职责链模式

职责链模式(Chain of Responsibility Patterm)，又叫责任链模式，为请求创建了一个接收者对象的链(简单示意图)。这种模式对请求的发送者和接收者进行解耦。
职责链模式通常每个接收者都包含对另一个接收者的引用。如果一个对象不能处理该请求，那么它会把相同的请求传给下一个接收者，依此类推。
这种类型的设计模式属于行为型模式

Handler:抽象的处理者,定义了一个处理请求的接口,同时含义另外Handler
ConcreteHandlerA, B是具体的处理者,处理它自己负责的请求，可以访问它的后继者(即下一一个处理者),如果可以处理当前请求，则处理，否则就将该请求交个后继者去处理，从而形成一个职责链

应用实例：职责链模式解决OA系统采购审批

应用实例要求
编写程序完成学校OA系统的采购审批项目:需求
采购员采购教学器材
如果金额小于等于5000,由教学主任审批
如果金额小于等于10000,由院长审批
如果金额小于等于30000,由副校长审批
如果金额超过30000以上，由校长审批

public abstract class Approver {

   Approver approver;  //下一个处理者
   String name; // 名字
   
   public Approver(String name) {
      // TODO Auto-generated constructor stub
      this.name = name;
   }

   //下一个处理者
   public void setApprover(Approver approver) {
      this.approver = approver;
   }
   
   //处理审批请求的方法，得到一个请求, 处理是子类完成，因此该方法做成抽象
   public abstract void processRequest(PurchaseRequest purchaseRequest);
   
}

public class CollegeApprover extends Approver {

   public CollegeApprover(String name) {
      // TODO Auto-generated constructor stub
      super(name);
   }
   
   @Override
   public void processRequest(PurchaseRequest purchaseRequest) {
      // TODO Auto-generated method stub
      if(purchaseRequest.getPrice() > 5000 && purchaseRequest.getPrice() <= 10000) {
         System.out.println(" 请求编号 id= " + purchaseRequest.getId() + " 被 " + this.name + " 处理");
      }else {
         approver.processRequest(purchaseRequest);
      }
   }
}

public class DepartmentApprover extends Approver {

   
   public DepartmentApprover(String name) {
      // TODO Auto-generated constructor stub
      super(name);
   }
   
   @Override
   public void processRequest(PurchaseRequest purchaseRequest) {
      // TODO Auto-generated method stub
      if(purchaseRequest.getPrice() <= 5000) {
         System.out.println(" 请求编号 id= " + purchaseRequest.getId() + " 被 " + this.name + " 处理");
      }else {
         approver.processRequest(purchaseRequest);
      }
   }

}

public class SchoolMasterApprover extends Approver {

	public SchoolMasterApprover(String name) {
		// TODO Auto-generated constructor stub
		super(name);
	}
	
	@Override
	public void processRequest(PurchaseRequest purchaseRequest) {
		// TODO Auto-generated method stub
		if(purchaseRequest.getPrice() > 30000) {
			System.out.println(" 请求编号 id= " + purchaseRequest.getId() + " 被 " + this.name + " 处理");
		}else {
			approver.processRequest(purchaseRequest);
		}
	}
}

public class ViceSchoolMasterApprover extends Approver {

	public ViceSchoolMasterApprover(String name) {
		// TODO Auto-generated constructor stub
		super(name);
	}
	
	@Override
	public void processRequest(PurchaseRequest purchaseRequest) {
		// TODO Auto-generated method stub
		if(purchaseRequest.getPrice() > 10000 && purchaseRequest.getPrice() <= 30000) {
			System.out.println(" 请求编号 id= " + purchaseRequest.getId() + " 被 " + this.name + " 处理");
		}else {
			approver.processRequest(purchaseRequest);
		}
	}
}

//请求类
public class PurchaseRequest {

   private int type = 0; //请求类型
   private float price = 0.0f; //请求金额
   private int id = 0;
   //构造器
   public PurchaseRequest(int type, float price, int id) {
      this.type = type;
      this.price = price;
      this.id = id;
   }
   public int getType() {
      return type;
   }
   public float getPrice() {
      return price;
   }
   public int getId() {
      return id;
   }
}

public class Client {

   public static void main(String[] args) {
      // TODO Auto-generated method stub
      //创建一个请求
      PurchaseRequest purchaseRequest = new PurchaseRequest(1, 31000, 1);
      
      //创建相关的审批人
      DepartmentApprover departmentApprover = new DepartmentApprover("张主任");
      CollegeApprover collegeApprover = new CollegeApprover("李院长");
      ViceSchoolMasterApprover viceSchoolMasterApprover = new ViceSchoolMasterApprover("王副校");
      SchoolMasterApprover schoolMasterApprover = new SchoolMasterApprover("佟校长");
   
   
      //需要将各个审批级别的下一个设置好 (处理人构成环形: )
      departmentApprover.setApprover(collegeApprover);
      collegeApprover.setApprover(viceSchoolMasterApprover);
      viceSchoolMasterApprover.setApprover(schoolMasterApprover);
      schoolMasterApprover.setApprover(departmentApprover);
      
      
      
      departmentApprover.processRequest(purchaseRequest);
      viceSchoolMasterApprover.processRequest(purchaseRequest);
   }

}

职责链模式细节和注意事项

将请求和处理分开，实现解耦，提高系统的灵活性
简化了对象，使对象不需要知道链的结构
性能会受到影响，特别是在链比较长的时候，因此需控制链中最大节点数量，一般通过在Handler中设置一个最大节点数量，在setNext(方法中判断是否已经超过阀值，超过则不允许该链建立，避免出现超长链无意识地破坏系统性能
调试不方便。采用了类似递归的方式，调试时逻辑可能比较复杂
最佳应用场景:有多个对象可以处理同一个请求时，比如:多级请求、请假/加薪等审批流程、JavaWeb中Tomcat对Encoding的处理、拦截器