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

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

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