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:

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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:

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//

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：

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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.