责任链模式 chain of responsibility（原）
动机：
在开发过程中有一种情况：一个事件产生一个请求，这个请求需要很多对象来处理它，更极端一点，我们不能直接访问这些对象。这种情况下我们有两种解决方案：
?????? 1、让这些对象变成public，创建这些对象的引用，然后处理这个请求
???????2、使用责任链模式
在责任链模式中，一组处理请求的对象构成一条链子，每个对象都可以处理请求而且持有下个对象的引用，请求从一个对象传递到下一个对象，直到处理完这个请求。
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意义或者作用：
????? 1、避免请求发送者直接依赖所有接受者对象，达到解耦的目的。
????? 2、所有接受者对象都成为链的一部分，请求在这些对象中传递直到某个对象解决掉请求
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使用场景或者条件：
????? 1、有多个handler可以处理一条命令
??????2、预先不知道哪个handler来处理，到底使用哪个handler应该由handler内部处理方法自动地确定
????? 3、请求应该发送到一组handler，而不是某个handler
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UML结构图：
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附上一个例子：
public class Request {private int m_value;private String m_description;public Request(String description, int value){m_description = description;m_value = value;}public int getValue(){return m_value;}public String getDescription(){return m_description;} }public abstract class Handler{protected Handler m_successor;public void setSuccessor(Handler successor){m_successor = successor;}public abstract void handleRequest(Request request);}public class ConcreteHandlerOne extends Handler{public void handleRequest(Request request){if (request.getValue() < 0){ //if request is eligible handle itSystem.out.println("Negative values are handled by ConcreteHandlerOne:");System.out.println("\tConcreteHandlerOne.HandleRequest : " + request.getDescription() + request.getValue());}else{super.m_successor.handleRequest(request);}} }public class ConcreteHandlerThree extends Handler{public void handleRequest(Request request){if (request.getValue() >= 0){ //if request is eligible handle itSystem.out.println("Zero values are handled by ConcreteHandlerThree:");System.out.println("\tConcreteHandlerThree.HandleRequest : " + request.getDescription() + request.getValue());} else{super.m_successor.handleRequest(request);}}}public class ConcreteHandlerTwo extends Handler{public void handleRequest(Request request){if (request.getValue() > 0){ //if request is eligible handle itSystem.out.println("Positive values are handled by ConcreteHandlerTwo:");System.out.println("\tConcreteHandlerTwo.HandleRequest : " + request.getDescription() + request.getValue());} else{super.m_successor.handleRequest(request);}}}public class Main {public static void main(String[] args) {// Setup Chain of ResponsibilityHandler h1 = new ConcreteHandlerOne();Handler h2 = new ConcreteHandlerTwo();Handler h3 = new ConcreteHandlerThree();h1.setSuccessor(h2);h2.setSuccessor(h3);// Send requests to the chainh1.handleRequest(new Request("Negative Value ", -1));h1.handleRequest(new Request("Negative Value ", 0));h1.handleRequest(new Request("Negative Value ", 1));h1.handleRequest(new Request("Negative Value ", 2));h1.handleRequest(new Request("Negative Value ", -5)); }}?
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