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.

加薪代码:

// 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.