学步园

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

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:

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

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;</p> <p>public abstract class Handler {<br /> //注意修饰符为protected，因为子类要访问他<br /> protected Handler successor;</p> <p> public void setSuccessor(Handler successor) {<br /> this.successor = successor;<br /> }</p> <p> public abstract void handleRequest(int request);<br /> }<br /> //<br /> package designpattern.chainofresponsibility;</p> <p>public class ConcreteHandler1 extends Handler {<br /> public void handleRequest(int request) {<br /> if(request>=0&&request<10){<br /> System.out.println("request handled by ConcreteHandler1");<br /> return;<br /> }<br /> if(successor!=null){<br /> System.out.println("ConcreteHandler1 can't handle this request, dispatch to next.");<br /> successor.handleRequest(request);<br /> }<br /> }<br /> }<br /> //<br /> package designpattern.chainofresponsibility;</p> <p>public class ConcreteHandler2 extends Handler {<br /> public void handleRequest(int request) {<br /> if(request>=10&&request<20){<br /> System.out.println("request handled by ConcreteHandler2");<br /> return;<br /> }<br /> if(successor!=null){<br /> System.out.println("ConcreteHandler2 can't handle this request, dispatch to next.");<br /> successor.handleRequest(request);<br /> }<br /> }<br /> }<br /> //<br /> package designpattern.chainofresponsibility;</p> <p>public class ConcreteHandler3 extends Handler {<br /> public void handleRequest(int request) {<br /> if(request>=20&&request<30){<br /> System.out.println("request handled by ConcreteHandler3");<br /> return;<br /> }<br /> if(successor!=null){<br /> System.out.println("ConcreteHandler3 can't handle this request, dispatch to next.");<br /> successor.handleRequest(request);<br /> }<br /> }<br /> }<br /> //<br /> package designpattern.chainofresponsibility;</p> <p>public class ConcreteHandler4 extends Handler {<br /> public void handleRequest(int request) {<br /> if(request>=30){<br /> System.out.println("request handled by ConcreteHandler3");<br /> return;<br /> }<br /> }<br /> }<br /> //客户端测试用例<br /> package designpattern.chainofresponsibility;</p> <p>public class Client {<br /> public static void main(String[] args){<br /> Handler handler1 = new ConcreteHandler1();<br /> Handler handler2 = new ConcreteHandler2();<br /> Handler handler3 = new ConcreteHandler3();<br /> Handler handler4 = new ConcreteHandler4();<br /> handler1.setSuccessor(handler2);<br /> handler2.setSuccessor(handler3);<br /> handler3.setSuccessor(handler4);<br /> handler1.handleRequest(60);<br /> }<br /> }</p> <p>

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.