用责任链模式实现军队指挥链路

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概述

本文将用用责任链模式实现一条指挥链路。首领发令给将军，将军传令给指挥官，指挥官再传令给士兵。首领、将军、指挥官、士兵形成一条指挥链路。

代码实现

首先我们定义Request和RequestType。

public class Request {
 
  private final RequestType requestType;
  private final String requestDescription;
  private boolean handled;
 
  public Request(final RequestType requestType, final String requestDescription) {
    this.requestType = Objects.requireNonNull(requestType);
    this.requestDescription = Objects.requireNonNull(requestDescription);
  }
 
  public String getRequestDescription() { return requestDescription; }
 
  public RequestType getRequestType() { return requestType; }
 
  public void markHandled() { this.handled = true; }
 
  public boolean isHandled() { return this.handled; }
 
  @Override
  public String toString() { return getRequestDescription(); }
}
 
public enum RequestType {
  DEFEND_CASTLE, TORTURE_PRISONER, COLLECT_TAX
}

然后我们定义抽象类RequestHandler及其实现类OrcCommander，OrcOfficer和OrcSoldier代码是和OrcCommander及其类似的，不再赘述。OrcKing不仅掌握了将军、指挥官和士兵，还实现了命令的传递。

public abstract class RequestHandler {
  private final RequestHandler next;
 
  public RequestHandler(RequestHandler next) {
    this.next = next;
  }
 
  public void handleRequest(Request req) {
    if (next != null) {
      next.handleRequest(req);
    }
  }
 
  protected void printHandling(Request req) {
    LOGGER.info("{} handling request \"{}\"", this, req);
  }
 
  @Override
  public abstract String toString();
}
 
public class OrcCommander extends RequestHandler {
  public OrcCommander(RequestHandler handler) {
    super(handler);
  }
 
  @Override
  public void handleRequest(Request req) {
    if (req.getRequestType().equals(RequestType.DEFEND_CASTLE)) {
      printHandling(req);
      req.markHandled();
    } else {
      super.handleRequest(req);
    }
  }
 
  @Override
  public String toString() {
    return "Orc commander";
  }
}
 
public class OrcKing {
  RequestHandler chain;
 
  public OrcKing() {
    buildChain();
  }
 
  private void buildChain() {
    chain = new OrcCommander(new OrcOfficer(new OrcSoldier(null)));
  }
 
  public void makeRequest(Request req) {
    chain.handleRequest(req);
  }
}

实际调用如下

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"));

控制台输出如下，这样就实现了一条完整的指挥链路。

Orc commander handling request "defend castle"
Orc officer handling request "torture prisoner"
Orc soldier handling request "collect tax"

使用场景

• 不局限于一个对象能够处理请求。
• 无需指定具体的请求对象。
• 处理请求的对象能够灵活切换。

类似案例

• java.util.logging.Logger#log()
• Apache Commons Chain
• javax.servlet.Filter#doFilter()

参考文献

• Design Patterns: Elements of Reusable Object-Oriented Software
• Head First Design Patterns: A Brain-Friendly Guide