责任链模式

责任链模式

1. 意图

   • 通过给对象一个机会处理请求来避免将请求的发送方耦合到接收方。链接接受对象，并沿链传递请求，直到对象处理它。

2. 解释

   案例：兽人王大声命令他的军队。最接近他并首先做出反应的是指挥官，然后是军官最后是士兵。指挥官、军官和士兵形成了一条责任链。

   简单说：它帮助构建一条对象链，一个请求从一个对象进入，并不断从一个对象进入另一个对象，直到找到合适的处理者为止。

3. 样例

   //请求
   import java.util.Objects;public class Request {private final RequestType requestType;private final String requestDescription;private boolean handled;public Request(RequestType requestType, String requestDescription) {this.requestType = Objects.requireNonNull(requestType);this.requestDescription = Objects.requireNonNull(requestDescription);}public RequestType getRequestType() {return requestType;}public String getRequestDescription() {return requestDescription;}public void markHandled(){this.handled = true;}public boolean isHandled(){return this.handled;}@Overridepublic String toString() {return getRequestDescription();}
   }

   //请求类型
   public enum RequestType {DEFEND_CASTLE,TORTURE_PRISONER,COLLECT_TAX;
   }

   //请求处理者
   public interface RequestHandler {boolean canHandleRequest(Request request);int getPriority();void handle(Request request);String name();
   }

   //指挥官
   public class OrcCommander implements RequestHandler{@Overridepublic boolean canHandleRequest(Request request) {return request.getRequestType() == RequestType.DEFEND_CASTLE;}@Overridepublic int getPriority() {return 2;}@Overridepublic void handle(Request request) {request.markHandled();System.out.println("Orc Commander handled request : " + name() + " : " + request);}@Overridepublic String name() {return "orc commander";}
   }

   //军官
   public class OrcOfficer implements RequestHandler{@Overridepublic boolean canHandleRequest(Request request) {return request.getRequestType() == RequestType.COLLECT_TAX;}@Overridepublic int getPriority() {return 3;}@Overridepublic void handle(Request request) {request.markHandled();System.out.println("Orc Officer collects taxes : " + name() + " : " + request);}@Overridepublic String name() {return "Orc Officer";}
   }

   //士兵
   public class OrcSoldier implements RequestHandler{@Overridepublic boolean canHandleRequest(Request request) {return request.getRequestType() == RequestType.TORTURE_PRISONER;}@Overridepublic int getPriority() {return 4;}@Overridepublic void handle(Request request) {request.markHandled();System.out.println("Orc Soldier torturing prisoner : " + name() + " : " + request);}@Overridepublic String name() {return "Orc Soldier";}
   }
   

   //兽人王
   import java.util.Arrays;
   import java.util.Comparator;
   import java.util.List;public class OrcKing{private List<RequestHandler> handlers;public OrcKing() {buildChain();}private void buildChain(){handlers = Arrays.asList(new OrcCommander(), new OrcOfficer(), new OrcSoldier());}public void makeRequest(Request request){handlers.stream().sorted(Comparator.comparing(RequestHandler::getPriority)).filter(handlers -> handlers.canHandleRequest(request)).findFirst().ifPresent(handler -> handler.handle(request));}
   }
   

   //测试
   public class Main {public static void main(String[] args) {var king = new OrcKing();king.makeRequest(new Request(RequestType.DEFEND_CASTLE, "defend castle"));king.makeRequest(new Request(RequestType.TORTURE_PRISONER, "torture prisoner"));king.makeRequest(new Request(RequestType.COLLECT_TAX, "collect tax"));}
   }
   

   

4. 结构图

5. 适用性

• 不止一个对象会处理请求，处理程序是位置的。应自动确定处理程序。

• 你希望在不显示指定接受者的情况下向多个对象中的一个发送请求。

• 应该动态指定可以处理请求的对象集。