successor.handleRequest(request);
		}
	}
}
//
package designpattern.chainofresponsibility;

public class ConcreteHandler4 extends Handler {
	public void handleRequest(int request) {
		if(request>=30){
			System.out.println("request handled by ConcreteHandler3");
			return;
		}
	}
}
//客户端测试用例
package designpattern.chainofresponsibility;

public class Client {
	public static void main(String[] args){
		Handler handler1 = new ConcreteHandler1();
		Handler handler2 = new ConcreteHandler2();
		Handler handler3 = new ConcreteHandler3();
		Handler handler4 = new ConcreteHandler4();
		handler1.setSuccessor(handler2);
		handler2.setSuccessor(handler3);
		handler3.setSuccessor(handler4);
		handler1.handleRequest(60);
	}
} 

责任链模式的关键点在于把请求的处理者连成一条链,一个处理者可以处理当前请求,也有权决定是否沿着链朝上传递请求.

优点：

1 可以简化对象之间的连接，他们只需要知道一个后继者就行了；

2 可以很方便的增加或修改处理者。

注意：

一个请求可以传递到末端都得不到处理，因此需要考虑全面。

经典案例：

jdk中我真的没有找到合适的例子,网上说Java 1.0版本中的AWT库采用责任链模式和命令模式来处理GUI的事件，我觉得有兴趣你可以看一下源码.DHTML中的事件处理机制可以看作职责链模式,这 种机制又叫事件冒泡(Event Bubbling)机制, 当页面中的一个dom节点捕获一个事件(比如鼠标双击), 当前节点的处理函数可以处理改事件,然后也可以决定是否向上冒泡让父元素继续处理这个事件.浏览器的事件处理机制也是这样的。

如果你想到了j2ee中的Filter,或者spring中的Interceptor和ExecutionChain, 那你就错了. 因为就实现方法上看, Filter和Interceptor以及ExecutionChain都不是职责链模式.